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 Post subject: Re: My Benjamin 392
PostPosted: Sun Jan 05, 2020 6:56 pm 
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Location: Coalmont BC
Here is the start of my preload adjuster, which may morph into an SSG.... I cut off the top of the trigger, which formed the safety interlock with the bolt.... This is designed to prevent firing the gun with the bolt open, but also meant that you could not uncock the gun, a feature I really didn't like.... I then took a 1.25" length of 3/4" OD 6061-T6 aluminum bar stock drilled a 21/64" hole through it so that it could be tapped to 3/8"-24NF for an adjusting screw, and then faced the ends off so that it was a drop-in fit into the recess in the top of the trigger housing.... It is a perfect fit, providing you get the length right.... Note that the potmetal casting has a slight taper to allow it to come out of the mould, so you have to measure the length at the bottom of the recess, not the parting face.... and on my trigger housing one side was about 0.005" shorter than the other.... ::)

Image

I will drill out the ends of the housing at some point when I get ready to make the adjuster, but for the next round of testing I don't want to disturb the spring seat, so that will come later....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Tue Jan 07, 2020 12:36 am 
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Today was trigger day.... I have been reading about tuning the 392 trigger group, some of the best info was from Timmy@Mac1, who makes the Steroids, of course.... I also read up on the AoA "Super Sear", which has undergone a downgrade, and asked some questions of others who have tuned their own 39X triggers.... I decided to do my own tune, with a bit different "flavour".... Let me make a warning right now about trigger safety.... Many attempts trying to make a conventional trigger into a 2-stage can leave you with an unsafe trigger.... If you don't understand what a "balk fire" is, then you need to read up on it, because it is VERY dangerous.... The short summary is that if you modify a trigger incorrectly, you may create a situation where pulling the trigger through part way (eg. to the beginning of the 2nd stage) can leave a trigger hanging because it doesn't return to full sear engagement.... Imagine a situation where a trigger normally has 1/16" of sear engagement, and without you realizing it, an aborted shot leaves you with the sear engagement only 0.005".... The gun could fire by bumping it.... It you don't know what you are doing, leave your trigger alone.... That means DO NOT copy what I did, because get it just slightly wrong, and your gun may be an accident waiting to happen.... This is a report on what I did, not a recommendation that you follow suit....

The stock 392 trigger is extremely safe, but a very heavy pull weight.... Most of this is because of the geometry of the sear.... When you slowly squeeze the trigger, you are actually moving the hammer back slightly against the hammer spring.... This is most of the reason for the heavy pull weight, but it also guarantees that the trigger will reset and not leave you hanging.... To get around this, I followed the lead of Tim's steroids, and reshaped the sear so that when it is pulled down, it does not move the hammer back.... This makes it much easier to pull, but it won't return if you don't fire the gun and release it.... Tim also grinds the engagement ring on the hammer to match the sear angle, and then rehardens both components for longevity.... I did neither, I'm willing to see how long it lasts, and repair it if it doesn't....

The sear only engages the hammer by the depth of the rear ring on the hammer.... The front ring has nothing to do with the trigger, it is there so that the SHCS on the bottom of the bolt can cock the gun.... There is a second bump on the sear that gives you a "1/2 cock" position if you pull the bolt back a short distance, which causes an audible "click" and leaves the hammer not touching the valve stem.... The angle on the rear sear is the one I worked on.... I changed it from about 17 deg. to about 5 deg.... Basically I made it almost part of an arc of a circle centered on the sear pivot pin.... I was going to make a fancy jig, but got lazy and just did it by hand with a diamond file, checking the angle until I was happy with it, and it didn't move the hammer back when I squeezed the trigger.... I then polished it, and I also polished the top of the tang that the trigger rubs on, and the bottom of the radius on the front part of the trigger that pushes down on the sear tang.... Here is a photo of the sear after I changed the angle and polished it....

Image

You can clearly see how much I changed the angle.... Now WHAT you are asking yourself is that SHCS doing there?.... :shock:

After getting the trigger working the way I wanted, including installing a MUCH lighter coil spring behind the trigger, I found out just how bad the "creep" in the trigger really is.... Since the sear is engaged well over 1/16", the trigger moves back a long ways during the period that sear is moving down on the hammer engagement ring.... I wanted to shorten the engagement, and since the lighter spring behind the hammer provides a "fake" second stage that was very short, I thought long and hard about how to make it "ACT" like a 2-stage by shortening the sear engagement.... The result is much like a worked over QB trigger, which can be quite nice.... It has a "fake" first stage where all that is happening is the trigger is moving against a light spring, and the sear adjustment allows you to set how far the sear moves once the trigger starts to move it.... The problem was, there was nowhere easy to put an adjusting screw in the housing, because it had nothing on the sear to push against to set the sear engagement.... So, I added that screw in the spring hole, and added a 1/8" long #6-32 setscrew in the top of the trigger housing to push against it.... Bingo, I had an adjustable sear engagement, that looks like this (sear shown upside down)....

Image

The setscrew "A" pushes against the head of the SHCS "B", and the further you screw it in, the less the sear engagement is.... Now HOW did I get that 6-32 SHCS into the spring hole in the sear?.... With great difficulty and a little luck.... :roll: …. If the sear was not hardened, I could have just tapped the hole, but an HSS tap wouldn't do more than scratch it.... The hole is 0.125", and a #6 screw is only 0.135", so I drilled a 1/16" hole through the screw for the tang on the spring, shortened the screw to a few thou more than the thickness of the sear, put some Loctite 638 on it, and using brute force, wound it into the hole in the sear from the left (under) side, opposite where the hairpin spring is.... I then peened over the end of the screw, and figure it won't come out.... That gave me a "bump" on the underside of the sear for the adjusting screw to push against, so all I had to do was mark where it needed to go, and drill and tap the housing (very close to the edge) for the adjusting screw.... I screwed the adjuster in flush with the top of the trigger group and assembled it for initial testing.... This gives me the minimum sear engagement, and at that adjustment, the assembled trigger looks like this....

Image

I didn't even know if the sear would catch the hammer, but to my surprise it did.... and the trigger felt GREAT !!! …. 8) …. The first "stage" was much longer and very light (because of the light trigger spring), and the force increases significantly when the trigger touches the tang on the sear, because of the fairly heavy (and stock) hairpin sear spring.... I didn't know how much engagement the sear had, and the answer was "not much", as virtually any movement of the second (actual) stage fired the gun.... I tried it several times, and it felt reliable, and I also cocked the gun and banged it around in every direction, thumping it against my workbench and with my fist, and it never fired once.... My conclusion was that the sear, with the adjustment screw flush with the top of the trigger housing was "safe", but with VERY little engagement.... I disassembled the trigger once more, lubed all the contact points with moly paste, and put a drop of blue Loctite on the sear adjusting screw and set it 1/2 turn above flush.... In this position it still misses the bottom of the main tube, and I love the way it feels.... It is nice and crisp, and you can move it a whisker on the 2nd stage pull, but for all intents and purposes it breaks cleanly, with no noticeable creep.... a great hunting trigger, IMO.... Incidently, set this way the "half-cock" does not work.... You can hear it click over the front sear, but with the sear in this position, it won't catch the hammer.... If you back the adjusting screw out all the way, it hits the main tube, but the sear would be fully engaged, just like a stock gun.... It looks like this backed out all the way, so that the head of the SHCS is touching the inside of the trigger housing....

Image

If you wanted to have the full sear engagement, you could just remove the adjusting screw so it wouldn't hit the tube.... I didn't see how far out I could screw it and still miss the main tube, but maybe 1 turn above flush?.... The point is, I can get a safe sear engagement and still have a nice trigger, so I'm a happy camper.... Remember, do NOT attempt this unless you know what you are doing.... YOU are responsible for your safety and that of others, don't blame me if you have a problem.... YOU HAVE BEEN WARNED !!!!

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 08, 2020 1:15 am 
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Location: Coalmont BC
I decided today to make my velocity adjuster, which may become an SSG.... Here is a photo of the assembled parts....

Image

The body is a piece of 3/4" diameter 6061-T6 aluminum 1.15" long (sized to fit the cavity in the trigger group)…. It is drilled and tapped 3/8"-24 NF for the front 3/4", and the back is drilled out with a Size "W" letter drill, which is 0.386".... That gives clearance for the shank of the adjuster, which is a 1.5" long bolt that has 1" of threads.... I drilled the bolt through with a #11 drill, which is 0.191" to provide a nice sliding fit on a piece of 3/16" drill rod.... There is a piece of it slid through the hole in the bolt in the photos.... If I decide to make an SSG, a longer piece of that drill rod will be the spring guide I will use.... The nice thing about using 3/16" rod is that I can thread it 10-32 on the ends for the front spring seat and adjusting nuts....

You can see a setscrew protruding from the top of the aluminum housing.... It is a #4-40, installed in a tapped hole with Loctite.... The purpose of it is to prevent the round bar from rotating in the trigger housing.... It fits into the slot in the top of the housing.... The next photo shows the adjuster sitting in place in the trigger group....

Image

I drilled the front of the housing out with the Size W drill, drilling through the original spring seat.... The back of the housing is drilled a bit larger, with a 7/16" drill, as it just needs to provide lots of clearance for the 3/8" adjuster bolt.... You can see another setscrew (a #8-32 that is 1/8" long) in the bottom of the adjuster housing.... It has a short piece of 1/8" plastic rod under it, pressed against the side of the 3/8" adjuster threads to act as a brake.... This allows me to change the position of the adjusting bolt, but prevents it from adjusting itself.... I do most of my adjusters that way....

The way the adjuster is currently set has the front of the adjuster bolt in the same location as the original spring seat.... so the gun will behave as if the adjuster wasn't there.... The 1.5" long bolt can move forward about another 1/16" to increase the hammer spring tension, and has more than 1/2" of adjustment to the rear, for a total of about 5/8".... If I need a more forward position for the spring seat, I can add a spacer/spring guide to provide that.... but in reality I will probably need to move the adjuster back, to allow for the installation of an SSG at a later date.... The ability to adjust the hammer strike will greatly enhance the ability to tune the gun as a retained air pumper (RAP, aka ACP - air conserving pumper)….

I assume that existing ACPs, such as that sold by AoA, must have some way of tuning the hammer strike.... but if so, I don't know what they use.... I think that this version will work great....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 08, 2020 10:34 pm 
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Location: Coalmont BC
Today I drilled out the barrel port, slimmed down the bolt probe, and drilled the holes in the tube for the side valve screws.... It turned out to be a LOT of hand work, mostly because I could not grasp the soldered barrel and tube assembly in my very small milling attachment on my lathe.... The side holes for the valve I had to drill undersize in approximately the right place, install the valve with the bottom screw, mark the position of the side screws, and then Dremel the holes over so that they fit the screw heads nice and snug.... It took a long time, but the results are great.... When I install the side screws, they are snug in the holes in the tube, and the bottom valve/trigger screw threads in with NO resistance, it lines up perfectly.... I am a LOT happier with the strength of the valve mounting now.... 8)

Drilling out the barrel port was likewise problematic, because I could not mount it in the lathe milling attachment.... I drilled it out one number drill size at a time until I got to 11/64" (0.172"), which is 79% of the bore.... Since the exhaust hole in the valve measures 3/16" at the top, because it was milled on an angle, I used a 3/16" center drill to put a slight taper in the barrel port, so that there is no step at the valve, but the port at the boreline stays at 0.172"....

Even the bolt probe was a problem, because with the bolt handle attached (it appears to be one piece) I could not chuck it in my lathe.... I ended up removing the O-ring, and sanding the probe smaller on the edge of a disc sander, and at the same time tapered the front of the O-ring gland, because it partially restricts the (now larger) barrel port.... Here is a photo of the thinned out bolt probe and the side valve screws (which are just above the stock)…. At the back of the tube you can see the hex head on the bolt for adjusting the hammer spring preload.... At the moment it is set for stock preload....

Image

While I was working on the tube, I investigated why the valve was so tight when in position.... I had to drive it back the last inch, and it was almost impossible to turn to get the screws lined up.... It turns out the the roll-stamp on the left side is so deep that it dented the tube on the inside.... I had to file and sand off the bump on the inside (not an easy job), but it turned out really well, and now the valve slides in to position the way it should.... :roll:

Here is a photo of the new front pins.... They have a single nut peened and loctited in place on one side, and double nuts on the other.... I will eventually get a couple of Ny-Loc nuts instead, they will be simpler and look better....

Image

The last photo shows where the pump handle sits with no pressure in the air tube.... I adjusted the gap in the valve to 0.050" to achieve this position.... It is easy to remember, because the top of the pump handle is level with the bottom of the stock....

Image

These changes took the whole afternoon.... After dinner I did some testing, the results will be in the next post....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 08, 2020 10:54 pm 
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OK, time to test this beast.... All the port mods are now done, both in the valve and barrel, including a slimmer poppet stem (compliments of the MRod poppet), larger valve throat and exhaust port, and a slimmer bolt probe.... The smallest port in the system is the barrel port, which is 0.172" (79% of bore)…. It was 0.154" stock, so I have increased the area by 25%.... Everything else in the system can flow more air than that.... I had the hammer spring preload adjuster set so that the preload was exactly the same as stock.... The gun does not retain any air even at 14 pumps (more about that later)…. I checked the velocity every 2 pumps, ending at 14, and here is the results....

Image

The dotted lines are the previous results, with the valve mods done, but not the barrel port and bolt probe.... Obviously they were hampering the airflow.... The larger ports make a big difference at higher pump numbers, as expected.... I then tried different pellet weights at 8 pumps and 12 pumps.... Here are those results....

Image

The solid lines are at 8 pumps, and the dotted lines are at 12 pumps.... The gun now exceeds 22 FPE with the 25.4 gr. Monsters at 12 pumps.... 8) …. I did one additional test, using my 27.4 gr. BBT HPs at 14 pumps.... I got 631 fps, which is 24.2 FPE.... I'm pretty happy with that, still using the stock pump....

I then backed off the hammer spring preload, because I was curious when the gun would start retaining air.... I backed it off a full 1/2" from stock, and it still showed NO velocity loss at 8 pumps.... but it was a LOT quieter.... I then tried 10 pumps, still no retained air.... At 12 pumps, it finally retained a small puff of air.... It looks like I can make an SSG, probably using the stock spring, and still have it dump 14 pumps.... That may be my next project.... ;)

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Wed Jan 08, 2020 11:09 pm 
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awesome work as usual Bob... 8)

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 Post subject: Re: My Benjamin 392
PostPosted: Thu Jan 09, 2020 12:20 am 
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Location: Coalmont BC
I hate doing things by hand.... too much chance of an error ruining a good project.... :roll:

I couldn't believe it when I saw the bumps on the inside of the tube from the roll-stamp.... :evil:

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Thu Jan 09, 2020 9:52 pm 
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This afternoon I made and installed an SSG (stopping spring guide).... Here is a photo of it, ready for installation....

Image

The concept is quite simple.... There is a 3/16" spring guide rod 4" long that slides through the adjusting bolt.... On the front is a nut threaded and glued in place, and then turned down to fit inside the hammer.... At the back the rod is threaded 10-32 and a pair of nuts are used to adjust the preload on the spring.... In this case, I have 0.50" of preload, which is about 3.5 lbs.... There is a small O-ring on the rod between the rear nuts and the head of the adjusting bolt, to absorb the impact when the spring guide stops, and reduce the noise of that collision.... The entire assembly threads into the aluminum mounting sleeve in the top of the trigger group.... I chose 3/8"-24NF threads so that the assembly may be removed to change the preload or spring without disassembling the gun.... Here is a photo of it installed....

Image

Turning the hex head bolt in towards the gun reduces the gap between the end of the spring guide and the bottom of the hole in the back of the hammer.... The idea is to have a small gap between them, so that the spring stops pushing the hammer just before it hits the valve stem.... The hammer then carries on from its own momentum, strikes the stem and opens the valve.... The hammer is NOT preloaded by the SSG, it can "rattle around" in the gap between the valve stem and the spring guide....

The magic happens when the valve closes after firing.... The hammer is thrown back against the now stationary spring guide, but because of the preload on the spring, and the mass of the guide rod, instead of recompressing the spring and getting thrown back at the valve and reopening it (the infamous hammer bounce)…. it just rattles around, without enough energy to open the valve a second (or third) time and wasting air.... Changing the gap adjusts how hard the hammer hits the valve stem, because as you increase the gap, you reduce the amount the spring is compressed when you cock the gun.... I tested the gun with the SSG in place, and I got a pleasant surprise....

Image

The solid blue line is the velocity with 15.9 gr. pellets as I increased the SSG gap.... As you can see, the velocity did not begin to drop off until I had 6 turns of gap (1/4")…. When I compared the velocity with what I got yesterday, without the SSG, I was surprised to see that with no gap (and indeed yesterday without the SSG) I got 659 fps, which increased to 666 fps as the gap was increased to 3 turns (1/8") and over the next 2 turns it returned to 659 fps.... I have no concrete reason why the velocity should increase about 1% with the SSG, but I suspect that without the SSG, the hammer is bouncing off the back of the valve, and that is actually reducing the dwell a whisker.... Anyway, the important thing is that I can have 5 turns of gap and not lose any velocity with the SSG installed....

Now remember that without the SSG the gun did not retain any air, all the way to at least 14 pumps.... Now it retains a puff of air at 8 pumps with the SSG installed, even at zero gap.... At 5 turns of gap, it is retaining enough air to produce a 2nd shot of over 400 fps (the dashed blue line)…. That is roughly the velocity the gun shoots at 3 pumps.... so in theory I could pump 5 times and get back to the pressure I had without the SSG with 8 pumps.... That is why they call this type of gun a "Retained Air Pumper" (RAP, aka Air Conserving Pumper or ACP)…. 8)

As I continued to increase the SSG gap, the velocity of the first shot dropped, and that of the second shot increased.... By the time I got to 9 turns of gap (3/8") the second shot was slightly faster than the first.... Not only that, but there was enough air left to produce a 3rd shot, with the gap set to 7 turns or more (dotted blue line)…. I repeated the testing at 12 pumps, the results are in red above....

If I adjust the SSG gap to about 8.5 turns, I should be able to tune the gun to have two equal shots of about 550 fps (10.7 FPE) without pumping in between.... However, since there is a bit of air left after the second shot, I can't just pump 8 pumps to repeat the procedure.... However, I might be able to pump it about 6 times after every 2 shots, and keep getting a string of pairs of ~11 FPE shots, for only 3 pumps per shot.... instead of 5 pumps per shot to get the same power without the RAP design and the SSG....

The SSG opens up a whole range of possibilities of different ways to tune my 392.... With 2 turns of gap, I could pump to 12 pumps, and get a shot of 755 fps (20 FPE)…. There would be enough air for a 2nd shot of about 550 fps (normally about 5 pumps), so I should be able to add 7 pumps (instead of 12), and keep shooting at 750 fps for as long as I want to pump.... The best way to do an RAP is to have a gauge.... That lets you know exactly how many pumps you need to get back to the pressure you need for the power you want.... The really cool part is that you have fewer pumps required to get back to full power....

If you need any further proof that an SSG really changes things, just listen to the difference.... Without it, you can hear the hammer bounce, with that telltale burrrppppp sound.... With the SSG, you just hear one POP.... and as you tune it to lower power levels it gets a LOT quieter.... 8)

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Sat Jan 11, 2020 12:47 am 
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Location: Coalmont BC
I pulled the 392 apart again today and measured it up for a new pump piston with a flat face and an O-ring seal.... and decided that would also need a new valve front end and check valve assembly.... The only thing that really makes sense is to make that front end as a 2-piece valve, from a time point of view.... That part of the project will wait until I get some pressure measurements with the existing valve and pump to provide a baseline....

I measured up the valve and cut two new O-ring grooves, one on the back half, just in front of the existing exhaust port and valve screws, and the other on the front half, far enough back to allow a gauge between it and the front O-ring.... I used a # 113 O-ring for the front one, and a # 016 for the back one.... The rear groove is just behind the valve seat (where the valve is solid).... and the front groove is just ahead of the shoulder where the washer sits between the check valve spring and the valve spring (where the valve wall is the thickest).... There is plenty of room between the front two O-rings for a gauge port and two side mounting screws, which will be necessary when I make the 2-piece valve.... Here is a photo of the valve with the distance between the O-ring grooves dimensioned....

Image

I clamped the main tube in my milling attachment on my lathe, centered off the bottom valve screw hole, and drilled a gauge hole 2.25" ahead of that.... Since I am using a 1/16" to 1/8" NPT adapter for the gauge, the hole in the tube is 5/16" which will just clear the adapter threads.... I installed the valve, with the two halves unscrewed with the same 0.050" gap as before, and scribed the gauge location between the front O-rings.... That looks after not only the location, but the clocking of the threads (at the gap selected)…. I then drilled and tapped the valve at the marked location to 1/16"-27 NPT.... The hole is in the check valve section so there is lots of thread depth.... I cleaned and reassembled the valve, and reinstalled it in the gun, and threaded in the gauge and adapter.... It looks like this....

Image


I laid out and drilled a 1" hole through the stock with a hole saw, and then using a Dremel sanding drum fitted it to a fairly close fit around the gauge.... Once the gun is assembled, the gauge sits in a hole in the stock much like on a Disco....

Image

I'm quite pleased with the appearance.... The gauge I had was a 3K gauge, and I should probably get one that has a 2000 psi scale.... However, it will at least let me find out what pressures I am pumping to now.... I don't know how accurate the gauge is, but it doesn't really matter, as all I want is to be able to pump to the same pressure for each shot, and to compare changes to the pump and when I change the valve.... It looks like 12 pumps with the stock pump, adjusted the way it is, and stock valve volume, is about 1200 psi.... The valve volume is slightly larger because of the gauge and adapter, maybe 10%.... It doesn't leak down while taking a couple of shots to test it, so I have put 5 pumps in it (just over 500 psi) to leave it overnight to see if it has a slow leak....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Sat Jan 11, 2020 3:07 pm 
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Well, the gun didn't leak at all overnight.... 8) …. However, I am getting a puff of air out the side screw holes on firing, particularly at high pump numbers.... :(

Today I tested the velocity and pressure at every pump stroke up to 15 pumps.... That was just over 1500 psi, which I am going to use as a self-imposed limit (at least for now) because of using the stock pump linkage, and because I want to maintain the safety margin on all components, particularly the valve screw pockets in the tube.... I did the same on my .25 cal Disco Carbine.... I pumped it to 1800 psi to test it, and then backed off to 1500, for longevity, and also because it was harder to pump than I liked at 1800.... :roll:

I calculated the additional volume from adding the gauge, which was about 0.4 cc, so the total volume has now increased to about 4.6 cc.... This means that for any given number of pump strokes the velocity and FPE have dropped a few fps because the pressure has dropped by about 10%.... This is because the swept volume is the same, but since the valve is larger the pressure gain on each pump stroke is a bit less.... In other words, it takes about 11 pumps now to get the same pressure as I had before at 10 strokes.... However, if you take that into consideration, the gun is developing more power at the same pressure than before, because of the extra valve volume, which causes a lower pressure drop during the shot.... I set the SSG at 1 turn of gap for this testing, and recorded the pressure and velocity for the first shot, and once the gun began retaining air, I did the same for the second shot (dotted lines)….

Image

This gives me a really good baseline for further work.... As I increase the valve volume, the pressure gauge is a necessity to know how many pumps to use to get back to the same pressure.... It will be equally important when I start to work on the pump, not only to measure any gains in pump efficiency, but to make sure I am not overstressing the components.... When I compare this pressure curve to the graphs in Reply #66, the closest match is a headspace of 0.3 cc (because I adjusted the valve length a bit longer) and an efficiency of about 75% (when corrected for the increased valve volume)…. The next step is to remove the valve and bore it out to increase the volume....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Sat Jan 11, 2020 7:20 pm 
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Location: Coalmont BC
Today's project was to bore out the valve, removing as much metal as I could safely, without cutting holes in the mid section.... Even though I currently have the two extra O-rings needed to do that, I wanted to see how much I could increase the volume and still have the valve look stock on the outside (except for the gauge port)…. At this point, the two extra O-ring grooves are not needed, even for the gauge, since it seals to the valve with Teflon tape.... Here is a photo of the outside of the valve at this point....

Image

The gauge port is exactly 2.25" ahead of the lower valve screw hole, measured center-to-center.... The first thing I did on the valve to increase the volume was to cut off some of the threads.... I left the stub 3/8" long, measured from the shoulder…. I then bored out the inside of the valve front to 5/8" ID, from about 1/16" ahead of the shoulder where the O-ring sits down to the shoulder where I had the spring seat.... The length of that bored out section ended up at 5/8".... I had to make a new double ended spring seat to hold the check valve spring and the front end of the valve spring, I just turned it out of aluminum.... It is 3/8" OD on the front end, 0.450" in the middle, and drilled to 0.29" to accept the outside of the much lighter check valve spring.... I also shortened the check valve some more, to make up for the extra volume of the new seat.... The back of the seat is 1/4" OD to fit inside the valve spring, and the through hole is 3/16".... Here is a photo of the "guts" of the valve, ready to install....

Image

The back half of the valve also got bored out to 5/8" ID, which just removed the threads inboard of what I needed to secure the two halves together.... The 5/8" ID carried on all the way back to the solid area where the seat is.... That section of the 5/8" ID is about 3/8" long, starting from 5/16" inside the valve, leaving the same threaded length as on the stub, once you allow for the 0.050" gap when assembled.... Here is a photo looking inside the valve halves, to show where they were hogged out....

Image

I calculated the increase in volume from the machining, and it works out to 2.1 cc.... That would be a 50% increase on a stock valve, but with my gauge port and adapter, my total volume is now 6.7 cc.... That is 50% larger than with the gauge installed, and a 60% increase over stock.... To get to the same pressure, it should therefore need 50% more pumps than the graph I posted in my last post.... We'll soon see, as the next thing to do is reassemble the gun and repeat the last set of tests....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Sat Jan 11, 2020 9:53 pm 
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Location: Coalmont BC
After dinner I ran the same tests I did with the stock valve with gauge.... I measured the pressure and velocity of the first and second shot every 2 pumps up to 20, where the pressure exceeded 1500 psi.... I was still using the same 15.9 gr. pellets as all the other similar tests.... As expected, I require more pumps to get to the same pressure as before, because the valve volume is 50% larger, at 6.7 cc.... I am still running the SSG at 1 turn of gap.... Here are the results....

Image

It looks like I have gained significantly in pumping efficiency.... With a 50% valve volume increase I had expected 50% more pumps to get to the same pressure, but these results are significantly better than that.... I got 1520 psi with 20 pumps, whereas I expected that to take 22 pumps.... This 2 pump advantage over what I expected occurred all the way along the pressure range, from 600 psi and up.... The first few pumps are MUCH easier, which I think is from increased pump efficiency from filling the larger valve.... There is probably also an efficiency gain because the headspace, which hasn't changed, is a smaller percentage of the larger valve volume....

The velocity and energy is significantly greater at the same pressure with the larger valve volume.... This is much like what happens when you install a larger plenum in a PCP.... In addition, the residual pressure for the second shot is also greater, and should increase even more when I dial back the SSG.... At 1520 psi (20 pumps) I got 827 fps with the 15.9 gr. pellets (24.2 FPE), and the 2nd shot was 622 fps (13.7 FPE)…. for a total of nearly 38 FPE.... I tried the 25.4 gr. Monsters at the same pressure, and hit 698 fps (27.5 FPE) with a 2nd shot of 524 fps (15.5 FPE) for a total of 43 FPE.... The next thing to test is to see what happens to the velocity of both shots when I dial the hammer strike back by increasing the SSG gap.... It will be interesting to see how many pumps per shot are required to get a continuous string of 20 FPE shots....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: My Benjamin 392
PostPosted: Sun Jan 12, 2020 7:30 pm 
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OK, guys, now we're into the meat and potatoes of this build.... :D …. Today I wore out my arm completely, shooting out all the air in the valve at 4 different pressures and up to 15 different SSG gap settings.... Here are the results, on 2 charts so they aren't so crowded.... In all cases the solid line is the first shot, the dashed line the second shot, and the dotted line is the third shot.... By the time the 3rd shot got to about 500 fps there was a 4th shot, and although I recorded it, I didn't graph it, it would have made the graphs even harder to understand....

Image

On the above chart, the red lines are at 6 pumps (about 600 psi), and the blue lines are at 10 pumps (about 900 psi)…. You will note that the plateau (665-670 fps) on the 10 pump curve extends to 8 turns of gap (over 5/16") and on the 6 pump curve (550-555 fps) past 10 turns of gap (about 7/16")…. Reducing the gap for more consistency by using a lighter spring and/or less preload would be better.... MAN is the gun easy to cock with the SSG installed, and would be even nicer with a lighter spring and smaller gap.... Now the details for these pressures....

At 6 pumps, all three curves cross at 14 turns of gap, giving a 3-shot bell curve of 385, 400, 360 fps (a 10% ES)…. This is only 5.1 FPE average, and I didn't explore it further.... At 10 pumps, I can get two shots of 555 fps (10.9 FPE) without repumping, at 10.7 turns of gap.... It takes 7 turns to repump back to the same pressure as I get at 10 from empty, or if I only shoot 1 shot, just 3 pumps.... and I can keep shooting at 550-555 fps with 3 pumps required per shot.... That actually works out to 1.02 FPE/CI, based on the volume of air pumped (58 cc or 3.54 CI per stroke)…. which is very good for a pumper.... 8)

Image

On the second chart, the red lines are at 14 pumps (about 1200 psi), and the blue lines are at 20 pumps (just over 1500 psi)…. The plateau at 14 pumps (742-748 fps) extends past 6 turns of gap (1/4"), and at 20 turns the velocity starts to drop from 824 fps (24 FPE) after 3 turns of gap (1/8")…. With the SSG set at just over 3 turns of gap, I still have maximum power available, but over half the air is retained, so I only have to pump 10 pumps to get back to 24 FPE, instead of 20 pumps.... The same thing applies at the 14 pump level, set for where the velocity just starts to decrease, it only takes 7 pumps to refill to 19.5 FPE, instead of 14 pumps.... For maximum power work that means the hammer spring and preload are OK, but I could still probably reduce the preload, or use a lighter spring, particularly for a 2-shot tune at 14 pumps....

At 14 pumps the first and second shots have the same velocity (640 fps) with the SSG set to 8.5 turns of gap.... That gives two shots at 14.5 FPE without repumping between, and takes 10 pumps (instead of 14) to repeat those 2 shots.... If I choose to fire a single shot, I have to alternate 4 pumps and 5 pumps to maintain the velocity as close as possible to that 640 fps mark.... Looking at the curves for 20 pumps, I can get two shots of 725 fps (18.6 FPE) by setting the SSG gap to 5.7 turns.... It takes 14 pumps to get back to just over 1500 psi, or I can shoot single shots and pump 7 times each.... Here is a short string, showing the velocity and the number of pumps in between, mixing 2-shots and single shots between pumping....

Pump 20 times
2 shots, 732 & 726 fps
14 pumps
738 fps
7 pumps
721 fps
7 pumps
727 fps
7 pumps
2 shots, 723 & 722 fps

The gauge makes this easy, I just pump until the pressure is just over 1500 psi, and I can take 1 or 2 shots, and stay within less than a 3% ES.... 8)

My conclusion is that this bored out valve, with a volume of 6.7 cc, is too small to be able to get a useful 3-shot bell curve without pumping.... By increasing the volume by using a 2-piece valve, that could probably be accomplished, at the expense of a lot more pumps for the initial fill.... However, it is entirely possible to get 2 equal shots at any power level desired from 10 FPE up to probably 20 FPE (by using heavier pellets), and be able to shoot a mix of single or two shots by repumping to the appropriate pressure level.... It requires roughly 1/3 as many pumps as the initial fill for each shot fired when tuned for 2 equal shots.... If tuned for full power, the SSG allows enough retained air to cut the number of pumps require to continue shooting at full power in half.... :shock:

The gun set up the way it is right now is pretty close to accomplishing my original goal.... I can get 2 shots within a 2% ES at 19 FPE (688 fps with the 18.1 gr.) at 20 pumps (1500 psi), with 7 pumps per shot required to refill.... I was hoping I could get 2 shots at 20 FPE.... :roll:

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


Last edited by rsterne on Sun Jan 12, 2020 9:11 pm, edited 1 time in total.

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 Post subject: Re: My Benjamin 392
PostPosted: Sun Jan 12, 2020 8:13 pm 
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Joined: Mon Sep 05, 2005 10:55 pm
Posts: 6882
Location: Rocky Mtn Hse Alberta
Love this thread Bob!
One thing that I struggle with on these soldered tube/barrel pumper is the barrel bend, 'Swoop' as I like to call it. The more the compression the higher they point. Would you have ideas on mitigating that?
Cheers

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Walter


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 Post subject: Re: My Benjamin 392
PostPosted: Sun Jan 12, 2020 9:02 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
Maybe melt the barrel off the tube, make a receiver, and start over on the design?.... :roll:

Seriously, I have no idea, Walter.... :oops:

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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