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 Post subject: Tranzfer port question
PostPosted: Mon Dec 16, 2019 12:37 pm 
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Just wondering what is the stock diamter of the transfer port on a benjamin classic .22? How do you measure or how would i measure mine?


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PostPosted: Mon Dec 16, 2019 5:50 pm 
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Nominal size is 1/8''. You can use a drill bit to measure it.

I deburred and polished the one on my own Benji Classic, it helped the velocity a bit, but really not that much. I seem to remember that over 9/64'' in size, there's a diminishing return effect :?:


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PostPosted: Mon Dec 16, 2019 7:30 pm 
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Ok cool thanks what did you get yours shooting at?


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PostPosted: Mon Dec 16, 2019 7:51 pm 
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I cover that in the guide, which I think you have? I know it's a big read but it's in there.
So yeah, .125" is oem, but it's often so rough that an 1/8" but won't go all the way in and/or it has a nasty burr at the end to impede flow as well as the bit. Since the port is super rough I always drill it out to clean it up, then I sand it smooth(er). I think it's prolly impossible to actually drill/sand all the imperfections out b/c. Not sure how they make such a nasty hole but they manage. I think the ideal size for a full power B18 is likely .125, but it's far from ideal to have it that rough, so drilling it is two steps fwd one back. The only other way to do it is drill it out much bigger and put an insert innit. I've never done it but people have.
The outside edge on size is imo .140", which will hurt power unless the gun is tuned to make max power and is making it, but even then it still hurts you some thanks to lower psi which increases piston impact. So i try to drill only one size up, which is a #30 bit, but it's only.1285" and usually doesn't do a whole lot to clean it up. Next is a #29 bit at .1360" which is still kinda big. I wish there was a .130-132 bit, which maybe I'll make one day by sanding down a #29. So, imo, the best thing is to use the #30, then sand it with that redneck sanding stick in the guide. If it's still nasty in there then it's up to you if you wanna go to #29. Most of mine are #29, maybe 30-40% are #30 and I think two are #28 (.140").
I also bevel the inlet side of the port b/c it's a sharp 90 degree turn and since it has to flow some rather thick air into the port inna microsecond. I can't really say how much I bevel it b/c I can't really measure it, or even get a good pix of it, but it isn't much. Not much is way better than a sharp 90 it had, which may still include a burr or two. I use a 45 degree diamond bit to do most of the work, then stones to round and smooth that. If I had to estimate, and assuming a .1285 hole, I'd guess the outer edge of the 45 I put in it is .145? Then the rest is just blending w/ stones as mentioned.
There's pix in the guide of how I did it. At the very least try to break that edge with something, a reamer, bigger drill, sandpaper or whatever.
Sanding the main seal per the guide will bump psi and reduce piston impact. By how much I can only guess, but it will easily counter whatever loss you have from a larger port. It's just better to net that free psi and not squander a big chunk of it on a big port.


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PostPosted: Mon Dec 16, 2019 8:28 pm 
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Ok going to try this again on my other tube. What # sand paper do you sugest i use on the tube? Also what can i use instead to bevel the transfer port and what #sand paper on the transfer port? I mean i had the other tube up to 745 fps.. i took it apart to put brand new seals on it and it fd it up? Went from 750 to 680? Dont get what i did wrong? Its almost like i sanded the tube to much or somthing? Maybe wrong sand.paper? Luckly i got two tubes so ima try this again


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PostPosted: Tue Dec 17, 2019 4:19 pm 
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Ahh yes, the brake hone tube.. So I generally use like 320 to start on either the tube or port, then 600. Sometimes I use something much more aggressive like 180 or 220 on the tube b/c it's usually tighter around the aft end and I want to open that up. It's not all that easy to damage the tube so I wouldn't worry about it too much. Like if you used a brake hone you're certainly heading in the damage direction, but it all depends on how much time you spent in there. Odds are you did not hurt it but I'd still use the hone in the guide to clean it up.
I think your power loss is likely something else, like the breech seal or barrel. Or maybe your gas spring is leaking. It was typical for my gas springs to give lower velocity every time I checked them. Some barely, some a lot, but all of the full power BT9 springs leaked on me. They're supposed to be better now but no guarantees, so that's still a suspect in my book.
Did you use a new oem breech seal? They need time to mate to the gun, and never actually seal which is why I prefer an O-ring. Is the new latch doing a good job of latching?
As mentioned, the other receiver tube may screw up your impressive accuracy, but only one way to find out.

As for the bevel in the xfer port, all my ideas are in the guide. Outside of that you'll just have to improvise. It's a pita no doubt, but my diamond bit and extension works great.


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PostPosted: Tue Dec 17, 2019 4:29 pm 
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Ravenairgun wrote:
Just wondering what is the stock diamter of the transfer port on a benjamin classic .22? How do you measure or how would i measure mine?

I don't recommend drilling your transfer port there more to loses than you will ever gain .Whats wrong with your tube ...


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PostPosted: Tue Dec 17, 2019 6:37 pm 
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I used 400 then i used 800 attachecd to a power drill wraped around a plastic tube for a nice fit. Driled the transfer port with a 1/8 and sanded. Brought it outside got 650 fps. Barrel was cleaned with isopropyl and barrel cloths. How do i check if my gas ram is leaking? I dont think it is altho it is oddly easy to cock. Maybe drom sanding the tube. Gun still shoota great just lost 100 fps :? New piston seal new breach seal both synthetic. Sanded breach face. Seems everytime i sand it it drops fps...


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PostPosted: Tue Dec 17, 2019 8:19 pm 
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Ravenairgun wrote:
I used 400 then i used 800 attachecd to a power drill wraped around a plastic tube for a nice fit. Driled the transfer port with a 1/8 and sanded. Brought it outside got 650 fps. Barrel was cleaned with isopropyl and barrel cloths. How do i check if my gas ram is leaking? I dont think it is altho it is oddly easy to cock. Maybe drom sanding the tube. Gun still shoota great just lost 100 fps :? New piston seal new breach seal both synthetic. Sanded breach face. Seems everytime i sand it it drops fps...


Did you lube it and with what and what made you do all this in first place


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PostPosted: Tue Dec 17, 2019 8:47 pm 
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Ravenairgun wrote:
Ok cool thanks what did you get yours shooting at?


IIRC, it was shooting at around 425 fps stock, and after it was just over 430, like 432-434 fps. It's not modded for full power.


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PostPosted: Wed Dec 18, 2019 12:22 am 
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I did a gas ram, conversion and polishsd the internals.and tryed to hone the tube at the same time. Had the gun shooting at 750 the first time then i went to replace the seals to fresh ones thinking if its at 750 now ill get more with fresh ones...took it apart misewell sanded it some more with 800 replaced the seals and it droped to 650...dont get it. And ya i lubed it with same lube i had it shooting at 750 with it. Its a yogurt thickish lube red in colour used for grease fittings on tracktors forget the name....herd molly paste is trash


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PostPosted: Wed Dec 18, 2019 8:30 am 
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Moly is the best. I bet it's sub zero and wheel bearing grease gets thick, making everything move slower. But I'm betting that it dieseled, and thus why getting 750 FPS, and now it's dropping.

But bigger port doesn't always mean more power. You get more pressure from smaller holes. Take a garden hose, with no attachment, start pinching off the hose, and you'll see the PSI increase,


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PostPosted: Wed Dec 18, 2019 8:49 am 
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Lol when it dieseled i got over 1000 fps first shot and sounds like a .22 rim fire.... it leveled out at 750-730 fps ever after 50 rounds. Your right it is sub zero butnit also was when it was shooting 750 fps. Also i checked to make sure took the gun while it was warm insise put the chrony outside went quick outside took a shot and it was still 650 fps reload tolm another 645 so the cold dint get time to effect the gun. When im outside for a while the gun will drop fps but evenualy sèttles back to orginal fps. Im almost 100% sure its from sanding or over sanding tje compresion tube.


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PostPosted: Wed Dec 18, 2019 2:04 pm 
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Put a thousand round down the pipe then check


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PostPosted: Wed Dec 18, 2019 11:18 pm 
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Easier cocking is a dead giveaway the spring has leaked. So my $ is between leakage or your breech seal, but now I'm leaning way more twoards the spring.
It won't do any good to shoot it more, but you're welcome to try. Many gas springs leak when used, so the more you shoot the quicker they die. If it has leaked then it's dead anyway, so it doesn't matter.
Fyi if it's that cold out I'd likely use 0wt motor oil on it. I've even ran no oil, just tungsten disulfide, which was to see how it worked and to disprove the nonsense that springers get their power from burning the oil. It's one several old wives tales about springers that won't die.
So, if you have a scale I'd use it. With an 18.5" barrel, measured 1" back from the muzzle, I'd expect to see at least 28lbs peak. More is friction, less is leakage. It is possible to have a leaker and the friction is bringing it above 28.
I use a digital luggage scale from ebay to measure, and I verified its accuracy. Bathroom scales are usually a fail at low levels like that. Or, if you recall how hard it was to compress the spring, you can pull it out and check. Since I do them by hand I can easily tell, but if you used a compressor then I guess you're screwed.
I spoze you could put a board on a bath scale, to protect it, then put your weight on the spring to see about where it is. Since I have no fully charged BT9 springs as a reference I can only say I "think" it should be ~160lbs? Cycle it as best you can, then measure it from just where it starts to compress.
If it is the spring, which was $pendy, you can try the NP2 spring which is more like $17CA. Imo the BT9, at full psi, is a bit stronger than needed so you get some negative drawbacks like scope issues. The NP2, while cheaper and less likely to leak, is even stronger so your gun and especially scope will suffer more. You may be just fine, maybe not, just saying the oem spring breaks scopes and this one is stronger. Heavier pellets will help counter that so imo I'd never use anything lighter than 14.3 and maybe consider switching to the 15.x gr JSB. And of course sand the main seal per the guide and stick to a #30 bit for the Xfer port. It may not all make sense but those things help counter the abuse of a stronger spring.
The NP2 spring is ~6mm shorter so you have to shim it. I'd shim so you have at least 3mm preload as with any gas spring. You can shim more but at some point the trigger won't catch the sear when cocking. Since the NP2 spring has less stroke you're window is much smaller, but shimming for 3-5mm preload should work.
You do have the coil spring to fall back on if you want, or even an aftermarket one like the Vortek 780125 or 780128.

Cold air plays a part in power so if you wanted you could put a pellet in the warm gun, but not cock it, then go outside and cock/fire it. That way you have a warm gun but cold air. Not sure how much power it'll net over cocking it inside, but we'll see... If your spring has leaked then at some point I'd imagine cold air will hurt power...


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