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PostPosted: Fri Sep 11, 2015 8:24 pm 
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Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
I've rec'd a lot questions lately on how to put a nitro spring in a standard Crosman coil gun, like Optimus, Benjamin Classic etc, so here is info on two different mods. Also note that the standard Crosman nitro (BT9M22-00-5) will increase power in most all guns so any 500fps models will no doubt exceed that. What power it will have depends on what piston length you use, but for the std 150mm piston it could exceed 18ftlbs. There is also a lower strength nitro available, part # BT5M22-00-1, which is ~2/3rds as strong.
There's the long detailed text version, and below are pix showing a much simplified version:
The most common way people do it is using just the nitro and its butt cup. I call it a butt cup for lack of a better description but it's part #BT9M22-00-2. This isn't a bolt in deal because the cross pin hole in the nitro guns receiver is ~.100” or ~2.5mm further forward than the coil version. So if you install the nitro butt cup in a coil receiver with the cross pin then the trigger/stock bolt will be .100" further aft. You have two options; one is elongate the hole in the receiver so the bolt will go in, which is easy but will move the rear stock screw and lug position back by .100” so you'll need to grind on the stock a bit for it to fit. It also causes the butt cup to stick .100” further aft so the oem plastic end cap won't fit. You can either do without the cap or cut/grind it down until the cap fits. One advantage to this setup is the move the trigger back 2.5mm too, which buys you about 1/2 ftlb power. If you want peak power then this is the way to do this mod. The other way to fit it is to drill/tap a new hole for the trigger/stock bolt in the correct position to match the receiver. You can do this by turning the butt cup around so the oem bolt hole is on top and you have a fresh surface to drill/tap. It's a better and cleaner fit, but to drill/tap that hole is a bit much for some people. Your next problem is the piston end. The nitro's shaft is pointed on the end which mates to a shallow hole in the piston, this centers the nitro in the gun which is important. So you can either buy a nitro piston $$, which has the hole, or mod yours to work. Buying a nitro piston will mean you need to add ~.100" of shims into the butt cup to get proper nitro preload. If modding the coil spring piston you'll end up needed more spacing because the coil piston is deeper inside, so closer to 1/2" shimming all together. If you simply drill that shallow centering hole in the piston then the piston will be ~1oz lighter than a normal nitro piston which may cost you a little power with mid to heavy pellets. So ideally you'll want a disc of metal that fits in the piston (piston ID is ~.790-.800), and ~1/2" thick. If you use thinner metal you can add shims under it or in the butt cup end to get your preload. Just be sure it's a good enough fit that stays centered in the piston. The end of the nitro shaft is pointed so it will self center in the hole if done correctly. An 1/8 - 3/16" bit drilled ~3mm deep should work, then use a countersink to bevel the edges and make a nice tapered surface to better match the nitro. I use an1/8" bit followed with the countersink seen in some other pix, like crowning. You could even use a stack of fender washers if you wanted. Say you have some 3/4" OD with a 3/16" hole, you could make a stack ~1/2" and wrap them with something to make then a snug fit in the piston and as long as that assy stays in place it should work. Or even a stack of nickels could work.
This next mod is probably better and the only Crosman part you need to buy is the nitro spring. The nitro will be reversed for this mod. Since the nitro body will be in the piston it needs something to keep it centered. The nitro body OD is ~.710-720" and the piston ID ~.790-.800". So if you're real lucky you'll have some tough plastic tubing to fit that. If not then maybe cut a long strip of plastic from a 2-liter bottle and wrap it tightly around like a bandage until it's a bit thicker than the pistons ID, then sand to a nice snug fit. Of course it will need to be super glued to the nitro and to itself as you wrap it. I suppose 1/2" wide would be fine but wider will have a better bond. Of course skuff the paint on the nitro and sand both sides of the plastic strip so the glue can stick. Apply glue as you go so the end result is one solid wrap. After sanding to fit the piston put a bevel on the edge of the plastic so the cocking linkage can't catch it. Do this all the way around because the nitro may rotate over time. I doubt it will catch or rotate but better safe than sorry.
For the other end you'll be using the original spring guide assy for parts. Remove the guide tube from the end block which can be done by using the spring spacer like a slide hammer, just slide the spacer down the guide hard until the end comes off. You don't need the spacer or guide tube, just the end block that the cross pin goes thru. This end piece/block end is where that centering hole for the shaft needs to be, and to make that happen you'll re-purpose the top hat which is the short spring guide that was inside the piston. Cut the guide section of the top hat down to 5mm long and slide it into the hole where the guide tube used to be. Then drill the shallow centering hole in that for the nitro shaft and that end is good to go. As it is there should be enough preload for the nitro, but if more is needed then add to the piston side under the nitro's base. As mentioned the coil piston is ~1oz lighter than the nitro version which may cost a little power and velocity deviation as pellets get heavier. If you want you want you can add a metal under the nitro to add weight, and the thickness will be determined by the nitro because too much and the nitro will bottom out before the trigger sets. So I'd trail and error it with washers to see where that happens, then make a sold steel disk. You could even make a lead one if you wanted. I haven't tried it but I think there may be enough room for 1/2".
Whichever mod you do I think it's also a good idea to add a sheet if rubber under the base of the nitro. It may help noise and vibration, but imo the idea is so the nitro is aligned better. If the butt end and/or part it sits on is slightly uneven, or the hole off center, then the nitro body will want to tilt to one side which will likely cause the nitro shaft to rub on its internal guides. So the point is the rubber will help it self center. The pressure the nitro will put on the rubber is ~700 psi, so it can't be any old rubber. The stuff I use is called "PacSkin" http://pachmayr.com/home/pac-skin.php which I bought the 5 x 20 sheet for something else years ago. It's maybe 1-1.5mm thick and very tough, like tire rubber tough and it survives just fine. I cut a little disc to fit in the butt cut or piston, but I suppose you could use two or more if you wanted. I tried another rubber that was soft like silicone sealant and it squirted out like grease. Rubber washers from a hardware store may survive, and I suppose leather would have a similar effect. If you don't have anything then no biggie, but I think it will help.


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PostPosted: Sat Sep 12, 2015 4:52 pm 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1184
Location: Eastern Townships
Thanks a lot for sharing that, Chevota. Reminds me of my neglected Optimus, it would be much smoother with a nitro ram like the one in my TR77 NPS.


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PostPosted: Mon Sep 28, 2015 6:58 pm 
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Joined: Wed Nov 27, 2013 12:54 pm
Posts: 518
Chevota,

Does a delrin top hat centering the gas strut would chip out or would help for smoothness in the shot cycle?

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Hatsan At44 Tactical .22
Steyr hunting 5 automatic
Ruger air magnum .22
Weirhauch HW100 .22
Gamo Expomatic .177
Benjamin Marauder DougP custom stock
Theoben SlR98 .22
AlEx Custom gun .22 (JezX built masterpiece) :)
Hatsan Atp2


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PostPosted: Mon Sep 28, 2015 10:38 pm 
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Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
Alex: If you mean in place of the steel one cut down? I'd say it's a bad idea but who knows. My worry is the nitro would press deeper and deeper into the plastic until preload is gone, then the nitro would pound itself until ruined. The nitro shaft is a bit pointed and it's a lot of force 24/7/365 so I just don't know. I think it's probably strong enough, especially if you cut and shape the end so the cross pin supports it, but hard to say so I don't want say yes. And no I don't think it will help shot cycle but it certainly wouldn't hurt, unless the nitro bottoms.
Using the oem steel top hat is convenient since it's there and perfect for the job, but you can use other things. A 1/2" hex head bolt with the threads slightly ground down to fit in the hole should work fine. Or an M12. I'd go with soft grade 2 since it's easier to work with. Or you could find/make a disc of steel or T6 alum that would center on the ID of the receiver. You could use a drill and sander to make one, which I can explain if needed. You could also use a stack of fender washers, or your plastic deal with a fender washer on top.
The oem top hat is cheap, I think $1.15US which I guestimate would be $3 in CA? I'm also sure there's people that would gladly trade you for their steel one.
If you want to soften the shot cycle I'd work on modding the main seal which I can explain if needed, ensuring the breech seal doesn't leak, and switch to heavier pellets, preferably JSB. Adding weight to the gun helps too, if done properly.
Airmec: It would help but it would also increase the sharp reverse recoil, I'd tune the oem coil setup first so you can see and compare, plus the tune and mods are basically free where a nitro spring is probably expensive in CA?


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PostPosted: Tue Sep 29, 2015 12:21 am 
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Joined: Wed Nov 27, 2013 12:54 pm
Posts: 518
Thank you very much.
Will try the above...

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Hatsan At44 Tactical .22
Steyr hunting 5 automatic
Ruger air magnum .22
Weirhauch HW100 .22
Gamo Expomatic .177
Benjamin Marauder DougP custom stock
Theoben SlR98 .22
AlEx Custom gun .22 (JezX built masterpiece) :)
Hatsan Atp2


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PostPosted: Tue Sep 29, 2015 6:00 pm 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1184
Location: Eastern Townships
I guess your right, Chevota :) . I'm just too obsessed with mods :rolleyes: . I should know better, after tuning my Beeman and Benji Classic .22 and some others, they are totally different rifles (in a better way!). Doing a full tune on my steel spring Optimus would yield pretty interesting results for sure :D . Think I'll keep that ''madness'' for my 1377 (BTW, the breach is coming good), but that's another story...


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