The Crosman Powermaster 760...Recapturing my youth !
- Thread starter GoldenStateAIRGUNer
- Start date
Hello again unionrdr. Would you care to sell one? 
Yes, it is 1:16, 10 lands, right hand twist.
Thanks,
Biz
Yes, it is 1:16, 10 lands, right hand twist.
Thanks,
Biz
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Hello again unionrdr.
I wanted to ask if you have a problem with a loss of air pressure at the breach end of any of your vintage 760's...the Power Masters when you pull the trigger. I've been trying to find a simple way to to stop the leak and I believe I've found one. I was curious if you have the problem and if so how you dealt with it. My 760 is a variant 3.
After I managed to get it sealed I gained 17 fps average over 10 shots with pellets. My son has a 1994 version of the Pump Master and what they did (as you likely know) is encase the breach bolt in rubber and that curtailed the pressure loss.
I sort of copied that method to successful ends. I put a piece of 3/32" ID soft rubber vacuum tube over the small portion of the brass bolt where the magnet is, then used a utility knife to slice the majority of the rubber from the OD of the tube. I then used my 6" power disk sander to reduce the remaining rubber tube to approximately the diameter of the brass bolt. The bolt slid into the breach and barrel quite snugly even with a bit of grease but it moves back and forth easily and there is absolutely no air loss now.
I pumped 10 times and triggered it...it sounded like a mouse sneezing. So I removed the bolt and cut about half the length of the tubing off (it was originally the length of the magnet portion) apparently allowing much more air to exit through the muzzle...nice
.
I've been trying all sorts of ways to do this and finally lighted on this. I thought if you had thought about stopping the loss in your guns but hadn't this might help and/or you could pass it on to your huge following.
Thanks for giving a listen.
Bruce Jennings (BIZNAWICH)
I wanted to ask if you have a problem with a loss of air pressure at the breach end of any of your vintage 760's...the Power Masters when you pull the trigger. I've been trying to find a simple way to to stop the leak and I believe I've found one. I was curious if you have the problem and if so how you dealt with it. My 760 is a variant 3.
After I managed to get it sealed I gained 17 fps average over 10 shots with pellets. My son has a 1994 version of the Pump Master and what they did (as you likely know) is encase the breach bolt in rubber and that curtailed the pressure loss.
I sort of copied that method to successful ends. I put a piece of 3/32" ID soft rubber vacuum tube over the small portion of the brass bolt where the magnet is, then used a utility knife to slice the majority of the rubber from the OD of the tube. I then used my 6" power disk sander to reduce the remaining rubber tube to approximately the diameter of the brass bolt. The bolt slid into the breach and barrel quite snugly even with a bit of grease but it moves back and forth easily and there is absolutely no air loss now.
I pumped 10 times and triggered it...it sounded like a mouse sneezing. So I removed the bolt and cut about half the length of the tubing off (it was originally the length of the magnet portion) apparently allowing much more air to exit through the muzzle...nice
.I've been trying all sorts of ways to do this and finally lighted on this. I thought if you had thought about stopping the loss in your guns but hadn't this might help and/or you could pass it on to your huge following.
Thanks for giving a listen.
Bruce Jennings (BIZNAWICH)
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G
Guest
Guest
I've tried filing ring lands for seals, but to no avail. Then tried coating the end of the bolt probe in black silicone rubber. No good either. But the New Old Stock 761XL barrel had a tighter tolerance on the breech end, so that cured it. And I only bought one of the two 761XL barrels he had. Another member bought the other one.
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I recall doing the same things to try and stop the leak. Some of them turned into a lot of work for a little thing and I began to consider that I was going overboard so I got myself back to basics.
What I finally did was simple and very repeatable using only 3/16" long piece of soft rubber tubing. If you still have the leaks in the non 761xl barrels give it a shot...no pun intended
Biznawich (Bruce Jennings)
P.S.- I wanted to ask if you ever thought about pushing a tungsten carbide rifling button through the smooth bores you have... just curious how it turned out if you have.
Biz
What I finally did was simple and very repeatable using only 3/16" long piece of soft rubber tubing. If you still have the leaks in the non 761xl barrels give it a shot...no pun intended
Biznawich (Bruce Jennings)
P.S.- I wanted to ask if you ever thought about pushing a tungsten carbide rifling button through the smooth bores you have... just curious how it turned out if you have.
Biz
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Mine has improved considerably since I last posted velocity figures. With BB's. 3 shot averages.
3 pumps 410 fps
5 pumps 500 fps
7 pumps 550 fps
8 pumps 595 fps
10 pumps 635 fps
I've noticed the air blast between the breech and bolt on mine. My idea was to use a lathe with a cutter tool designed to produce a groove for an "O" ring. Problem is the groove would have to be very small and deep enough to retain the o-ring. The o-ring itself would have to very small. Say .040" thick with a outside diameter of 3/16" or less.
3 pumps 410 fps
5 pumps 500 fps
7 pumps 550 fps
8 pumps 595 fps
10 pumps 635 fps
I've noticed the air blast between the breech and bolt on mine. My idea was to use a lathe with a cutter tool designed to produce a groove for an "O" ring. Problem is the groove would have to be very small and deep enough to retain the o-ring. The o-ring itself would have to very small. Say .040" thick with a outside diameter of 3/16" or less.
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Gold...
Yeah I filed a groove in the brass deep enough to hold a tiny o-ring and wide enough for two of them. It worked very well for about 12 shots then they wore down and tore. I'm thinking 90d o-rings to for durability but considering the trouble I had stretching the standard rings over the bolt it would seem impossible with the 90d's.
I watching the rubber tube I put on the bolt (mentioned above) to see if/when it's going to tear and fall off. When/if it does I'll need to use something tougher like nylon beer line or something.
Biz
Yeah I filed a groove in the brass deep enough to hold a tiny o-ring and wide enough for two of them. It worked very well for about 12 shots then they wore down and tore. I'm thinking 90d o-rings to for durability but considering the trouble I had stretching the standard rings over the bolt it would seem impossible with the 90d's.
I watching the rubber tube I put on the bolt (mentioned above) to see if/when it's going to tear and fall off. When/if it does I'll need to use something tougher like nylon beer line or something.
Biz
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Kind of a hassle to disassemble the gun everytime you do a bolt modification. Wonder if epoxy ( like JB Weld) built up on the bolt and then lathe turned would work ? Only 17 fps gain on 10 pumps is minuscule. The highest velocity I hit was with 12 pumps @ 670 fps. I have found that velocity consistancy suffers dramatically at 10 pumps. One shot might to 645 and the next was 615 fps. I think I know why. When the pressure in the storage receiver is high there is a battle between the pressure built up in the compression stroke cycle and the storage receiver. The check valve begins to open during the pump stroke and causes the PSI in the pump cylinder to drop. At some point the stored pressure exceeds the compression chamber pressure causing a loss of stored air. I only notice this at 9-10 pumps. At 3-4 pumps the compressed air is always much higher than the stored pressure so it's not a issue. My 3 pump velocities only vary 5-8 fps. Just a theory on my part but it makes sense.
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Hello Goldstate...been a little while. Well the rubber tube I had on the brass breech bolt came apart. It split...guess the rubber wasn't made for the sustained stretch however....
I figured it out finally. I inadvertently broke off the magnet (used for BB's) that was embedded in the end of the brass bolt. I was filing the groove I mentioned above widening it preparing to use a longer piece of some kind of tubing. What I settled on was surgical tubing...very soft and pliable, to use as the plug. I needed something that could compress enough to get into the breech and pliable enough to expand and block the air from exiting when the gun was fired. It worked perfectly if only by accident.
I hadn't planned on ever shooting BB's through it again since they bang up the barrel so I won't miss the magnet.
I was filing to widen my original groove and ended up with no magnet and the brass bolt filed down to approx. 2.5mm dia. (measured with a digital caliper) in the form of an approx. 5/16" long pin. The brass got too thin and broke off right at the end of the magnet.
The size tubing I finally settled on is called 1745 natural rubber "sling shot" tube on Ebay and is cheap. It measures 1.7mm inner dia. and 4.5mm outer dia....nice since 4.5mm is the size of the barrel inside and a 1/32" wall. Slide it over the hand filed pin remaining on the end of the bolt (with the magnet removed) and leave about 1/8" hanging over the end. Put a little oil on it and slide it in the breech end of the barrel. It will meet a tiny bit of resistance which is what you want and it will block the leak when fired. Be sure not to file the brass bolt any smaller round than 2.5mm...the slight bulge created when the tube is slid over it is what blocks the air. Give it a shot.
Thanks, Bruce J (Biznawich)
I figured it out finally. I inadvertently broke off the magnet (used for BB's) that was embedded in the end of the brass bolt. I was filing the groove I mentioned above widening it preparing to use a longer piece of some kind of tubing. What I settled on was surgical tubing...very soft and pliable, to use as the plug. I needed something that could compress enough to get into the breech and pliable enough to expand and block the air from exiting when the gun was fired. It worked perfectly if only by accident
.I hadn't planned on ever shooting BB's through it again since they bang up the barrel so I won't miss the magnet.
I was filing to widen my original groove and ended up with no magnet and the brass bolt filed down to approx. 2.5mm dia. (measured with a digital caliper) in the form of an approx. 5/16" long pin. The brass got too thin and broke off right at the end of the magnet.
The size tubing I finally settled on is called 1745 natural rubber "sling shot" tube on Ebay and is cheap. It measures 1.7mm inner dia. and 4.5mm outer dia....nice since 4.5mm is the size of the barrel inside and a 1/32" wall. Slide it over the hand filed pin remaining on the end of the bolt (with the magnet removed) and leave about 1/8" hanging over the end. Put a little oil on it and slide it in the breech end of the barrel. It will meet a tiny bit of resistance which is what you want and it will block the leak when fired. Be sure not to file the brass bolt any smaller round than 2.5mm...the slight bulge created when the tube is slid over it is what blocks the air. Give it a shot.
Thanks, Bruce J (Biznawich)
Upvote 0
Hello Goldstate...been a little while. Well the rubber tube I had on the brass breech bolt came apart. It split...guess the rubber wasn't made for the sustained stretch however....
I figured it out finally. I inadvertently broke off the magnet (used for BB's) that was embedded in the end of the brass bolt. I was filing the groove I mentioned above widening it preparing to use a longer piece of some kind of tubing. What I settled on was surgical tubing...very soft and pliable, to use as the plug. I needed something that could compress enough to get into the breech and pliable enough to expand and block the air from exiting when the gun was fired. It worked perfectly if only by accident.
I hadn't planned on ever shooting BB's through it again since they bang up the barrel so I won't miss the magnet.
I was filing to widen my original groove and ended up with no magnet and the brass bolt filed down to approx. 2.5mm dia. (measured with a digital caliper) in the form of an approx. 5/16" long pin. The brass got too thin and broke off right at the end of the magnet.
The size tubing I finally settled on is called 1745 natural rubber "sling shot" tube on Ebay and is cheap. It measures 1.7mm inner dia. and 4.5mm outer dia....nice since 4.5mm is the size of the barrel inside and a 1/32" wall. Slide it over the hand filed pin remaining on the end of the bolt (with the magnet removed) and leave about 1/8" hanging over the end. Put a little oil on it and slide it in the breech end of the barrel. It will meet a tiny bit of resistance which is what you want and it will block the leak when fired. Be sure not to file the brass bolt any smaller round than 2.5mm...the slight bulge created when the tube is slid over it is what blocks the air. Give it a shot.
Thanks, Bruce J (Biznawich)
I sold mine just the other day. Needed the funds for a RWS 34
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