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PostPosted: Sun May 29, 2016 7:06 pm 
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Joined: Sun Nov 30, 2014 8:22 pm
Posts: 286
Location: Hill Spring, Alberta
It seems that each day my air rifles provide me with more interesting things to think about. 8)

Unfortunately, sometimes they also provide me with more interesting things to buy. :shock:

My Crosman Nitro Venom for which I recently installed a .22 caliber barrel was shooting great, averaging about 726 fps on my Chrony, when suddenly cocking it resulted in a nasty "grinding" sensation.

I disassembled it, and discovered that the piston galling on the side of the cylinder remote from the cocking slot was causing the sensation. Luckily the galling doesn't extend forward into the compression area of the cylinder.

The piston is cracked all the way lengthwise to within about 1/2" of either end. The little bits I'm guessing were coming out of the crack and causing the galling because that's exactly where the damage is. The piston also appears to be cracked about 1/3 around the circumference about 1/4" from the piston seal, about where the insert to seat the nitro piston is seated in the piston.

1. I bought it new and the gun hasn't been dry fired.
2. It hasn't been constantly dieselled, although I can't guarantee it didn't happen on a few isolated occasions.
3. The gun has been de-burred and lube tuned, and thoroughly cleaned before re-assembly. I've put several thousand pellets through it before changing to .22.
4. There is a small amount of wear on the side of the end of the nitro piston where the piston comes out.
5. My only thought has been that perhaps when the end for receiving the piston rod was force fit into the piston, it resulted in a stressed area on the piston wall, but otherwise I HAVE ABSOLUTELY NO IDEA WHAT MIGHT HAVE CAUSED THE CRACKS. DOES ANYONE ELSE?

Peter :D


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PostPosted: Mon May 30, 2016 2:21 am 
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Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
Fascinating. Pix please...
The piston's tube body appears to be rolled from stamped sheet steel, and welded together at the spot you describe the crack is. I do not believe it was anything you did, I believe it was defective to begin with and normal use caused the crack to grow. I imagine the dovetail end was a force fit as you mentioned, which started the crack, and having been rolled and welded it'll be under great spring tension to open up. Picture it as a giant a roll pin that was squeezed together and welded.
Double check it didn't scratch down in the chamber since the piston does go into it ~95+mm, so if scraping mid way on the piston then it certainly scratched where the seal rides.
If I were you and if it's under warranty I'd tell Crosman to send a new piston, maybe a receiver too if scratched too much. If not under warranty I'd still tell them, along with pix, and say; clearly a mfg defect from day one that I only noticed until now, please send a new one. Or maybe offer to send it to them for inspection and quality control reference or whatever, if they send a new one...
I personally would likely just send them pix of it for they know, then fix it by welding it back in a few spots and calling it good. Optionally I image there are many people with longer pistons that are willing to sell cheap, which you could then cut down to normal size. If the receiver is scratched then some sanding can most likely fix that or at least make it a non-issue.
To avoid confusion I call that part the main seal attaches to the piston, and the gas spring is the spring. A nitro piston to me would be a piston that was made to accept a gas spring. It's easier than what most people and Crosman says, like the piston (or nitro piston) is whatever adjective, but to which part are they referring?


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