Fit of SVT-40 gas piston and cup

GunsNotPuns

GunsNotPuns

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A question for my SVT-40 pals out there: How is the fit between your gas piston and cup?

The original that came with my rifle the piston moves very easily in and out of the cup but the stainless steel version I recently acquired sees it quite difficult to place the cup inside of the piston. It caused a few issues this morning as the bolt couldn't close properly on the next chambered round and I had to cycle the bolt a few times to get the end of the operating rod back far enough to close the bolt. Removing the handguard I saw that the piston wasn't going fully forward (perhaps a little more than an 1/8th of an inch away from where it was supposed to be) which was preventing the action from fully resetting.

Which is it supposed to be? Easy or hard.
 
GNP: While on some I can feel a bit of roughness or very light drag, I would say there is supposed to be a fair bit of operating clearance between piston and moving cylinder. You might attempt to smooth out the inner surface with some sandpaper wrapped around something you can chuck up in your hand drill. Or, why not just use the original. I've had no corrosion problems with mine using corrosive ammo. Takes no time to remove the piston and flush out with hot water, along with the inside of the cylinder.

milsurpo
 
When I first got a stainless steel set (the original is still fine, but I want to keep it that way, and the cup and piston are an easy to swap high wear item), the stainless set felt "gritty" when dry fitting it. But everything moved relatively freely, and it sat as deeply as it needed too.

After 50 or so rounds, I cleaned it, and worked them dry, and the parts had "self sanded" basically, and worn themselves quite smooth.

They're supposed to fit quite snugly, with little room for gas leak, but enough for the two parts to move smoothly. For an early Soviet era firearm, it's a remarkably tight tolerance item.

I'd avoid sanding, because it would be easy to take away too much material... Maybe try removing the cotton off a Q-Tip and replacing the cotton with a bit of steel wool and giving it a rub just to smooth out the interior of the cup.
 
To check the proper fit you would need to hold the cup hole down, insert the piston from the bottom all way up and let it go. If it falls down right away then it's too loose, if it stays there then it's too tight. It should slide down in a smooth slow motion to be a perfect fit.
 
Horilka said:
To check the proper fit you would need to hold the cup hole down, insert the piston from the bottom all way up and let it go. If it falls down right away then it's too loose, if it stays there then it's too tight. It should slide down in a smooth slow motion to be a perfect fit.
Click to expand...

The new one would definitely be too tight then.

D-Leach said:
where are you buying these from? i'd like one for mine
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I got mine at http://www.ppsh41.com/SVTitems.html, guy is in Eugene, Oregon. $10 for shipping to Canada.
 
It's more than roughness, it's tight. Tight enough that brand new recoil springs don't have enough force to push the gas piston back into its reset state. I have to action the bolt two or three times in order to get the op rod far enough back in place to have the bolt fully closed.
 
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