Help with M305 gas piston/cylinder problem

[dagarb]

I think I've diagnosed why my M305 doesn't cycle, and why it fails the gas piston tilt test. The flattened end of the gas piston is too big to protrude out of the gas cylinder towards the op rod. The whole piston can be removed from the cylinder the other way (towards the muzzle), but even with a good shove I can't get the gas piston to stick out of the cylinder towards the op rod.

Has anyone fixed this problem before? Maybe the cylinder is too small, or maybe the gas piston or cylinder is shaped incorrectly? What can I do before I buy replacement parts?

I tried to take helpful pictures, but I can certainly take more if it will help those that are generously helping me.

Thanks all

 

[Pr589]

This is the first time I've ever seen this. Is the piston or the gas cylinder aftermarket or are they both OEM?

If this is a new rifle, send it back to the vendor. If not, I would try replacing the piston and/or the cylinder. Note that USGI parts and Norc parts don't always play well together (i.e. USGI cylinder with Norc piston) so if you have to order USGI parts, you would need the cylinder and piston. The Norc gas cylinder lock needs to be retained to fit the threading on the barrel (which is non-USGI).

 

[michael s]

I had pretty similar issues with my gas piston, I cleaned it and the cylinder with fine emery paper. The cylinder is not easy but with patience doable. I concentrated mainly on the piston, not rounding the edges at all and plenty of lube and test fittings. It did take quite a bit of time, but in mho if it is a new rifle take it back.

 

[t-star]

I would agree with the above.

- or buy a new norinco piston from marstar if he has them

 

[LeftFootOfDoom]

that might not fix the issue, if the pistons are correct and the gas tube exit hole is too small.

I've also seen this with 2 of the newer norc 14's, very tight fit of the piston and gas tube. I would think that just shooting would wear in the metal fitment over time.

 

[jeepcjaholic]

Try to put it in backwards. Threw the butt stock end if this makes sense. To see where the obstruction lies. May be a easy fix? Or like said, send it back. Myself, I would not shoot this rifle till addressed. You could creat a much bigger issue. Test fit try not to force and see the best you can with a light where it's hanging up.

 

[dagarb]

Thank you all for the input.

Is the piston or the gas cylinder aftermarket or are they both OEM?

If this is a new rifle, send it back to the vendor.

Thanks Pr589. I haven't changed any of the parts and AFAIK everything is straight from the factory. It's not worth trying to return it.

I had pretty similar issues with my gas piston, I cleaned it and the cylinder with fine emery paper. The cylinder is not easy but with patience doable. I concentrated mainly on the piston, not rounding the edges at all and plenty of lube and test fittings. It did take quite a bit of time, but in mho if it is a new rifle take it back.

Thanks michael. The cylinder generally fits well within the piston except for the flattened end of the piston and the little raised part at the back of the cylinder that is supposed to match the piston. Did you file these parts?

Try to put it in backwards. Threw the butt stock end if this makes sense. To see where the obstruction lies. May be a easy fix? Or like said, send it back. Myself, I would not shoot this rifle till addressed. You could creat a much bigger issue. Test fit try not to force and see the best you can with a light where it's hanging up.

Thanks jeepjunky. I tried this and the flattened part of the piston is larger than the gap in the cylinder it's supposed to fit through.

 

[jeepcjaholic]

Hmmm, don't like jeeps?

 

[Odd Shot]

Can you take a clear close up shot "head on" of the stem of the piston and the hole in the cylinder (receiver end) it passes through, side by side?

Are you fully lining up the flat area of the stem of the piston with the flat portion of the hole it needs to fit through?

 

[jonh172]

Have you tried taking some measurements of the stem vs the D slot?

Shouldn't be much harm removing or polishing some material out of the slot since this area doesn't need to be air tight.

 

[dagarb]

Have you tried taking some measurements of the stem vs the D slot?

Shouldn't be much harm removing or polishing some material out of the slot since this area doesn't need to be air tight.

Thanks Jon. And now I can use names for these parts: the "stem" of the piston and the "D-slot" in the cylinder. Also, no d-slot jokes allowed!

So I'm better off leaving the piston as-is and shaving down the D-slot?

Can you take a clear close up shot "head on" of the stem of the piston and the hole in the cylinder (receiver end) it passes through, side by side?

Thanks Odd Shot. I'll try to get that picture over the weekend, but my only camera is my phone.

Hmmm, don't like jeeps?

I've got no problem with jeeps. Jeep people on the other hand...

Junky was actually a reference to your CJ addiction, not Jeeps.

 

[notsorichguy]

Looks like someone put the piston in upside down and then torqued on the gas plug.

I have seen this before, and its a bear to fix.... easier to replace the gas system....

I'm not pointing any fingers, but it may not have been the manufacturer nor the seller/retailer who did this....

You should be able to find a decent, complete, Nork system for $80-100. Or you could really splurge for a M1 NM system from Brownelles for $110 USD, of course you will need a new US thread gas plug....

 

[jonh172]

Notsorichguy makes a great point.
If the face of the stem is undamaged then try removing some of the finish off the cylinder.
If the edges of the face of the stem are raised then hold (not clamp) the piston in a padded vice and buff the raised edges off, like polishing a boot, with some 400 grit paper.
Go slow and test fit along the way.
If it's beyond repair, pick up another one and get shooting!

 

[eaglesnester]

Looks like the gas cylinder and piston are galled. The gas system on the M305 is a precision piece of kit on the rifle. Small scratches and tiny dings in either the walls of the gas cylinder or the sides of the gas piston will mess up your accuracy big time. The USMC goes so far as to try out different gas pistons to improve accuracy. In this case the armors are unable to see any flaws in either the piston or the cylinder. You have obvious problems. The photos appear to have the cylinder and the piston in bad shape indeed.

 

[45ACPKING]

judging by the pics, ans speaking from experience, the only solution I see is new parts.