Thread pitch for gas cylinder?

[CHW2021]

What is the screw thread type, pitch and diameter for a M-1 gas block piston nut?
My carbine had a loose nut (besides me) and blew out the piston and nut upon firing. I want to clean up the threads before installing a replacement. I will need to find a correct tap and just do not want to guess or trust the helpful ace hardware guy.

Help please.

[Tuna]

You won't find the tap at your local hardware store. It is an uncommon one. I do not have the information you need but just wanted to let you know it will be a special order from a national type supplier. The military had a tool that included the tap but few were made. They do appear on some of the auctions now and then.

[CHW2021]

Rather what I figured, there are industrial supply houses about, but I need to know what I am looking for.

[phil441]

The only info I can provide is from Eric Nicolas' compilation showing the drilling of the Gas Cylinder separate part.

1/2 -32NS-3
MAJOR DIA .5000 MIN
PITCH DIA .4797 +.0026
MINOR DIA .4662 +.0034

I'd try to verify these numbers but there's no way I'm gonna break loose a staked-in part.

I'm sure this is useless info but thought it might give you a starting place.

[Varmintpopper]

CHW2021
I may be able to help with a thread chaser.
Email Me: miltonetime (At sign) aol (dot) com.

Good Shooting

Lindy

[seagoatami]

you might try a thread file, each file has 8 differnt sizes they come in sets of 3, I believe you can get them from a tool truck that stops at factories for the mechs. to buy tools Mac tools is one and Snap on is another. you can also buy the individual flies. use is pretty much self explanitory.

[Marty T.]

What you are looking for is a tap size 1/2 in dia. with 32 threads per in. NS stands for National Special. The 3 is class fitting. You can order them from companies that deal with taps, MSC Supply or McMaster-Carr are a couple. It is a standard thread, and checking with MSC is listed around $30. And yes, I am a machinist so this is accurate info. You will need a bottoming tap so the thread will go to the bottom of the hole in the gas cylinder.
Just make sure you START IT STRAIGHT!!! If not, you will crossthread and that is not what you want to do. Let us know how it goes.
Marty

[emmagee1917]

Wouldn't a tapered tap be better here ?
Any screwed up threads will be at the outside edge where they were punched . Behind the nut you have that space where the piston sets and moves, and behind that a small air cavity with the port hole into the bbl.
The taper tap will be less likey to cross thread and will keep you from going too deep.
IIRC , this is what I use , but I can't lay my hands on it .
Chris

[Marty T.]

That could be used. Reason behind what I was thinking with the bottoming tap is that there are not many threads there to begin with, and since some on the outside edge of the hole may be gone from what the original post stated as "blown out", if using a taper tap and not started straight, it would cut the top off the remaining threads in the hole which would have the same results as stripping the threads. The bottoming tap would use the existing hole where threads may have been removed to help center and then let it follow the small amount of threads already there. BUT, if there is room for the taper tap, I would agree with you there. Good point.

[jimb]

Armorers had a special tool for replacing the piston nut and dressing the threads. It attached to the front receiver lug and was spring loaded to provide a light pressure on the nut and the threads. I have a set here.

[CHW2021]

Jimb, please check your messages.

[Richard H Brown Jr]

Unless it's a collectable m-1 carbine, the fact that you blew out the gas piston, might indicate more damage to the gas piston assy that's braised/welded to the barrel. Cracked? The Gas Piston nut was staked on so it shouldn't have come loose. Have you had a *GOOD* carbine gunsmith look? or magnafluxed or other metal inspection tests done to see if the gas piston housing is hosed? A n. o. s. in packing barrel replacement might be a better idea.

RHB

[Tuna]

Not all carbine castle nuts were or are staked. If it was not staked and just shot loose then the damage will be minimal if any at all. But as stated a staked castle nut coming off would do damage to the gas cylinder threads if not damage to the cylinder itself.

[CHW2021]

The carbine is a Universal, it is one of the "clones" using surplus style parts and is interchangeable with military guns. The gas block was drilled so close to the edge of the block that it did crack and allow the nut & piston to blow out. I had the block welded, it is a good job done by a experienced pro; the threads for the piston were not improved by the whole process. I can not justify the cost of a new barrel on this gun (and am out of work) so I am in the position of trying one last thing before shelving the rifle for better times. Yes, I am aware that the gas block could fail again, so be it if it does, the gas escaped into the stock and no one and nothing was harmed. Not a perfect solution to my issue I know, but my world is not a perfect place at this time. Thank you gentlemen for your input, your experience is very helpful.

Charles

[Tuna]

Charles, As to your Universal being compatible with USGI parts. It all depends on your serial number. The only ones that are 100% compatible are the first series which is up to about the 25,000 serial number range. From there on the amount of USGI parts that are useable drops. The later ones have really no compatibility left as Universal made all their own parts by then. The gas cylinder is different then a USGI one. The castle nut is not staked in a Universal and the piston is reversible to turn the rifle into a bolt action. Welding of the gas cylinder does not work very often. The cylinder often cracks again where it is weakened beside the weld as the weld is stronger then the surrounding metal. This happens in USGI carbine barrels as well as Universal carbines. If it does crack again the only way to fix it is to replace the barrel. Hope yours turns out OK.