Trying to create an adjustable gas valve for semi auto rifle.

Topical Discussions include anything pertaining to Gunsmithing.

Moderators: Harold_V, JackF, GeorgeGaskill

TheGreenMan
Posts: 22
Joined: Tue Feb 23, 2016 12:57 pm

Trying to create an adjustable gas valve for semi auto rifle.

Postby TheGreenMan » Tue Jan 03, 2017 3:52 pm

I'm looking for some help with the design.

I know there is an easy way of doing this, just copy the on/off only valve drill two holes for the gas flow, drill a center hole and thread it, screw a bolt in and out to adjust gas flow.

But I wanted to try something a little more fun to try and make.

The original gas flow valve is pretty much a simple cylinder held in place with a lock ring. Where the gas comes out of the barrel and into the gas tube the cylinder has a grove cut into it about halfway though and a little wider than the holes the gas comes in/goes out of. When an external lever is pulled the cylinder rotates 90 degrees blocking the gas port, keeping any gas from entering the gas tube.

My first idea was to drill two holes in a cylinder at 90 degrees. The cylinder would not move to adjust the gas flow, so the holes would always line up with the gas ports. Instead I would drill a hole length-wise through the cylinder until it intersected the 90 degree holes. I'd then make a thumb screw that was threaded only on the end. I'd make a threaded cap that would thread onto the end of the thumb screw and be the same diameter. To keep the cap from just spinning along with the thumb screw I'd cut a groove in the cap and put a pin through the cylinder into the groove so it could not rotate. Then when the screw was rotated it would screw the cap in and out. Another pin or two would keep the screw from backing out.

I've encountered two main issues with this idea, the size of the valve is fairly small (the cylinder is 0.275'' in OD and the thumb screw/shaft is only about 0.185'' in OD though that may be able to get slightly bigger) so doing some of this is a bit delicate and may end up with too delicate parts that will break easily in use or as I make them. And if I do what I'd like and have the cap go from full open to full close in one turn of the screw I would need to use a 7 TPI thread. A 'bit' aggressive for the small size I would think.

If I could come up with a way to tell if you were on the first/second/etc. rotation of the screw I could use whatever TPI I'd like and that would solve the thread problem nicely but I can't think of a good way to indicate this.


The second idea I had was to eliminate the cap and the threading on the thumb screw, just making it a shaft. I'd cut a groove a little wider than the gas ports and have it go from very shallow to cut almost halfway through the shaft within about 3/4 of a rotation of the shaft.

The issues with this are that I'm not sure how well you could do any fine adjustments with it and I don't know if cutting through that much of the shaft would weaken it too much.

Any ideas or advice? 'Just do it the simple way stupid' is acceptable as well :-).



Since I'm not the best at descriptions (and I'm operating on 0 hours of sleep, been up for 26 hours so far with another 4 or so to go at least) here are some quick sketches I made up. Not to scale (or correct threading) of course.

This one shows the two holes for gas flow and three holes for the pins.
The upper left and right are badly done end on views.
Image

This one shows the thumb screw and cap idea.
The upper left is a cutaway though the shaft where the groove is cut in it.
Image
Last edited by TheGreenMan on Sat Jan 07, 2017 2:53 pm, edited 1 time in total.

User avatar
steamin10
Posts: 6487
Joined: Sun Jun 08, 2003 11:52 pm
Location: NW Indiana. Close to Lake Michigan S. tip

Re: Trying to create an adjustable gas valve.

Postby steamin10 » Tue Jan 03, 2017 9:31 pm

When it come to pneumatics, there are a ton of ways to skin the buffalo.

Paint ballers, and nail guns use a pre-chamber, to collect the gas charge, and a pop valve to release it.
A needle valve can regulate flow in a crude way, but really offers no advantage, as it will flow at diferent rates, at different pressures. A simple spring loaded ballcheck, can read both sides, and become an adjustable regulator. Look up a coupla designs and follow through on how they work, before you try to re-invent the wheel. There is a lot of good stuff out there that can work for you.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

TheGreenMan
Posts: 22
Joined: Tue Feb 23, 2016 12:57 pm

Re: Trying to create an adjustable gas valve.

Postby TheGreenMan » Tue Jan 03, 2017 10:37 pm

steamin10 wrote:When it come to pneumatics, there are a ton of ways to skin the buffalo.

Paint ballers, and nail guns use a pre-chamber, to collect the gas charge, and a pop valve to release it.
A needle valve can regulate flow in a crude way, but really offers no advantage, as it will flow at diferent rates, at different pressures. A simple spring loaded ballcheck, can read both sides, and become an adjustable regulator. Look up a coupla designs and follow through on how they work, before you try to re-invent the wheel. There is a lot of good stuff out there that can work for you.


A touch more info: This is the gas valve for a Mas 49/56 that was rechambered to .308 by the importer.

Yeah, I do not want to reinvent the wheel, I just don't know what is out there already or what to search for. It is annoying to know there is information out there but not know what terms to use to search for it.

I'm not sure that what will work in a paint ball gun or pneumatic nailer would work in this situation. Then again I'd need to actually look at what they are doing.

The PSI is different. A nail gun is what, ~100 psi? A paintball gun is close in the neighborhood of 800 psi. I would guess that the .308 is going to be more in the low to mid 1,000 psi. Though right now all I can find is data on the 30-06 being in the 1,100-1,200 psi range in the M1 Garand.

I don't think I want any delay between the pressure being bled off the barrel and entering the gas tube (anymore than the tube distance already does).

If a checkball would work I'd think that some manufacturer would have used them already. Also, I'd think that the bullet itself acts somewhat like a checkball since the pressure doesn't hit the valve until it is passes (unless you have bleed by). Mmmmm, maybe make the valve bleed gas into the atmosphere after a certain psi is reached? I think they do something similar with the M1 Garands that can shoot commercial ammo with no (little) over pressure worries.

Any help pointing to search terms or designs to look at is good help. I'll take a gander at the coupla, nail gun, and paintball gun designs.
I think my biggest limitation is going to be the small area I have to work with.

TheGreenMan
Posts: 22
Joined: Tue Feb 23, 2016 12:57 pm

Re: Trying to create an adjustable gas valve.

Postby TheGreenMan » Sat Jan 07, 2017 2:31 pm

steamin10 wrote: Look up a coupla designs and follow through on how they work,

I think I'm going to need some more specific pointers on where to look as when I search all I seem to get are architecture and iron furnace designs.

User avatar
steamin10
Posts: 6487
Joined: Sun Jun 08, 2003 11:52 pm
Location: NW Indiana. Close to Lake Michigan S. tip

Re: Trying to create an adjustable gas valve for semi auto rifle.

Postby steamin10 » Sun Jan 08, 2017 10:38 pm

My mistake. I was thinking in terms of a gas gun, not a gas operated bolt.
Look to the simple SKS and AK47 designs for your basis. They are simple balance of kinetic forces to operate the sliding piston, and bolt to get the full power of the charge.

Just as some blowback designs will fail to operate with subsonic rounds, it seems you are trying to solve a similar problem.
The low pressure pneumatic s are to slow and wont help you. Sorry.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

Patio
Posts: 1050
Joined: Wed Oct 06, 2010 3:14 pm
Location: Centralia Wa

Re: Trying to create an adjustable gas valve for semi auto rifle.

Postby Patio » Sun Jan 08, 2017 10:56 pm

The FAL has an adjustable gas port. It allows one to turn it down to the point that the action will cycle but not tear up the action any more than necessary.
Live for the moment!
Prepare for tomorrow!
Forgive the past!


Return to “Gunsmithing”

Who is online

Users browsing this forum: No registered users and 5 guests