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Having gas (block) problems


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#1 Dagomob

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Posted 24 June 2017 - 02:53 PM

Hi all. Fairly new member here, and I have a question about the fitting of the gas block to the barrel on an S12. I know that these are press fit , and I am wondering if anyone knows what the tolerances should be between the block inner diameter and the barrel outer diameter. My gas block was a real pain to remove, unlike some others that I have heard of, and I was wondering if I should try to sand down either the barrel or the inside of the block to make the fitment a little more manageable. At this time, my block i.d. is +/- .9035", and barrel o.d. ranges from .9060 -.9090" front to rear. If anyone has any suggestions or info on this topic, I would greatly appreciate the enlightenment. Thanks in advance.

Edited by Dagomob, 24 June 2017 - 05:20 PM.


#2 machinist

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Posted 24 June 2017 - 08:37 PM

You can buy a brake hone to use in a hand drill or drill press to inlarge the bore in the gas block. I have used these to open a gas block bore to make the re-installation easier. Also polish the barrel. Just measure carefully so you keep the interference fit.

Edited by machinist, 24 June 2017 - 08:38 PM.


#3 Dagomob

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Posted 24 June 2017 - 11:19 PM

You can buy a brake hone to use in a hand drill or drill press to inlarge the bore in the gas block. I have used these to open a gas block bore to make the re-installation easier. Also polish the barrel. Just measure carefully so you keep the interference fit.

Thanks for the suggestions on how to make the block diameter a little bigger. This is the kind of information that I was looking for. Do you have any recommendations as to how much of an interference fit that I actually need. I know that it took an air hammer to get the thing off in the first place, and I would think that that would be rather excessive, considering that the block is held on with pins in the first place.



#4 G O B

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Posted 25 June 2017 - 09:14 AM

Try polishing both surfaces with crocus cloth and using a light oil for assembly. A light press fit is what is needed. It should NOT slip on, and neither should it take a big fucking hammer either!


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#5 JohnnyE

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Posted 25 June 2017 - 09:46 AM

I had great success reinstalling a tight gas block by first heating it up in a 350 degree oven. It expanded and dropped right over the room-temperature barrel. Once the block cooled, it was very snug. I used the same trick to install an AR-15 gas block.


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#6 machinist

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Posted 25 June 2017 - 02:12 PM

You can buy a brake hone to use in a hand drill or drill press to inlarge the bore in the gas block. I have used these to open a gas block bore to make the re-installation easier. Also polish the barrel. Just measure carefully so you keep the interference fit.

Thanks for the suggestions on how to make the block diameter a little bigger. This is the kind of information that I was looking for. Do you have any recommendations as to how much of an interference fit that I actually need. I know that it took an air hammer to get the thing off in the first place, and I would think that that would be rather excessive, considering that the block is held on with pins in the first place.
I use an airhammer to remove and install the gas block. You want a tight seal. When you rapid fire the shottie the gas block will get very hot, drums empting are are a blast! If you make a small adjustment to both diameters the block with a little lube will slide back into place easily...Just take your time when installing and I always place the screw back into the gas block before I remove or install the block (don't tighten the screw, back off 1/2 turn from the barrel) I use the ledge on the bottom of the gas block with a piece of brass or aluminium to protect the gas block when I remove or install with the airhammer.
I also use 400 grit,600 grit and 800 grit wet sand paper you can buy at the auto parts store to polish the barrel. Start with the 400 grit and step up grits until the final polish with the 800 grit.

Edited by machinist, 25 June 2017 - 02:30 PM.


#7 Netpackrat

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Posted 27 June 2017 - 04:23 AM

It helps if you are building an SBS when you need to remove the gas block; you can just drive out the pins, support the block in a vise, and beat on the muzzle with a BFH.  You're just going to cut it off anyway if it gets bashed a little.  8^)



#8 GunFun

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Posted 30 June 2017 - 03:08 PM

It helps if you are building an SBS when you need to remove the gas block; you can just drive out the pins, support the block in a vise, and beat on the muzzle with a BFH.  You're just going to cut it off anyway if it gets bashed a little.  8^)

 

While I wouldn't support it entirely on the muzzle, a thread protected muzzle supported against a block of soft wood or lead can help a lot. 

 

Light machine oil and a heated gas block for reinstall. Use soft metal to pad against marring from hammer, and alternate sides.

 

Do NOT beat against the gas plug, or you will be googling for threads about repairing the threads in your gas block.


Also a solid anvil goes a long way. i.e. Hardwood block on a cement floor , receiver supported by vice jaws at the front edge of the front trunion. 

 

I managed to do mine with the cement floor, and block at the muzzle, using a crappy wood vice table thing to hold the receiver upright. I've done others in camp grounds with a person holding the gun , and the muzzle brake against firewood anvil.

 

Some guns need a shop press and a fixture to support more of the trunion 



#9 Dagomob

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Posted 01 July 2017 - 04:03 PM

Thank you all for the info. I'm going to try for a fairly snug, without being insanely tight, fit. I'm figuring that the interference fit that I will aim for will be somewhere around .0015-.0020". That, with the pins, should be quite sufficient to keep the block snug and in place. I will still need to beat it back on, but hopefully not as violently as was the removal. Thanks for all of the input, and I'm sure I'll have more questions in the future.
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