12 replies to this topic

[JimFromFL]

What are you using as a gauge when installing a new barrel?
I don't want to over tighten.

[Arthur Fliegenheimer]

Reference the alignment marks on barrel and receiver.

[gijive]

Arthur,

The new manufactured NAC Gun Parts barrels don't have witness marks on them.

Jim,

You have to tighten the barrel until it doesn't move anymore. I think it's usually about 1/4 turn past hand tight.

[JimFromFL]

I have it set and hope it does not need to be tightened any more as this will off-set the compensator.

[Sgt]

Jim--
I was curious about that too. Should you fire it for awhile before you set the compensator and sights? I was also wondering if the tightness of the barrel affects headspace.

[Chopper28]

You had better consult PK about this issue or you are headed for problems. Barrel tightness, head spacing, and comp. placement are all inter related. You need the advise of an expert. It is not like screwing on an oil filter.

[LIONHART]

Sgt,
Indeed you can fire it before the Comp is fitted. No problems whatsoever. Afterall, Colt 21A's were turned into Colt '21AC's by Threading and Pinning. Though, as Jim pointed out, if one over tightens the Barrel, the Pin for the Comp will be off set. If that were the case, I'm guessing that in order to fix the problem, one would need to extend the Pin Groove on the right hand side using a file. Thread on the Compensator, Pound in the Front Sight, and install the Pin. Though I haven't done this before, the way I just wrote it, sounds right to me! Please understand, I'm no PK!

[Sgt]

Lionhart--
I was thinking that by not putting the comp on too soon, it would give you more options in setting your sight, in case you had to retighten the barrel. PK already said that barrel replacement should be done by a pro. I'm just not brave enough to tackle it. I think I'll box my Kahr up this weekend and send my SBR job out to him. However, Jim's question stirred up my mechanical curosity.

[LIONHART]

O'h come on Sgt, go and play with that Pipe Wrench! I know you want too!!! I did at one time took a Barrel off a '27A1. Not that difficult. I simply took a Wrench and unscrewed it. I wasn't careful, as the Barrel was a real mess anyway. Indeed, I used a Pipe Wrench! No joke..I screwed the new Barrel in, and used Lock Tight. At the time, I had a Universal AR-15 Barrel Wrench. I have to say, everything worked out fine. I installed the WH Comp with no problems, but it was a Screw On Version, not a Pin/Threaded Version. But, as Chopper said, headspacing IS important. Though that was something I overlooked. I must have gotten lucky, as my Gun fired fine afterwards. Live and learn. I tend to learn from my mistakes..We all have to start somewhere I guess. However, if not done properly, it could be an expensive fix. My advise? If you feel up in trying it, by all means. If not, send to PK! He'll do a FANTASTIC job.

[Barry]

Would someone explain the concept of head space as it would relate to a Thompson. I've always thought that the bolt sent the round in the barrel , the round stops ,primer hit then the action repeats. It seemed simple but what do I know.
Barry

[JimFromFL]

If the head spacing is too small, the level/hammer will not hit the receiver which in turn would not hit the firing pin.

If the head spacing is too large, the bullet is pushed into the barrel, sure the hammer hits the firing pin, but the primer is too far away from the firing pin to ignite.

[PK.]

Barry,

There was a lengthy discussion on the topic of headspace on the old board, it would be worth your time to search it out.

Headspace is the relationship between the breech (bolt) and the part of the chamber that stops the forward motion of the cartridge. In the TSMG these features are the face of the bolt that supports the back of the cartridge and the step in the chamber that contacts the mouth of the cartridge and stops it from going father into the barrel. The flat surface of the “square” bolt body also comes into play at one end of the tolerance.

On a properly installed barrel, the cartridge will be “pinched” between the fist two features by .002-.007” on a maximum cartridge, a minimum cartridge will allow a bit of play as the bolt body contacts the receiver..

This is a crucially important dimension that is dependant on the dimensional integrity of the three major components involved in its measurement; the receiver, the bolt and the barrel. The tolerances for these parts are tightly specified. Unfortunately, many aftermarket parts are not made to standard. Some GI parts (including receives) are not either.

[Barry]

Thanks again for the info. I found the old post which well answered my question on head space.
Barry