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Discussion Starter · #1 ·
As with most Poly's, my recent acquisition was a bit long on headspace. For the past few months I have been looking around for a TRW bolt to fit to the rifle. Found a pretty good deal on a NIW one. Naturally, it was not a drop-in deal and some fitting was necessary. After alot of comparative measurements between the original ChiCom and the TRW bolts, I determined that the back side of both lugs and the front of the left bolt lugs would need to be ground and lapped. No other machine work was necessary to either the bolt or receiver.

I designed and built this fixture to precisely hold the bolt at the proper angles during the lug grinding process. It uses a mini air die grinder with an 1.250" diameter cutoff wheel to grind and polish the lugs. A mandrel goes through the firing pin hole of the bolt and rides on 2 Delrin "V" blocks. Worked perfectly and only took about 30 minutes to fit the bolt to my receiver / barrel assembly. I started the process with an extremely fine grit wheel and finished with a green Cratex wheel for the final polish. The lugs were cut pretty nicely in the receiver and hand lapping wasn't even necessary. I got 100% contact right out of the fixture. I am now at 1.632" headspace and can safely shoot any .308 or NATO ammo.










 

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How much did you have to take off? I was under the impression that if you just lapped the bolt lugs to fit that you'd cut through the surface hardening and comprimise the strength of the bolt lug.
 

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Discussion Starter · #5 ·
Tea:

I removed .0085 from the back of the right lug, 0.004 from the back of the left lug and 0.005 from the front of the left lug. If TRW heat-treated the part in accordance with the bolt drawing, there should be plenty of case hardening remaining. I did not Rockwell hardness check the thing either before or after, but will certainly be keeping an eye on wear patterns and/or increases in H.S. as it gets some rounds through it.

I guess I was lucky and only needed to mod my new GI bolt slightly to get a perfect fit and function. No major changes were necessary to the barrel or receiver lugs. I did polish and deburr things a little bit. The whole thing is much less sloppy now.

By the way, my H.S. (in what was basically a new rifle) was about 1.645 which is just about NATO "FIELD GAUGE" length before the retrofit.
 

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You are far braver than I am, I keep finding used bolts that drop in or need next to no work to go in, then re-park them. I just can't stand the idea of wrecking a new bolt. I'll save the new ones for the Heavy barrel project.


FB
 

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If you ground the lugs on the bolt with a grinder, you will ruin the hardness of the bolt.
The heat created will soften the metal and need to be hardened again.
If the bolt become soft, you'll be back where you were with the chinese bolt.
Not worth the effort and cost.





 

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Discussion Starter · #10 ·
Warbird:

I respect your observation and comment, but I am NOT concerned about any annealing in this case. The surface (much less core ) temps experienced during this work weren't even close to being an issue. This bolt will live a long and happy life in my rifle despite your skepticism!
 

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I'm not the skeptical type. :lol:

Anytime you remove metal, regardless of the method used, alot of heat is created.

Being a machinist for 20 + years, I know that first hand.

Steps to eliminate the heat, or very least, lower the heat generated, is a must.
If you use machining coolant, then your method would be a sound approach.




 

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This is a very creative approach to fitting a bolt. It is encouraging to see a fresh look at a topic having considerable past discussion. I appreciate the idea and setup photos being shared freely on this forum.

Intuitively, you wouldn't think that with the very small amount of metal removed from the bolt lugs and the depth of hardness, that there would be any problem with this approach.

Are there others out there who would comment and provide their experience with metal removal and reducing surface hardness?
 

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Discussion Starter · #13 ·
Range Report:

Finally took the subject rifle to the range yesterday and put the first 250 rounds through it since installing the new bolt. Functioning was perfect with no problems whatsoever. The first 10 rounds of WCC NATO ammo through it looked perfect. No case abnormalities, no set-back primers, ejects spend brass about 8 feet at the 1 o'clock position. After ever 50 rounds I would remove the TRW bolt and do a visual inspection of the back of the bolt lugs. The B.C. cold bluing I used to touchup the back of the bolt lugs after grinding and polishing showed nice uniform mating patterns with the receiver recesses. After 250 rounds, the cold bluing barely showed any signs of "wear".

Couldn't be happier with the results of fitting and installing the new bolt in my Commie rifle.
 

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I have hear of issues of the chi com barrel hood being longer than USGI barrels. Did you have to lap the hood of barrel tail or see any signs of wear on the top of the bolt where it mates with the top of the barrel. I started hand lapping a USGI bolt about a year ago but stopped when I heard that it may be hitting the tail of the barrel. It is very close to full lock up now. I may start hand lapping again after I research this a little more. Bolts are getting expensive and harder to find. Keep us posted.
 

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Discussion Starter · #15 ·
Jack:

I do not own a US GI barrel nor a reputable replacement (Krieger, etc.) to actually determine the variations that may exist between the dimensions of the depth of the recess you are talking about. What I did do is measure the dimensions of the ChiCom barrel shroud and the mating area of the ChiCom bolt to determine what kind of clearance the Chinese built into the original configuration before I started any work to fit the TRW bolt to the gun.

Don't have the dimensions here in front of me but I do recall that the Chinese axial (fore & aft) clearance between the front of the bolt and the rear of the chamber was in the neighborhood of 0.020" when the bolt was in battery. With the TRW bolt "adjusted" to the desired 1.632 headspace, the same clearance is now around 0.005".....by no means an interference fit.

The radial clearance around the outside of the Chinese bolt and the I.D. of the Chinese barrel shroud was pretty significant. As I recall it was around 0.015" to 0.020". With the TRW bolt it is now 0.002. Much tighter, but still no interference.

As I mentioned earlier in this thread, just about every clearance is now tighter, but everything functions really nicely so far. After 250 rounds, there are no significant wear marks in the original phosphate coating that would indicate an impending problem or premature wear.

I'm happy with the results so far!!!!
 

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I also Agree with Warbird, you dont want to grind on anything that is Case Hardened I dont care how much. :!:
 

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I started had lapping my H&R bolt to the Poly Tech again today. Man, what a pain in the neck, literally. A little closer than I was when I gave up last year, but still no cigar. I'll either just have at it for a few hours every month or so, or get feed up and send it out. I hate giving my children to them fellows with the brown shorts :D . I looked at my dremel, but better sense got the best of me. You may indeed have the skill Russ, I know I don't :( . As old Clint once said,"A man has to know his limitations" 8) .
 
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