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Discussion Starter · #1 ·
Hopefully someone can help me out...

Does anyone know if bolt head rollers are surface hardened or through-hardened? Reason I ask, my brother has a CA 89 that is a PDW clone. It is an sbr. I'm in the process of transferring it to me. I've done a lot of work to this gun in the past to get it running. I have a suppressor and want to run it on this gun. It came with a HK number 16 LP in it. I put a RCM number 28 in it, but the bolt gap shot up to .039 with the standard 8mm rollers.

I can't get proper bolt gap with even -8 rollers that are available. A German 28 lp seems to be impossible to find and ridiculously expensive even if i were to locate one. I made a set of rollers (just to try) from some scrap stainless that I have on-hand. Turned them down to .3075 and got .017 bolt gap. Gun ran like a champ and suppressed well. My try-rollers aren't hardened and definitely won't last any amount of time.

If the rollers are through-hardened, I can grind them with a tool post grinder to the dimension needed. If only surface hardened, I'll have to make my own, and harden them. Just trying to find out before I jump in, and have to do twice the work.
 

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I would suggest sending the gun to one of the awesome smiths here and have the barrel repressed to achieve proper head spacing and bolt gap
 

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Above is the best solution.

I would venture the rollers are only surface hardened to allow them to be easily produced.
 

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Discussion Starter · #4 ·
I didn't clarify... With the German rollers and the German number 16 lp, the bolt gap is in spec at .015. Bolt gap shot through the roof with the RCM lp. I believe the new RCM lp is out of spec. I can repress, ream and install an oversize pin myself. I don't believe the gun needs it. I can run it with undersize rollers until I can find a German 28 lp. I have all the equipment to make said rollers and harden them, just wanted to find which way to proceed.
 

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I just bought an RCM #28 LP. Here are my bolt gap numbers :

MKE 110 degree LP (new) = 0.33mm gap
H&K 100 degree LP (used) = 0.35mm gap
RCM 80 degree LP (new) = 0.60mm gap

I expected bolt gap to grow but hoped it would stay within the nominal range. I just ordered some -2 rollers to hopefully get into the 0.50mm range but probably should have also ordered -4 as well.
 

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Discussion Starter · #6 ·
-2 rollers should get you right at .50mm. My gap measurements above are in inches. Converted to metric, my bolt gap with the RCM LP is .99mm. No gauges needed to tell that it's not even close. I'm not complaining. I'm eternally grateful that an American company is making parts for these guns at a very affordable price point. Just figuring out how to work around my particular problem...
 

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Maybe a call to RCM is in order for you? I would ask for another LP, something does not seem right. And maybe they will understand the appropriate heat treatment of the rollers if you want to continue with what you have.

That said, I would be worried about using your workaround, only because I do not understand the ramifications of the bolt/trunnion relationship with super small rollers. Like you said, LP might be out of spec.

I did get my -2 rollers in today and the gap is down .50mm as expected. I am months away from the suppressor but I think I am going to see how it functions with 124gr/147gr and the #28 LP.
 

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I dont know but dang...you are grinding your own rollers and such? Seems hardcore and a lot of effort to me but to each his own. You obviously have access to equipment and are knowledgeable.

Interestingly...here are a couple (one bad, one good) factory rollers that my MP5 managed to mangle a while back. Never have seen anything like it...and the gun ran fine. I just noticed this upon cleaning. Recesses and all else are fine. Go figure.

Screw Auto part Nut Metal

Technology
 

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Discussion Starter · #9 ·
That definitely looks like a roller that suffered from improper hardening. The "try" rollers that I made weren't hardened, and were starting to get dimples after 30 or so rounds.

Yes, I have access to equipment. My dad is a blacksmith/bladesmith/knifemaker. Also have a small, home machine shop. I have a day job as a public servant, so my time spent in the shop is my therapy. I actually love projects like this.

As a side note, it's amazing what a difference just a few thousandths roller size makes in regards to bolt gap. There's also a big difference in how quiet the gun is suppressed when the bolt is properly delayed in its blowback cycle. It's much quieter at 17 to 18 thousandths bolt gap ("try"rollers) than at the 24 thousandths I get with -8 rollers.
 
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