My Zenith Z-5RS had a bolt gap of .013” with its original locking piece and bolt. It has worked just fine for 500 rounds so far in its stock configuration, but as an upgrade, I bought a M.A.D. MAX bolt. With the new/unfired M.A.D. bolt and original MKE locking piece, the bolt gap opened to .015”. After looking at the roughly-hewn original bolt and locking piece, and comparing them to the excellent M.A.D. bolt, I decided that a new locking piece would be in order. I ordered an RCM 100* locking piece, which is flawlessly machined. The only thing is, my bolt gap with the unfired M.A.D. bolt and unfired RCM locking piece is right at the maximum of .020”. A .021” feeler gauge is too tight, and a .022” won’t fit at all.

Since both the bolt and the locking piece are new/unfired, should I fire off a few rounds and re-check the bolt gap, or buy some -2 rollers to drop the bolt gap down to .016”? I don’t really want to run it with the MKE locking piece, but realize it might be an option. Or, am I overthinking this and a .020” bolt gap is “in-spec.” and therefore A-OK?

Thanks for reading this, and please feel free to comment.

I would drop it down to 15

No reason to have it at max, more wear/ tear.

why did you get a new locking piece?

The machining on the M.A.D. bolt, which is apparently made by RCM, is much (MUCH!) better than the original MKE bolt. So I decided that I wanted to pair it with a locking piece produced with similar high-quality machining. The MKE locking piece has tool marks aplenty, but at least it isn’t as crude as the MKE bolt. The RCM locking piece is extremely high-quality, that’s for sure. However, I would rather not run this Z-5RS at the maximum bolt gap.

Although I didn’t want to re-use it, l’ll probably put the MKE locking piece back into service, and keep the RCM locking piece as a spare part. It’s easier than buying and swapping rollers, and will work just fine I’m sure.

I’m thinking that you’ve got the right idea here.

When I build roller lockeds I set bolt gap at .020

They work great

*shrug*

why would you set it on the max to start?

On a new roller lock, the bolt gap will typically “shrink” some, once it is fired a bit. This happens as the new parts wear-in, and if the barrel moves forward a tiny amount against its retaining pin. Also, over many thousands of rounds, as actual wear accumulates on the gun, the rollers, locking piece and trunnion wear will result in a smaller bolt gap. So it makes sense to start with a large bolt gap on a new roller lock, as it gives the gun the longest service life possible.

The flip side to this is that a roller lock with a large bolt gap is running in a “more unlocked” state than one with a smaller bolt gap. This can result in a more vigorous operating cycle, which could cause receiver damage.

On an MP5, a bolt gap of .010” (technically 0.25mm) is the minimum spec, .018” is considered “ideal”, and .020” (technically 0.5mm) is the maximum spec., per H&K.

Came to post this, .20 is fine, I would worry with the tighter .14< gap

I think you're fine with either combo. Monitor the back sides of your receiver channel. If you get any roller marks, then the gap is too great (or your recoil spring is worn out, which shouldn't apply here). Gap should shrink a little though, because you have new parts.

I wouldn't have bought the RCM locking piece. You think your gun cares it has tool marks?

I'm not one to ask that question of.  I've owned numerous HKs, SAR3/8's, Zenith's, Vector's, INDEPs and PTRs - oh even a few Century guns.  I have never ever in my life measured a bolt gap in any of them.  I just shot them.  They didn't blow up and I didn't die.

then why set it on max to start, you'd just beat your gun to death.

A new gun with all new parts will typically lose a bit of bolt gap over the first few hundred rounds. This is due to the new parts breaking-in and possibly the barrel shifting forward against its cross pin ever-so-slightly. We’re talking about the cumulative effect of minute amounts of wear on multiple surfaces here. A new gun built with a bolt gap of .020” might drop down to the “ideal” bolt gap of .018” over the course of a 500 round break-in, or it might not. Usually, a new roller lock will lose some bolt gap during the first few hundred rounds, then they will hold that new bolt gap for thousands of rounds thereafter.

I get that, but you still produce more wear/tear over the first few hundred rounds vs setting it lower-not to mention the crap recoil.

I guess I would rather not damage my receiver, have less recoil and maintain my rifle

The one bright spot to that matter is that the internals of the new gun are not as smooth as they will be once the gun is broken-in. The rougher internal surfaces will create more friction, which will slow down the bolt’s cycling speed. This will reduce the likelihood of damaging the receiver from the bolt over cycling. The bolt gap will drop to its “broken-in” dimension while the gun’s internal parts are smoothing-out. This is all happening simultaneously, while you’re shooting the first few hundred rounds.

Theoretically speaking, that is...

If you are in spec, why would you damage the gun?  HK recommends a higher bolt gap (0.18" or 0.45mm) which is close to the max.  I imagine it means a more forgiving gun when it gets dirty or a round is not quite full power.

On the flip side, if you set bolt gap low that means instead of the energy transferring into the recoil spring (which is easy to replace), it gets transferred into the rollers, then into the trunnion.  Assuming all parts are to spec, they should last most normal people FOREVER.  BUT - you decide, replace recoil springs or a trunnion?

I shot suppressed at both max gap and then something closer to min gap, and I did not notice a major difference in recoil.  There is a major difference between AE Suppressor 124gr and Lawman 147gr, in both felt recoil and how far the brass gets flung.  I bet shooting the 158gr PPU/Fiochi probably ups the ante quite a bit.

Ok so how do you gauge what is too tight.  On mine .017 slips in and out with small very resistance.  .018 goes in with a small amount or pressure and is captive I.E. wont fall out on it own upside down.   .19 can be pushed in and is very snug.   .020 is a no go.  How much pressure is adequate?

Sounds like your gap is a perfect 0.018" and you are here to humble brag!!!

Below is from a MP5 Armorers Manual

MEASURE BOLT GAP
Assemble the weapon and do a function check.

After the function check the bolt is to be locked to the rear.  To check the gap, point the weapon toward the floor, release the bolt and let it snap forward.  Put the selector on semi. Pull the trigger to let hammer fall.  Turn the weapon upside down and insert a feeler gauge through the magazine well into the gap between the bolt head and bolt carrier.  Record the measurement once a firm drag is obtained on a gauge.

No bragging here.  I am in the same situation as the OP.  With my original bolt I seem to have a .012 gap and with the new bolt I bought I have what seems to be .018 or.019.  It shoots fine with the original bolt but I also though it looked rough and thought I might improve the gap a bit..