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Discussion Starter #1
I have an early 80’s near new condition HK91 (IB date code) that I have owned since new. Along the way I ended up with a complete spare bolt carrier w/ bolt-head.

Today I was reading about how important it is to check bolt-gap and that it should be between .012 and .020. So I checked it for both bolt carrier assemblies. One complete assembly has a .011 bolt-gap and the other complete assembly has a .009 gap. If I swap locking pieces the gap measurement swaps between assemblies (gap follows locking piece). I find this strange because this gun only has fired about 2000 rounds since new. Oh well. The measurements don’t lie.

So I need to order new rollers for both bolt-heads. Given these bolt-gap measurements (.009 and .011), which number rollers should I order for each bolt-head? I assume I want to get as close to being slightly under .020 as possible, correct? Do I need any other replacement parts to do the roller job other than the 2 rollers?
 

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Get a +2 rollers. This theoretically should get you somewhere around .013 to .016 depending on the LP.
For a .308, I tend to like being around .015 although of the gun cycles fine, even .011 is ok. The closer you get to .020" the harder the gun is going to hit you and the more stress it puts on the receiver.
 

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57strat, on the Files I recommended +4 but seeing as how you got a reply from Joe here, take his advice over mine. He's wiser than me on these guns and even I will defer to his wisdom. +4 will keep you in spec but he has a point about be gap being at the top end.
 

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Who are these people saying the spec is .012 to .020? I just checked the G3 Armorer's Manual and it says .004 to .020. Yet I find sources on the Internets and HK Pro "experts" saying the spec is .012 to .020. WTF???? It turns out my rifle is totally in spec.

 

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You are in spec, as far as I,m concerned; but some like the quicker opening HIGHER bolt gaps. I personally think .012 to .016 is the sweet zone. just my ten years of EXP. +2 rollers and and a NEW hk locking piece[you may not need] will bring you right where you need to be in my opinion. The gun will work at lower bolt gaps,but either end of this range has pro,s and con,s. To low is not good and to high is not good either. Hope you get it.
 

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I've never seen bottom end spec being .004. Interesting. That's so close to zero that a good chunk of crud would lead to zero gap, a very bad thing which equals excessive head space and bolt velocity enough to damage the receiver.

Based on that I'd call it good and forget about it.
 

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My 91 shoots moa out to 200yds and I had to put in +4 rollers to get it to .17 I think sometimes it depends how well the barrel was indexed to the trunion at the factory. I mean I have a very good barrel and I am already at +4 so its kinda weird..I had a clone that had zero gap and it shot fine but it worried me. I like it on the high end anyway. Make sure the locking piece is ok, they go first.
 

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.015 to .019 is considered optimal. The more gap, the easier the bolt will come back, meaning you're gonna feel it more as felt recoil. My 51 had gotten down to something like .002/.004 and was cycling in full auto pretty slowly. I even had an SW5 that had NO gap. It ran just fine in full auto and we referred to it as a slowfire gun. No kidding. The 9mm SW5 was running at about 550/600 rpm. Never failed to work, however and zero damage on the SW5. The 51 was a different story. It was hammering the gun so severely, that the stock mounting reinforcements welded to the receiver on the 308 cal gun had several cracked loose and just waiting for a chance to blow the stock off the back of the gun. Dyer did the repair for me (goodbye gun for 9 months or so) but it came back gooder than new and this is what it looked like last summer in Eden.

 

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Who are these people saying the spec is .012 to .020? I just checked the G3 Armorer's Manual and it says .004 to .020. Yet I find sources on the Internets and HK Pro "experts" saying the spec is .012 to .020. WTF???? It turns out my rifle is totally in spec.

The reason for some people giving a spec of 0.012" to 0.020" is two fold, one you would not wish to setup a new rifle at 0.004 inch as would have no wear in (compared to a field check which you manual is covering) and because that is the spec for the MP5 in the HK Armorer's manual (also available on line). Bottom line is you probably don't need to change your rollers at this time, however, for the current cost it would not hurt to have a set of oversize rollers and locking piece in stock (sooner or later the German parts will tend to dry up).

You came asking for advice instead of doing your own complete research first, which is fine and what many of us also do. Having found more information you could have come back with the updated information and came across as a helpful and useful resource, instead your reply is "Yet I find sources on the Internets and HK Pro "experts" saying the spec is .012 to .020. WTF???". Which does tend to make you come across a little bit less professional. My 2 cents, which is worth the bandwidth cost (i.e. nearly nothing).
 

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Who are these people saying the spec is .012 to .020? I just checked the G3 Armorer's Manual and it says .004 to .020. Yet I find sources on the Internets and HK Pro "experts" saying the spec is .012 to .020. WTF???? It turns out my rifle is totally in spec.
Part of the problem is that these documents have been revised many times and that what you find isn't always the current. For example, the MP5 manual widely available states that ideal gap is .018" where the current "operational" spec is .014".

Understand that bolt-gap controls the amount of delay before the rollers unlock, hence the name roller delayed blow back. Over the decades, I assume changes in spec are driven from the delay affect from various types of ammo. Ultimately, as long as the bullet leaves the muzzle before the rollers unlock, HK considers that as safe.

The closer you run towards the bottom of the range, the greater risk of cycling failure because more energy is spent unlocking the system. Conversely, the closer you run to the top end, the greater the risk of the system unlocking too early. The low and high are for extremes, but most importantly please realize that the specs have gone through revisions. At the low end, as long as the gun cycles, you are fine, assuming the gap is not zero.
 

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Discussion Starter #11
This is posted on RTG, and is regurgitated numerous times on this forum. It is incorrect information and wasted hours of my time yesterday. I am glad someone pointed me to the HK G3 armorers manual before I replaced my rollers for nothing. My HK 91 has only 2000 rounds through it since new. Logically speaking, there is no way it should need rollers replaced.

Q: How to check your Bolt Gap?
A: For a HK91, G3 or Cetme to run reliably and safely it should have a bolt gap that measures between .3 and .5 millimeters (about .012 - .020 Inch). If your HK/G3 or Cetme has less bolt gap or no bolt gap, then you need to change your rollers and use HK + Size rollers which are available for sale on our G3 Parts page. Do not think that just because your rifle is working that your bolt gap is OK. I myself didn't notice the bolt gap had totally disappeared on my favorite FMP G3 and I was shocked to see I had no bolt gap. A set of HK +2 Rollers got me back up to spec and now I can once again safely shoot my favorite G3. The lack of bolt gap can be dangerous. It also increases the amount of wear that your rifle endures while firing.
1, Remove Magazine.
2, Pull back cocking handle and let the bolt slam shut, make sure it's shut all the way.
3, Turn rifle over so you are looking into the bottom of the mag well.
4, Using a feeler gauge measure the gap that is between your bolt head and bolt carrier. If you lack a feeler
gauge you can use one or two pieces of paper, though a feeler gauge is needed to obtain a proper measurement.
5, If you have less than .3mm of gap than you will need to acquire + size rollers which are available on the G3 page of this website. Then
follow the instructions for the above topic titled "how to remove rollers" and replace your rollers.
 

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This is posted on RTG, and is regurgitated numerous times on this forum. It is incorrect information and wasted hours of my time yesterday. I am glad someone pointed me to the HK G3 armorers manual before I replaced my rollers for nothing. My HK 91 has only 2000 rounds through it since new. Logically speaking, there is no way it should need rollers replaced.

Q: How to check your Bolt Gap?
A: For a HK91, G3 or Cetme to run reliably and safely it should have a bolt gap that measures between .3 and .5 millimeters (about .012 - .020 Inch). If your HK/G3 or Cetme has less bolt gap or no bolt gap, then you need to change your rollers and use HK + Size rollers which are available for sale on our G3 Parts page. Do not think that just because your rifle is working that your bolt gap is OK. I myself didn't notice the bolt gap had totally disappeared on my favorite FMP G3 and I was shocked to see I had no bolt gap. A set of HK +2 Rollers got me back up to spec and now I can once again safely shoot my favorite G3. The lack of bolt gap can be dangerous. It also increases the amount of wear that your rifle endures while firing.
1, Remove Magazine.
2, Pull back cocking handle and let the bolt slam shut, make sure it's shut all the way.
3, Turn rifle over so you are looking into the bottom of the mag well.
4, Using a feeler gauge measure the gap that is between your bolt head and bolt carrier. If you lack a feeler
gauge you can use one or two pieces of paper, though a feeler gauge is needed to obtain a proper measurement.
5, If you have less than .3mm of gap than you will need to acquire + size rollers which are available on the G3 page of this website. Then
follow the instructions for the above topic titled "how to remove rollers" and replace your rollers.
I do not frequent the RTG website so I am unware of what they publish. If you find false information on RTG's website, perhaps you should send them an email indicating your findings. It may be helpful in preventing people in the future from experiencing the same.

What I do know is current HK specs for bolt-gap for roller-lock firearms is the following:

Shipping spec from the factory:
Min 0.004"
Max 0.020"
Ideal 0.014"

Recommended "in-service" range:
Min 0.010"
Max 0.020"

You inidicated your bolt-gap to be 0.009". As I think I noted earlier, that is fine. It is my personal recommendation to add +2 rollers as I like to run my guns with a bolt-gap in the mid-teens.
 

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Can someone tell me why there are two different degree shoulders avaialble for the G3 on most web sites? Do some trunions need a differnet angle on the shoulder than others?
 

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Standard G3 vs G3k. Barrel length. There are many different locking pieces depending on caliber, barrel, suppressed or not, and in some cases ammo specific for a given caliber. Go to the home page and look up locking pieces. Use search. Theres volumes of info here for those willing to look.
 

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Above does not say to squeeze trigger and release hammer. I have seen elsewhere that is part of the procedure. What is correct??
Yes after cocking the rifle give it the HK slap and then point the gun in a safe direction and squeeze the trigger. Then turn the gun over and measure the bolt gap with feeler gauges.
 

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Thanks. That is what I did and on a new Century C308 am at .027. The other 3 we got in were all between 010 and 018 and shot well. We opted not to shoot this one, our of caution, as we are new to this pattern rifle.

Happy to install the "-" rollers but not sure what size I would need to hit high teens, as apparently recommended. They are a little expensive for experimenting.
TIA
 

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Thanks. That is what I did and on a new Century C308 am at .027. The other 3 we got in were all between 010 and 018 and shot well. We opted not to shoot this one, our of caution, as we are new to this pattern rifle.

Happy to install the "-" rollers but not sure what size I would need to hit high teens, as apparently recommended. They are a little expensive for experimenting.
TIA
I am not sure what size rollers you would need. These rifles tend to drop a bit during the first few hundred rounds and then hold steady only wearing slowly over time. During that initial break in the different parts of the system that determine bolt gap mate together and settle shrinking the bolt gap. Sometimes the movement is a bit more on a rifle like the C308 where both new parts and surplus parts are used. However I have had some rifles that only close up a few thousands. Surprisingly one of the G3 clones that I had built from all new parts had a 18 thousands bolt gap when I got it from the builder and closed to 12 thousands after about three hundred rounds. Even though the gap on this rifle shrank by 6 thousands within the first few hundred rounds I would expect it to take thousands of additional rounds for the gap to shrink that much again. RTG has -2, -4, -6 and -8 rollers, with a gap that large you should probably start with -8 rollers just be aware that the gap may shrink quite a bit after the first couple hundred rounds. Make sure you check the gap often for at least the first five hundred rounds.
 

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If you order +4 rollers from RTG, expect to get the kind with the hole through the middle, or if your lucky like me, one of each. It's all they have left.
 

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I don't think you guys mentioned cocking tube gap, I read that it should be the same as the bolt gap +15. Is this true?
 
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