HK91 Bolt Gap
The gap between the bolt head and bolt carrier should be as follows:

• Ideal:         .25mm (.010") to .45mm (.018")
• Maximum: .25mm (.010") to .50mm (.020")

The factory spec is:

• No more than 0.5mm (0.020")
• No less than 0.1mm (0.004")

HK USA advises armorers that they should adjust to an ideal of between 0.25mm (0.010") and 0.45mm (0.018").

If it's out of desired range you correct it by changing the size of the locking rollers.  There are 5 variants of rollers:

• Positive 4 (8.04mm) marked II increases gap 0.2mm/.008”
• Positive 2 (8.02mm) marked I increases gap 0.1mm/.004”
• Standard (8.00mm) unmarked
• Negative 2 (7.98mm) marked -2 decreases gap 0.1mm/.004”
• Negative 4 (7.96mm) marked -4 decreases gap 0.2mm/.008”

To increase (loosen) the bolt gap, replace with larger rollers.

To decrease (tighten) the bolt gap, replace with smaller rollers.

The HK advice to armorers is:

1. Never mix rollers of different sizes.
2. When adjusting bolt gap it is always better to be too high (loose) than too low (tight) as the bolt gap will tighten (decrease in size) with extended use.

Symptoms:

• Too LOW (tight) - excessive chamber pressure shown by separated case heads, blown primers, deep flute marks on ejected cases, excessive recoil.
• Too HIGH (loose) - below normal chamber pressure shown by anemic recoil, poor extraction, ejection and chambering.

Any chance of getting a picture of where exactly the measurement is supposed to be taken?

Also, is it the same for an HK 93 style weapon (Vector V93 Rifle and V53 Pistol)?

Same measurements?

Symptoms: Too LOW (tight) - excessive chamber pressure shown by separated case heads, blown primers, deep flute marks on ejected cases, excessive recoil. Too HIGH (loose) - below normal chamber pressure shown by anemic recoil, poor extraction, ejection and chambering.

Isnt this part backwards if I am reading it correctly.  Bolt gap is essentially the measure of how much the action is already prematurely open.  

As the gun wears and the bolt gap decreases (becomes smaller) the guns start to show anemic ejection due to the rollers having to overcome an ever larger mechnical advantage as they are placed farther back on the locking piece.

The reverse, when the bolt gap is excessively large (i.e the more the action is even more prematurely open) the greater the recoil and tendacy for blow case heads along with all the other wonderful stuff that can happen when the action unlocks with too high a chamber pressure.


James
Austin, Texas

Carbine Man, how many rounds fired should you expect to fire before having to replace rollers? I know that how you treat the weapon, fire it, maintain it , has a lot to do with appreciable wear. Just an approximate amount please. Thanks.

Quoted: Symptoms: Too LOW (tight) - excessive chamber pressure shown by separated case heads, blown primers, deep flute marks on ejected cases, excessive recoil. Too HIGH (loose) - below normal chamber pressure shown by anemic recoil, poor extraction, ejection and chambering. Isnt this part backwards if I am reading it correctly.  Bolt gap is essentially the measure of how much the action is already prematurely open.   As the gun wears and the bolt gap decreases (becomes smaller) the guns start to show anemic ejection due to the rollers having to overcome an ever larger mechnical advantage as they are placed farther back on the locking piece. The reverse, when the bolt gap is excessively large (i.e the more the action is even more prematurely open) the greater the recoil and tendacy for blow case heads along with all the other wonderful stuff that can happen when the action unlocks with too high a chamber pressure. James Austin, Texas

I have the same trouble you do with this!  I am not a gunsmith nor do I have experience de-bugging roller-delayed blowback actions.  I have just tinkered with my PTR rollers.

I got this stuff over a period of about a week searching the Web.  A lot of it comes from HKPro.  I didn't just take it at face value.  When I found a page with some information, I did a search on the author to see if they were blowin' smoke or really had some facts.  There are several 'smiths around the Web who build and debug these rifles.  The above is just compiled from what they said.

The last part about the symptoms assumes that if the locking piece disengages the rollers too fast and the bolt comes back too soon (gap too big), the pressure in the chamber is lower.  If it takes longer for the locking piece to disengage the rollers and the bolt comes back later (gap too small), the pressure is higher.

I have trouble with this because in my mind I imagine the action of the bolt being very slow in comparison to the combustion inside the case and the movement of the bullet.  But I have also seen stop-action pics of a 1911 which shows the bullet just exiting the end of the barrel, and the slide is already coming back.

So I guess it really is possible that the bolt can come back before the pressure has reached its peak.

Like everything on the Internet, you should take it with a grain of salt.

Some good information here.
I hope I can find this later when I need it.

Big picture