HK Bolt Gap
Understanding Bolt Gap
The bolt gap found in roller delayed systems is the gap between the bolt, and the bolt carrier when the firearm is in battery. This is most commonly used on HK firearms such as the MP5, G3, HK33, etc. It is important to check the bolt gap periodically to make sure the tolerances are in spec.
Operation of the Roller Delayed Firearm Cycling:
- Hammer hits the firing pin and strikes primer
- Round gets sent down the barrel and the rearward force pushes on the bolt
- The bolt being pushed rearwards pushes the rollers into the groove inside the trunnion
- The rollers riding in the groove are squeeze by the trunnion on to the locking piece
- The locking piece, since it is locking into the bolt carrier, pushes the bolt carrier rearwards
- The bolt carrier is allowed to move rearwards where it then grabs the bolt and pulls it back
- The bolt carrier group as a unit is moving rearwards as the round is extracted from the chamber
- The bolt carrier group trips the ejector lever, pushing it up and ejects the round
- As the bolt carrier group moves towards the barrel it strips a new round off from the feed paw location on the bottom of the bolt
- New round is chambered and the cycle starts over
Why Proper Bolt Gap Is Important
Over time a firearms bolt gap can drift through wear of the rollers, the locking piece, or the trunnion. The most common place to start adjusting this gap back into a safe range is through the rollers. As the Bolt Gap Table below shows, the standard size for the rollers is 8mm(.315in). If the bolt gap goes under the minimum spec you will need to put in a larger sized roller to increase your bolt gap. If a larger roller does not fix your issue you may need to look into replacing the locking piece. In extreme wear cases you might even have to replace the barrel and/or trunnion. The majority of the time adding plus sized rollers will fix the issues.
The bolt gap being above or below the acceptable range can cause several issues. The main issue encountered is unnecessary wear on the firearm. This is less of an issue on the MP5 due to the lower pressure pistol caliber it fires, but on the rifle caliber guns such as the HK33 and G3 it can be a big issue. If the gap is below the minimum measurement then the rollers and trunnion have worn down, and means the rollers are not being pushed into the trunnion and are not engaging into the trunnion grooves. When the firearm cycles the rollers are slammed into the trunnion causing excess wear, and changing the roller delay timing. This changed timing causes the bolt to travel at an accelerated rate rearwards. The increased speed can cause reliability issues with the firearm as well as extra stress on the receiver and bolt carrier group.
Bolt Gap Table
| Bolt Gap Table | ||
|---|---|---|
| Min | Max | |
| .25 mm (.010 in) | .50 mm (.020 in) | |
| Roller Size Table | ||
| Size | Measurement | Markings |
| Standard Diameter | 8mm (.315 in) | No Marking |
| One Size Larger | 8.02mm (.316 in) | - |
| Two Sizes Larger | 8.04mm (.317in) | = |
| One Size Smaller | 7.98mm (.314 in) | -2 |
| Two Sizes Smaller | 7.96mm (.313 in) | -4 |
| The minimum and maximum bolt gap and roller sizes are the same for all HK roller delayed firearms. From the MP5 to the G3. | ||
One thing that is often fixated on is the actual measurements of the bolt gap instead of looking at them as a Go/NoGo indicator. There are HK Armorer tools that only have the .25mm(.010in) and .50mm(.020in) feeler gauges on them. If the .25mm(.010in) fits and the .50mm(.020in) does not, you are ok. If the .25mm(.010in) does not, or the .50mm(.020in) does; then something is wrong and needs to be addressed. It doesn't hurt to check the gap to track trends and spot any wear before it’s a problem, but hyper focusing on a 0.14 gap vs another gun that's .019 is getting worked up for no reason.
Checking Bolt Gap
To ensure your bolt gap measurement is the most accurate reading as possible it is important to make sure the firearm is as clean as possible. This includes the bolt, the locking piece, the chamber, and the trunnion. It all needs to be as clean as possible. Once you are happy with how clean it is, check to make sure the firearm is empty, send the bolt home and drop the hammer. From here you are ready to start with checking the gap. You will want to take the feeler gauge, A good place to start is right in the middle .38mm(.015in). If that fits you need to go larger, if it doesn't fit then go smaller. For best results it's important to cycle the bolt and drop the hammer between each test as there is a chance you can slightly widen the gap when pushing in the feeler gauge in.
Locking Piece
Looking at the chart below you can see a few of the HK firearms and the locking piece sizes they use. The locking piece is the workhorse of the whole system. It sets the dwell time which is how long it takes for the firearm to unlock the bolt and start cycling the weapon. The higher the angle, the less forces are required to get things moving.
| Firearm | Marking | Barrel Length (in.) | Angle | Notes |
|---|---|---|---|---|
| MP5/94 | Blank | 8.85 | 100° | |
| MP5SD | 5 (old), SD (new) | 5.73 | 120° old / 115° current | |
| MP5k | 16 | 4.5 | 110° | |
| MP5k-PDW | 80 | 5.5 | 80° | |
| MP5/40 | 26 (old) | 8.85 | 80° | |
| MP5/40 | 24 (Low Impulse) | 8.85 | 90° | |
| MP5/40 | 25 (High Impulse) | 8.85 | 60° | |
| MP5/10 | 24 (Low Impulse) | 8.85 | 90° | |
| MP5/10 | 25 (High Impulse) | 8.85 | 60° | |
| HK33/93 | 3 | 15.4 , 16 | 58° | |
| HK33/93 | 7 | 15.4 , 16 | 55° | |
| HK53 | 8.3 | 65° | ||
| G3/91 | Blank | 19.7 | 45° | |
| PSG1 | 19 | 25.6 | 40° | |
| MSG90 | 23T (Titanium) | 23.6 | 40° |
Locking Piece Angles
The locking piece angle is measured at the start of the shoulder. It can be a little deceptive the way the locking pieces are shaped. Here is an image that shows how the angles are measured. See the above chart for the locking piece angles.
Further Reading
Thanks
A special thank you to Salted for making the images of the locking pieces with the angles on them.
