Interesting read here. May want to look at locking piece first to see how worn it is being a non 'HK' weapon. Also, just a thought, you may want to "size" the rollers that are in your bolt presently. 8mm is nominal. Considering they are only available in certain sizes, it would be best to know where you are currently to get where you need to be. They are available in +4, +2, 0, -2, -4. Plus sizes add gap while negative lessons. I'm stating this because one would think from the factory, there would be 0's in there. But not being "HK", the MFG could have done anything to get the gap into spec including a non-preferred roller size.
Disclaimer, I am a nobody sitting behind a screen using a keyboard. Hopefully an armorer will chime in to maybe add validity my words. @Chopstix Kid maybe?