Always, always, ALWAYS chased your threads before installing new hardware. When chasing the threads, use a tap and not a thread chaser/cleaner.

 

KTM summed it up well. It sounds like you have dirty threads in the holes. Clean and blow out the holes, brake cleaner and compressed air work great for the cleanup.
Remember the nuts are pinched so they lock instead of useing lock washers.

Good luck and thanks for your business.

 

safe to say i should use a M8x1.25 tap

 

That sounds right but you can buy a thread guage & measure the bolt. The question will come up again as you fix something else. Use an open end wrench if you don't have any other metric measuring device.

 

Make a thread chaser from a bolt that is clean and w/ good threads. Just take a die grinder and cut 3 or 4 vertical slots in the threads perpendicular to them. Then, take a die and clean up the threads. Why? Because:
1. the bolt will be softer than a tap and won't remove as much metal;
2. the bolt will not be tapered like most standard taps and will clean out the hole deeper;
3. Easier/faster to work with than a tap handle in confined spaces.

N.B.: The chaser is NOT substitute for a tap and should only be use to clean dirt out of a good threaded hole. Good for spark plug hole too using an old plug.