Only bolts I had were the ones in the brakes. They wouldn't thread together at all. Barely start, tons of running torque. Felt like it was cross threading. I spent the past few hours troubleshooting and I found the solution.
So they are both M6-1 threads. I found this out with calipers, thread standard sheets online, and some pitch gauges. The Shimano Ultegra bolts had included a thread locker. The female threads in the frame ended up having a tiny amount of resin in them. These two things combined made it almost impossible for them to thread together. I carefully (by hand for 80% of the time) ran an M6-1 thread tap through the frame threads to clean them up. Once I had done this, they actually went together.
Tomorrow I drill out the hole for the rear brake cable and file down the interference with the crank spindle. Wish me luck!
See my earlier post on the drilling with the carbide bit & finish with the sanding bit
My Chinese dremel was a pain to use & the company seems to be defunct so I used the bits on my Milwaukee rotary M12 cordless which was easier to use
For direct mount brakes I had a miserable time mounting the front one & removed the thread locker from the bolts (campy) & cleaned out the inside of the mount point with isopropyl alcohol & a soft rag & used a touch of asc-1 (anti sieze) on the bolt thread to get it started & carefully tightened it
Yes, I know, you should not use grease ppl-1 or asc-1 on this, but I was afraid of damaging the fork etc & I can always use a medium strength blue loctite later if it loosens out