surprised to believe a dull tap caused that much heat to cause cracking or the cause was tapping too fast, but I've been surprised before.
Now, I do not know anyone who rebuilds those mouse motors with that weak link still intact, i.e. pressed in studs. You stated a balance issue but not sure how bad that was and how you found out or the crank was not machined properly, are you referring to the past engine build where you ran it for 4 years or recently they machined it incorrectly. What was wrong with the crank? Now I'm not picking as you sound just like me some years past.
The machine shop I normally use is one of the best, but he was closed for a few weeks and I couldn't get the heads to him in time, so I went with the guy who put the engine back together after checking it out 4 or 5 years ago. I have since heard that he's had many issues with doing bad jobs.
I stayed with the pressed in studs, originally, because I never planned to take it over 6k. When the throttle stuck and I had to push in the clutch, I watched in horror as the tach went to 8500.
The engine always seemed to shake and not feel very smooth and I suspected it hadn't been balanced. The only way I found out about the balance issue was because when I found the head to be cracked I knew it would be a while before I would be able to get them back on. Since it was approaching winter I decided to tear the engine down and check out if everything was ok because of the rep with the guy that did it. The good machinist is the one that found all the problems. He said it was the worst out of balance he had ever seen. Of course with hindsight, if I had just gone to him in the beginning, all I would have had to do was put the screw in studs in the head. He said he uses a new tap on every job and said if it gets too hot while tapping, it can crack the bosses.
He showed me the crank and it had rub marks on the sides of the counter weights and the rod journals were rough around the inside edges. He said it was welded (badly) and the machining process was sloppy and gouged the inside of the journals which is why my bearings were grooved. It also was wearing unevenly. So the combination of the bad machine work and the out of balance really made the engine shake at idle and above 5k.
Everything is good to go now, but I'm having a hard time getting all the parts needed from one place like Summit.