I wanted to change the bearing in my 70mm idler pulley. I pulled the pulley off, I briefly felt a slight increase in friction When I was taking the bolt out. When I went to put the pulley back in, the bolt won't thread in the hole . I took a closer look at the bolt and there was a piece of aluminum on the threads.

I want to get a tap to clean the threads. Does anyone know what thread is? The only tap I have that is close is 1/2X13. That was too course of a thread.

 

After some measuring.
Bolt dia: .389" (9.90 mm)
Threads: around 17 tpi. My thread gauge had a 16 and 18 gauge. The 16 was too small and the 18 was too large. 17 tpi has a pitch of .0588" which is close to 1.5 mm .

I am going to get a M10 tap. Hopefully the threads aren't too messed up.