I was at pep boys today replacing nuts and bolts on my exhaust system, as well as a replacement stud and to get the bolts to release my brake rotor. Anyway, the measurements of the bolts included M8x1.25x35mm, M10x1.25x40mm (cat bolts), and the wheel stud was M12x1.25x.... I understand the M__ rating as the width of the bolt. What is that 1.25? the third dimension is obviously the length. I dont think its the threading either cause theres the course and fine designations as well. Nissan looks like they use alot of the M_x1.25 fine threading size. From my experience with Toyota, they use it too.

On a side note, I FINALLY got out one of the catalyst studs that was stuck stuck stuck. The way I did it was to heat up the one end with a roadflare, and then vice grip the other side. It came out finally and I was real excited. I then put my new bolts through a few times in order to clean out the threads. Now I'm gonna sandblast the cat and paint it with high temp paint. Another question, I can't seem to find (havent tried the dealers yet) a suitable replacement stud for the cat bolts that face the Y pipe side. Is it acceptable to just dispense with the stud and just use a bolt and washer?

 

The 1.25 indicates milimeters per thread.

As far as your other question of using a bolt instead of a stud, I have no idea.

 

grade of the bolt x pitch x thread

yes to the 2nd question. squirt on an anti-seize grease to make it pull off easier the next time around.