What size should the starter bolts for a 72 390/C6 be? 3/8 or 5/16?


5/16 came out but 2 holes are damaged. One had a nut the other silicon holding it in.
5/16 Helicoils or tap up to 3/8?

 

The original starter bolts part number 352464-s8 were 5/16" -18 X 1 1/8". If you drill and tap for 3/8" the starter holes will need some massaging also. I vote for helicoils.

 

The starter puts a lot of twist on the bell housing. I am not smart enough to know how helicoils stand up against the twist. Methinks that they are for static problems like manifolds and water pumps. So methinks redrilling and tapping oversize would be a better solution. Hogging out starter holes should not be much of a problem if you have ability to drill and tap.

 

Would grade 8 studs be a better choice over bolts? It might make changing the next starter a little easier next time.

 

Grade 8 is overkill, but studs aren't a bad idea...

 

Helicoils if it were mine. Studs wouldn't work on my T18/390 because I have headers and there's no way to slide/move the starter back far enough to clear the headers and still go over the studs. Although I 'could' remove the headers but...

Dan

 

I believe that once the bolts are torqued down the mating surfaces of the bell housing and starter case transfer the torque to one another and the sheer load on the bolts themselves would be minimal.
The holes in the starter/spacer/seperator plate are already large enough for 3/8 hardware.
I went ahead and installed helicoils. They went in without a fight, which was odd.
Studs aren't a bad idea.

 

Yeah, I think you are right. It is the static friction between surfaces that keeps stuff in place. It was just a thought.

 

If you are concerned with a helicoil loosening up from vibrations, I doubt they will. I would be more concerned about them backing out when you remove the bolt. Just use some high temp thread locker. I repaired the threaded starter bolt holes on my I-300 using a helicoil. It worked perfect.