Help! Rethreading Alternator Mount in Exhaust

[Grizzly]

A few weeks ago the lower alternator bolt backed out on my 300hp '67. The belt kept it from coming completely out. When I took it apart today to recheck everything I found that the threads in the exhaust manifold were pretty much gone. My guess is that it worked lose before and tore the threads out and the last owner jamed the current bolt in the hole and it held for several years/thousand miles. I can't seem to get a die on the threads of the bolt I took out, but it looks like a 3/8 NF. As I write this I wonder if it might be a metric size chosen to fit tightly in the hole.

Now, I'm looking for opinions. The easiest would drill it bigger and rethread it. Second would be to helicoil it. Have done this on small engines with aluminum blocks. How hard would it be to do a cast iron exhaust manifold? I have everything cleared away so getting a drill in would not be to difficult.

Any suggestions or cautions?

Thanks ................................ Griz

 

[JohnZ]

The original threads in that hole were 3/8"-16; I'd Heli-Coil it and never worry about it again.

 

[Grizzly]

Thanks John, good to have your first choice confirmed by someone else

.............................. Griz