I had some time to inspect that hole. I cleaned it out really good and tried to get a good look at the threads but I had a hard tome getting a clear view. I went ahead and tried to run a thread cleaner up the hole and only made it a few turns in. When I pulled it out I had a look and saw metal...been here before! I grabbed one of my odd shaped pics and started following the threads up and was able to peel the steel threads out of the cast iron threads.
I cleaned the hole out as best I could then ran the thread cleaner up and it screwed all the way in like it should. I pulled it out and get my eyeballs up close with a flash light and the threads in the block actually look pretty decent. I did not run a tap up the hole as I didn't want to damage anything with a cross thread issue given the location and angle I had to work in.
The buggered up bolt I pulled out of the block appears to be original to the motor and was 5/16-16 1/2" long. I grabbed a 5/16-18 3/4 inch flange bolt I had in stock and ran it up. There is enough room and threads in the block to get a 3/4 long bolt in and comfortably seated.
I think I'll go with a 3/4" standard bolt and lock washer because it will grab more threads unless there is a good reason to use 1/2" only?
Also, is there any issue with using zinc coated bolts for this application?
I have had a hard time finding plain steel fasteners anymore locally. Zinc and Stainless is pretty much all they have at my local shop. There is some parkerized stuff but that's mostly grade 8 flange bolts.
