Lonnie, I see "John_M" has already informed you that 5/16 -32 is a standard thread.

Still, you may want to consider having a good lathe operator single point cut the threads.
Due to the root to crest diameter differences it may be beneficial to index the threads to one another to maximize potential wall thickness.

Note that Genuine Timeserts use indexed threads. , if the inserts in your engine were not indexed, this could result in a wall as thin as 1/64".
May have contributed to the fracture.