It has been discussed on this forum at length over the years....
It depends on the year and which bolt. For all B18s if the bolt screws into the engine
block the thread is 7/16"-14UNC. You should be using the high-strength bolt that was
already there rather than what you can buy at the hardware store.
If the bolt screws into the aluminum bracket on the lower part of the bellhousing
(1968 or later) used on a few very late B18s and all B20s, the thread will be 3/8"-16UNC.
In this case also the high-strength bolt normally in this position should be used rather
than a hardware store bolt.
For VERY late B20 blocks (which have been used in some re-engine jobs in 122s) the thread size
in the block is M12-1.75 grade 10.9

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George Downs Bartlesville, Oklahoma