Buick Flywheel Pressure Plate Bolt Size?

[Kevin & Deana Brown <MGTRAutoXr@sprintmail.com>]

Does anyone know the the thread pitch of the bolts used to clamp the
pressure plate to a stock Buick flywheel?  They appear to be 5/16 x 24
but I'm not totally sure that they are not 5/16 x 18.  I chased one with
a 5/16 x 24 tap and more material came out than I thought should though
this flywheel has been sitting around for a while and had a little
surface rust on it.  Thanks.

Kevin Brown