Block Drain Tap Thread

["Edward R. Purcell" <epurcel@bellsouth.net>]

Listers,
My block goes to the machine shop Monday for installation of cam
bearings.  The drain tap had been replaced with a plug with incorrect
thread pitch.  I would like to have the shop chase the threads which are
corroded and the size and pitch is difficult to determine.  I have
ordered a new drain tap, but will not be here in time to help.  I
suspect the threads are pipe taper.  Can anyone help with the proper tap
to use on these threads?  Thanks.

Ed Purcell