9/16"x 20 TPI CEI, is a whitworth form thread. See BS 811. As are all the 20 TPI series of CEI. ONLY the 26 TPI are 60 degree thread angle.Steve
9/16" 20 TPI taps available on ebay - RDG Tools.
I think this was a common thread in the 50s known as cycle thread and if memory serves me correctly it was 60 degrees.
One word of warning - the taps on ebay seem very cheap at £7 ish, but it may be a starting point - a cheap tap broken in the hole can be a real problem.
H
Are you using original studs ? that`s the first thing I would ask.
New castings, if not properly heat treated will have a tendency to close up a little after initial tapping. This could be a reason, as could using original studs.