JimT wrote:1" pipe thread. I have used the neighbors tape a couple of times to clean out my cadet and the farmall cub manifold. 1" plumbing pipes thread right into the exhaust holes on both models.
Correct thread designation is 1" x 11-1/2 NPT
If you are out out and find pipe taps at garage sales/flea markets etc, either NPT or NPTF would be acceptable. The NPTF designation is made to a tighter tolerance to fit dry ( direct metal to metal -- no telfon tape or "plumber's putty )
If you find one marked NPS
use it as it is a straight pipe tap and will ruin your block ( by making the threads oversize )