Hayterette discs are threaded with 7/16s unf. The bolt that holds the disc on is a smaller diameter of 3/8. This means that if you get a long 7/16 unf bolt (about 2 1/2 to 3 inches long) and wind it into the disc it will draw the disc off the shaft. I know this as I did 2 only yesterday, an early 70s 3.5hp one that had never been off and an early 80s 3hp one. They came off quite easily. The only drama I had was that I had to run a 7/16 bottoming tap through both discs quickly as the beginning of the thread was slightly fouled on each
