1 5/8"-16 UN thread
Darkzero made a spider in this thread.
http://www.hobby-machinist.com/threads/my-pm1236.11475/
"As Gary mentioned in his thread, the spindle thread is 1 5/8-16 which is what I came up with too when measuring. On my spider the outside diameter matches the spindle, thread depth is .75", OAL is 2.25", & the bore is 1.30". I used M8x20 set screws cause I had extras laying around, plus I prefer to keep all fastners for the lathe & mill metric so I don't need to keep an extra set of tools nearby."