On mine a 3/4" x .035 4130 tube. On the wood prop it was a tight fit in the hole in the prop hub. Whirlwind also has a 3/4 hole but the tube is not a tight fit. Lots of options on keeping the front fitting tight. Easiest might be a AN490 rod end welded to the tube with a lock nut. Wittman used a short steel cone and I think he used a plain nut with a jam nut.
Regarding possible cracks in the Whirlwind hub:
I tightened the bolts finger tight and measured the distance between hub halves. Adjusted bolts so that the parts were exactly parallel. Then? tightened bolts 1/4 turn at a time in a cross pattern. When the got reasonably tight I switched to a torque wrench and torqued the bolts a few # at a time. I think I used two torque wrenches to avoid changing sockets and settings for two different size bolts.?
