It's a standard fine-threaded bolt. Either 5/8-18 NF (15/16 wrench) or 3/4-16 NF (1-1/8 wrench). I can't remember which one it is, however. I know it's the same size as the center bolt of my Lisle power steering pulley puller and balancer puller.
Tip:
Use the longer center bolt from the balancer puller with a stack of washers and the thrust bearing from the PS pulley kit to press on stubborn balancers like a Romac