3/4"x16 is the standard for the Cub & Avon, but don't assume that it is.
My Cub is 1"x8 with 2MT in both head & tailstocks. I got mine from the original owner, who told me he was offered upgrades when he ordered it.
I logged in with facearse no problem. But I hate the thing. The wife made an account for me so she could manage a page for the charity we are involved with. When she retires from her voluntry fundraising role at Chrisrmas I will be trying to deactivate my account. And as for Twatter :evil...
Common sense would suggest the biggest guage that will fit in the hole, and as long as possible without poking through t'other side.
So probably something like 1 1/4" x 10 roundhead.