O.K. I thought I knew everything but alas I'm just another dummy.
Here's the story and question.
Recieved a "broke" O.S. 91 C-Spec heli engine from a friend. (Long story). Rebuilt it from scratch. Test ran it on a bench with a 3-bladed acft prop (12x6). Tuned and it ran great! SO I wedged it into a GP Lancair cowl with a Bisson/Pitts and the fit is right on. After deciding on a couple of 3 bladed props from 14" to 16" dia. the prop shaft is too short!! Add a spinner backplate and the prop shaft threads don't reach out far enough to grab a prop washer or prop nut. What now?? Counterbore the prop hub?

Doofus