I've been running my 6 v starter on 12 v for years in my 46. The starter does spin faster and my starter does not crank more than a couple of times before it dis-engages. Sounds allot like you describe, only no grinding sound. One thhing I did a couple o months ago, put an HEI ignition. Got it from Jim Carters, what a difference! It's unusual to get a couple of cranks before it starts!