Besure to check the treads with a thread gauge, as there is a 3/4"-10 taps out there also. I would think that it is really a 11/16"-11 or a 3/4"-10 not to say they don't but most auto companys in that time period just standard type threads for ease of avaiablity and cheap tooling. I just find it hard to think it's not a standard. Post back and let us all know what you've come up with. Tom