I have an old Elgin bike mostely in pieces that I would like to re-assemble into something I could occationally ride. Can I use crank bearings, and head sets from newer bikes on this frame? Are more modern Ashtabula style parts and bearings compatable?
May depend on the year. Mine is a 1940 and I'll give you what I know from it. I think the bottom bracket was a standard 10 ball but might have 26 threads per inch threads. Most likely the standard though.
The headset uses road bike sized bearings.
I would take the crank, race and bearings and the headset bearings to a bike shop to be sure to get the correct ones.
One thing I am sure of is that you can buy brand new ones meant for more common bikes.