Join Date: Jun 2005
Bikes: Raleigh, Rudge, James 3spds., and a cast of many
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Park Tool says 33.8 or 33.9mm inside diameter of the headtube, for an interference fit of the bearing cups. This is for both threaded and threadless headsets.
I'm thinking that some work with a caliper will be needed to select the closest tubing size. It may be necessary to ream to final size.
Your construction plan sounds interesting. I'd like very much to hear more of the details.