Well, I’m already facing the sort of incompatibility issues I expected to come with a ’70s Taiwanese knock-off frame.
The steering head bearings aren’t standard metric, but rather 1-13/16″ OD. [1.8125″ / 46.04 mm]. Not simply a weird size, these bearings are extinct. I couldn’t find a single bearing, ball or roller, anywhere. Not McMaster-Carr, not the Timken catalog, nowhere. Not that I was terribly keen on non-standard ball bearings, anyway. But what to use? There isn’t enough meat on the flanges of the steering tube to fit standard 47mm OD bearings. And the idea of making up bearing adapter cups doesn’t sit well with me. But wait! Digging through my cache of steering bearings, I found a pair of 25x45x12 bearings that I’ve had, unused, for almost a decade. These are an extremely unusual size, designed for updating Bultaco frames to tapered roller bearings. I had them shipped from Pyramid Products in Aussieland Limeyland, only to discover that my late Pursang frame actually uses larger, perfectly ordinary bearings.
They are almost perfect for this frame. “Almost” is the operative word. Unfortunately, they are 0.0408 undersized — enough to drop in with a little bit of play.
My original solution was to order some stainless .021 feeler gauge stock, which should be here by Christmas. I thought would provide a suitable shim ring. While that would create a really firm press fit between the races and the frame, someone suggested that I’m just worried that the shim’s spring steel would — with vibration over time — wear into the mild steel of the steering head. I don’t know if that’s correct, but it’s plausible enough that I’m looking for other options.
All Balls Racing does list 25x46x13 bearings, P/N 99-3526-5. That would get me pretty damn close (.04 mm / 0.0016″), but it’s out of stock everywhere, and evidentially has been so for quite some time.
