I did know this! A few early-mid 80’s MTB’s used the 1” threaded BMX standard which takes a 21.1mm quill. Given how hard it is to find stems for them I reckon a steerer tube would be pretty much impossible. Sorry that’s no help. Good luck with the repair!
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Just sent ya a DM regarding your pivotal topper thing!
Hoping that this is not out of place here.
Long before I built frames and worked in the bike industry, I was a dedicated BMX rider and tinkerer. The tinkering got me into component and frame design under the moniker Eject BMX. For a period I was lucky enough to design and do some CAD work both for myself and a number of different brands.
A while back I decided to dust off my old instagram account and use that to post up some historiocal stuff as a little nostalgia project. Since it’s BMX-related, I figured I could share the account here. Hoping that some of you will enjoy it and maybe one or two of you will even recoginse some of the stuff.
Here’s a snippet from the latest post.
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My first batch of Bike Frames:
Really stoked on how the bike turned out, 21,666“ top tube, 14“ chain stays, 11.47“ BB height, 10“ stand over, 74* head tube angle, 71* seat tube Angle, 120mm headtube (no spacer needed). Perfect for trails and transition.
It was a really weird but cool expirience to give my „baybys“ away…
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Hell yeah! Love the dropouts. What’s the end result on the long seat tubes? Pivotal or??
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That’s awesome!
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Really cool frames!
If I may ask: What tube dimension did you use for these chain stays and how did you get them bent with those two radii so close to one another?
Asking because I’m looking for a solution to build something similar 
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They have 20x1.5mm wall thickness, thats why they bend so well. My bending set up is pretty ghetto:
I use this pretty cheap die and a wheel for a gate with a 20mm radius for my Set up.
The bends are center to center 70mm apart, if that helps for planning.
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New cruzer race bike finishing up “the drawbridge”. Lower bb, slacker hta longer tt..
Kinda dj and cruzer geo mix up. Big doubles are in its future..
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Hijacking this thread to some degree as I logged on to ask if anyone has a drawing for a pivotal seatpost head. I’ve done some reverse-engineering but I’d feel more confident with an actual standard (if one exists). I’ll be making some layback posts in titanium for a client once I figure that out, so if anyone want’s some, let me know. TIA.
Yeah. shoot me a message with your email and I’ll send it to you. I’ve used it and I’ve given the file to a bunch of folks.