I found a nice older stem riser with a long 120mm extension.
Trouble is, the threadless clamp size is 1" instead of the usual 1 1/8".

Does anyone make a quill stem adapter that has a 21.1 lower section and a 1" upper section?

Do you mean 22.2 lower section? That's the standard road/mtb quill size.

If so, Nitto made some two piece stems in the mid 80s that were used on higher end MTBs. They were similar to modern threaded->threadless converters + a threadless stem, but in a 1" size.



If you really meant 21.1 (bmx size, I think), that may be even harder to find. If you're lucky, Nitto may have made one of those since some very early MTBs used the bmx size stems.

Edit: Just searched, if you meant 22.2, this may possibly work for you:
https://www.amazon.com/Profile-Design.../dp/B001CJX1XG

My steerer tube is indeed a 21.1mm inner diameter...it's an early 70's Schwinn Super Sport. I am rebuilding two of them, one for my son-in-law and one for myself.

I don't think there is a 21.1mm adapter to a 1 inch threadless. The 21.1 is a bit of an oddity as it, and the only threadless adapter that exists (as least that my search has revealed) is the Manowar adapter from Crank2. A good machine shop can apparently turn down a normal 22.2 adapter.

Ebay also has 21.1 adaptors. Just type in 21.1 stem and make sure to include title and description. You'll get a few hits. Good luck on the search. You might even be able to get complete 21.1 quill stems.

Also enter .833 in your online stem searches. You'll see a somewhat different assortment of 21.1mm parts to choose from.

It's like my search for clipless 1/2" pedals. I entered both "clipless bmx" and "clipless 1/2" to search for the auctions I needed, and came up with Wellgo, Redline and Bulletproof-branded variants.

It's taken a while, but I now have all of the clipless 1/2" pedals (and 21.1mm, 11cm quill stems) that I need.
The EZSniper bidding also helped me secure the goods.

Oh, and I've looked, but never have found a 21.1mm-to-1" threadless quill adapter.

I can find an adapter with a 1" threadless top, but a 22.2 bottom: https://www.ebay.com/itm/BICYCLE-STEM...item3cc67ed048 and https://www.ebay.com/itm/BICYCLE-QUIL...item43b6f301da

Or, I can find an adapter with a 21.1 bottom but a 1 1/8" top: https://www.ebay.com/itm/New-Stem-Ada...item20cc70fe46

But, so far I cannot locate one with a 21.1mm bottom and a 1" top.
Looks like it's off to the local machine shop...

Thanks, as usual, for all the help and replies!