Reddit mentions of Origin8 Quill Adapter, 22.2/25.4 to 28.6 Threadless, Silver

Here is the link for the product I purchased:
Origin8 Quill Adapter, 22.2/25.4... https://www.amazon.com/dp/B0057C3EGI?ref=ppx_pop_mob_ap_share

The one titled “22.2/25.4 to 28.6” is the one that includes the shim. Should work with all standard threaded forks.

The dimensions can be quite confusing so here’s how it goes. 1” (referred to as 22.2mm) refers to the OUTER diameter of a threaded fork. The actual INNER diameter of the fork (or outer dimension of quill) is ⅞” because the thickness of the metal is 1/16”. For a larger fork with a 1-⅛” (referred to as 25.4mm) outer diameter, the inner diameter is actually 1”. And for threadless forks, the outer dimension is 1-⅛” (28.6mm).

So, to go from a 1-⅛” threaded headset to a 1-⅛” threadless system, the adapter’s physical dimensions are actually 1” to 1-⅛”.

I’ve seen some errors on some online listings regarding this and I actually ordered a product that didn’t fit before I did the math. Hopefully this long explanation helps.

Yeah 580 is extremely narrow, even for back in those days - and why it has such a long stem. With bars that are much wider you would almost certainly want to swap for a shorter stem at the same time.

This would give you the advantage of being able to buy modern 31.8mm diameter handlebars rather than having to look for used/rare 25.4mm bars. The type of stem you would want is very rare now as the standard of attachment for stems changed not long after your bike was new, so you are probably best off buying this $15 adapter so you can run a modern threadless stem like this (I'd probably try 100 or 110mm).

The wider the bars are the more air resistance you'll have to slow you down on high speed sections, but wider bars do improve control quite a bit, and bars as narrow as yours are at an extreme end of the scale.

My thoughts are the sweet spot without drastically changing the set up of the bike, utility of bar ends and the speed of the bike would be about 660-700mm, though you can always cut down a wider bar to some extent.

These bikes don't have geometry designed for much shorter stem/wider bars, if you go too far it can get harder to stop the front wheel lifting up while climbing steeper hills and harder to get enough weight over the front wheel in corners.