1 1/8" forks in a 1" headttube, have a look at my IS41 adaptor and let me know what you think!

what depth would you have the outer sleeve of these new cups? it looks like about 2mm on your drawing, but presumably 10mm would be significantly better?

Of course it completely depends on the frame. In the drawing it's 3mm, on my frame here I can go 5mm deep at the top and 3mm at the bottom. Paint will of course be removed and cleaned.

i mean more the bit that extends over the headtube. there's no effective limit to that, right?

That's what I meant too. Yes of course there is a limit as there are toptube and downtube in the way?

ah yes. well that's a factor. i was looking at your drawing and not reality :)

Paint will of course be removed and cleaned.

You're going to have to go a bit further than that if you want a solution which lasts more than one ride. The headtube needs machining so that the two ends are fairly accurately cylindrical and coaxial, and sized to create the necessary press fit in the cups. This is relatively simple when dealing with the ID, since you just have to hold the head tube fairly close to coaxial with the mill spindle and run a boring head into it. Holding a finished frame in a way which allows you to perform similarly accurate finishing work on the OD is pretty challenging

Just make it on the lathe and bash it on then go ride. Video it too.

The headtube needs machining

My idea is that the softer aluminium will be headset-pressed onto the paint- and residue-free but not-so-perfectly cylindrical steel heattube simply by brute force as soon as ID and OD roughly match. According to Park Tool, the difference of ID and OD can be up to 0.3mm for a press-fit to work.

Will report how it goes. If it's shit and goes loose all the time, I will machine another set from steel and simply braze it on.

By the way, what I really don't get: as there is no doubt that the brazed-on version would work nicely, why isn't this already available as a product in framebuilder catalogues.

You could machine it for a tighter interference fit and then heat the cup before pressing it on, especially since aluminium expands more than steel with heat.

This is actually a brilliant idea.

Don't crack the cup though! It might be more of a risk if the headtube is slightly oval. I can't think of a way of making it more cylindrical without turning it on a lathe, but it might be worth half an hour with your best calipers and some very fine abrasive paper once you've taken the paint off.

i think we're in micrometer territory here

I could convice Park Tool to manufacture an external reamer. For mass production 1" to 1 1/8" conversions ;-)

i think we're in micrometer territory here

Nah, decent vernier caliper will get you in the desired tolerance range for the diameters, an interference anywhere in the 0.1-0.2mm range will be acceptable. The harder thing than getting it on size and roughly cylindrical is getting the two ends sufficiently coaxial.

isn't that what HT facers are supposed to do (well)?

facers only face the top but not the sides of the tube

facer+reamer = yes, but I think gbj_tester is talking about coaxial external sides of the tube

is getting the two ends sufficiently coaxial

what is the typical accepted coaxial tolerance when using (for example) IS41-compatible bearings? I can't imagine that I will end up with more than 0.1mm dislocation over 200mm headtube length, and that should really be fine? Also given the fact that (as you said yourself before) components generally deform under load here and there.

oh, i thought he meant coaxial as in if the ends weren't parallel, they would be on different axes

what is the typical accepted coaxial tolerance

Dunno, a bearing manufacturer will have tables for that kind of thing. Since the bearing separation is about 10 times the bearing radius, you will want the axis of one bearing to intersect the plane of the second bearing inside a circle with a radius of about 10 times the maximum permitted clearance on the lifted side of the bearing and centred in the second bearing. I think I'd want the target circle to be R<50μm just to make the bike somewhat ridable and durable.
The further out you are, the more the bearing bears on only a subset of the rolling elements as the inner race is canted relative to the outer race. This is a perennial problem in designs where the bearing seats cannot be cut with a single tool without moving either the work-piece or the spindle axis, and for this reason designers try to avoid such layouts. For example, the circlips in a BB30 bottom bracket seem like an annoying and unnecessary complication compared with just machining a shoulder into the shell, but they make reaming the two bearing bores coaxial a piece of cake.

I think I'd want the target circle to be R<50μm

Well ok, so you propose less than 0.05mm. Anyway, we'll see how it goes.

I just realized I could also simply braze on the cups (if machined from steel) and THEN apply a IS41 facer+reamer. Then everyone should be happy?

I just realized I could also simply braze on the cups (if machined from steel) and THEN apply a IS41 facer+reamer

That's the correct order of operations. Of course, you could also use the same tool to tweak in the pressed-on aluminium cups if you leave some stock on the inside to allow for the finish cut to be done in situ.

Yes but then Senor Bear will still complain that the cups are stretched loose by the steerer forces. I still really have no idea if that would happen or not.

Senor Bear will still complain that the cups are stretched loose by the steerer forces

The point of the interference fit is that when the shaft pushes the bearing to one side, the interface load between the parts never drops to zero on the relaxed side. That's why it's important (if you want to avoid fretting) that the degree of interference is properly calculated relative to the stiffness of the parts and the anticipated service load.

is properly calculated

as I can't do that, this will be done by rule of thumb. Do you think it's feasible at all (also taking into account the idea about the pre-heating of the cup before pressing)?

this.