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So I have a steel road bike which orignially had a 1" threaded fork. I replaced it for a ahead fork and have been using this with a 1" Ahead stem without issues.

The stem was just a temporary solution and somewhat too long for my comfort, so I decided to replace it. I found it much easier to find lengths and angles of ahead stems for 1 1/8" steerers, so I opted to get one of those(FWIW Tune "Geiles Teil 4.0").

I was aware, that I was going to need a reducer and I was initially just going to buy one, but then I saw that at 15€ I consider them too expensive for what they are(a piece of shim) and that there are different heights and it was not trivial to find one to exactly match the 39.5 mm height of my stem.

Anyhow: Seeing that I have access to a bicycle workshop and scrap parts, I took an old 1 1/8" suspension fork with an alloy steerer, reamed the inside of the steerer to exactly 25.4mm inside diameter, cut 39.5mm off and made a 1mm wide cut, along the tube, all the way through, to allow for some compression.

TBH it fits like a glove, it's definitely stiffer than the previous stem, when riding hard out of the saddle and I am pretty happy with having made my own solution, however one Question arose:

Is it even correct to make a reducer with exactly the inside and outside diameters one is going for, or should, the outside diameter for instance, be slightly larger than the inside diameter of what it will be inserted into.

This might be a silly question, but seeing that I don't have access to a quality reducer like this one to measure myself, I thought I'd ask.

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    Interesting question. I don't know the engineering but I'm guessing the answer is that due to fatigue concerns you don't want the stem to have to splay open at the sides, at the 3 o'clock and 9 o'clock spots if you're looking down at it, and if the shim wall thickness was greater than that (28.6-25.4)/2 number, that's what would happen when you jammed it all together. I think the right answer is probably go for the closest wall thickness you can get to that, bias the tolerance in the over direction to prevent the stem binder from bottoming on itself, and make the slit as narrow as possible.
    – Nathan Knutson
    Sep 6, 2018 at 16:51

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There are off-the-shelf products satisfying your needs at sub €2 price tag
stem tube adapter:

This one is actually an adapter for stem tube but I don't see a reason that it shouldn't be used as stem quill adapter. Which means you simply manufactured this part yourself, even using the same material (the one depicted is aluminium).

Coming back to your initial question - since you've also put a length-wise cut on your adapter, the inner diameter varies within some range - it needs to be slightly larger to be able to slide it on the steerer tube. Of course, when clamped, the inner diameter is exactly what it needs to be. So in my opinion, yes, it is correct to make a reducer the way you made it.

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The reducer must have a slot cut in it to allow it to transmit the stem clamping compression force, exact inside and outside diameters therefore don't matter too much.

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