OK, so I have this old Gary Fisher MTB ($5 CL find!), with a small frame that I'm putting my son on. The problem is that the stem is too far of a stretch for him, so I was looking around for a shorter stem, but couldn't find one that fit.

Now that I'm looking more deeply into various sizing issues, I'm worried that, according to Sheldon Brown's threaded headset cribsheet, I'm beyond the "oversize", and into the "mainly tandem" department. I can't find my calipers to be absolutely sure, but with a tape measure, I could swear that the INNER dimension of my headset/steerer tube hole is a lot nearer 1 1/8" than 1".

Now obviously, I don't want to go to some tandem specialty place and pay >$100 for a quill-to-threadless adapter and also a 60mm stem -- not for a $5 bike! But I got this crazy idea and I wonder if you geniuses could give me any feedback how this would work:

I get a "regular oversized" quill stem, meant to fit in a headset/steerer tube with ID 1" (and OD 1 1/8").

I take the over-over-size stem that I have, totally unscrew the long bolt and remove the diagonal/expansion thingy from the bottom.

I take a hacksaw or a dremel cutting wheel and cut me a 1"-2" section of that pipe, which I already know perfectly fits inside my headset.

I insert that into the headset as a sleeve to take up space between a "regular oversized" quill stem (OD 1"?) and my over-over-sized steerer tube. Given that these sizes seem to have exactly 1/8" increments, it seems that we are working with 1/16" thickness metal here, so hopefully the regular-over-size stem would fit into this "sleeve" just right.

Make sure the new stem's expansion bolt is fully below the sleeve so when it expands it will be pressed only against the headset

Tighten that sucker up; the expansion bolt would expand more than usual, but hold the bottom of the quill stem securely against the inside of the headset, and hopefully the sleeve would keep the top end from rattling around.

Any ideas how well this would work, or would I be signing up my son for a deathtrap where his stem may suddenly and catostrophically break free from the headset at any time?

OR, what if I get a regular-over-size stem and just use it as-is (no makeshift sleeve)? If I tighten it up, I'm thinking it would make the stem come out of the steerer at a little bit of an angle, but would that make any difference in terms of security? I mean, this kid's only 8, it's not like he's going to put a lot of stress on this thing...

OR, what if I make a long sleeve from my over-over-size stem and put a regular-oversize stem inside and keep the expansion-bolt inside the sleeve, would that be able to tighten?

OR, what if I make a long sleeve, and cut a slit all the way down one end -- THEN would I be able to take full advantage of the gripping-properties of the expansion bolt of a regular-over-size quill stem?

OR... (you guys are the experts, what would you do?)