Purists look away now, but this actually works.

Head-tube reaming does not actually have to be all that precise. So long as the diameter averages out to somewhere around 33.8 or 33.9mm the headset will squash in there nicely without splitting the tube or being loose. It's not the cylinder bore of a Formula 1 engine. You probably do want to face it properly though.

Like anything there is a proper tool for the job but it costs $$$.




The drum in the drill is a piece of Al turned down to 31mm with a slot cut in it at angle. The drum is threaded for the M8 bolt which is then all clamped up with a nylock nut (as it happens because that was what I had). This leaves just enough room to wrap a bit of emery tape (80 grit or 120 grit is a good choice) but have it fit fairly tightly inside the HT. A close fit means you have no choice but to sand in something approximating a circle of about the right size.

Measure the HT twice at 90 degrees and average the measurements. If it's a little oval you can concentrate the sanding where you need it. Otherwise sort of precess the tool around evenly inside the joint while running the drill on its high speed setting. The other bit of Al there has been turned down to 33.7mm. If that fits in the HT reasonably comfortably by hand it's about the right size and you're done. This should take about the length of time it takes for the drill battery to run out.

This actually seems to work pretty well.