Musings on headset reaming tolerances

The Park tool headset reamer for standard 44mm headset is 43.95mm. This means, assuming the headset cup is exactly 44.0mm, that there is an interference fit of 0.05mm (0.002") between the bore of the head tube and the headset.

In any normal machining context, an interference fit of 0.05mm on this diameter would be considered an extreme or even impossible amount of interference. The rule of thumb that I was taught in my machining career for interference fits, was "0.0005 inch interference per inch of diameter" for a strong interference fit. That means the Park reamer is giving ~5 times more interference than I would have expected.

Clearly, the reason for this large amount of interference is that the bicycle tubing is elastic. So the rules must be different for tubing than for normal bores in castings or forgings. So, we can get away with more interference, or maybe we even need the extra interference because of the elastic tubing.

Headset tubing is also not round, either after machining or after brazing. Even after reaming, I find head tubes are often 0.05mm or more oval, because they spring back a bit after reaming. But this isn't my major concern. The tubing will stretch back to round when the headset cup is pressed in. It does mean that getting a good diameter measurement is hard and may require taking at least 2 or preferably more measurements and averaging them.

I make my own head tubes usually, and I have the freedom to bore them however I want. Yes, they don't end up perfectly round after brazing, but as I mentioned above, they aren't perfectly round after reaming either because of springback. So I'm considering stopping the reaming altogether and just boring them to final diameter when I make them in my lathe. So should I target 43.95mm, like the Park reamers? Or something else?

Is 43.95 really the best for all frames, and has anyone used different tolerances on purpose? I've seen pretty big differences in head tubes...such as thin steel road-bike head tubes with the barest hint of a reinforcement ring. Or titanium head tubes, which is twice as springy as steel. Or gnarly thick Aluminum fat bike head tubes, or thick steel BMX head tubes. It seems unlikely that they all work well with the same amount of interference. Does anyone deliberately deviate within the 43.95-44.0 range?

I was taught that for bicycle headsets a 0.1mm of diameter is the undercutting/interference amount one wants. This is why for a 26.4mm crown race seat the Campy tool is speced to cut to a 26.5 diameter.

One thing to remember is that making bicycles is not rocket science and does have a vast range of tolerance in the alignments and fitments. (And this is one of the reasons why bicycles have continued to use angular and adjustable bearings, to better handle these imperfect conditions. I have used the very same cutters on dozens of bike over the years and have seen results in diameters on either side of the goal depending on their manor of use and condition.

Two added stories- When I got my surface plate and some dial indicators I found I could measure to the thousandth of an inch. On repositioning, a second time, the indicator to the same location I get indicator readings a few thou different. What I learned was that just because I could measure to a thou doesn't mean that it means anything in real bicycling life. The other story is when I attended my first frame building class. The instructor got tired of most of the students continually asking if this or that (we were at the mitering stage then) was "right". Albert called us around and said that none of us had perfect bodies. A curve to one's spine, leg/arm length differences, flexibility issues and dominate sides of the body with the imbalance of the muscle strengths as obvious examples. "Show me a perfect body and I will make a perfect frame" was his comment. After 44 years I still believe this to be true. Andy

A thought came to my mind yesterday.

If head tubes use undersize tolerances because of the greater amount of elastic stretch they have compared to normal bearing bores, then it stands to reason they also stretch more when reaming.

In other words, if you have a thick BMX head tube or a thick, soft aluminum head tube, the reamer will probably cut a hole close to the reamer size, because stretch will be minimal. So you will get a bigger hole, but you need it because the headset doesn't stretch as much.

On the other hand, if you have a thin steel headtube, it probably stretches when you ream it, resulting in a hole somewhat smaller than the reamer. But it works out because the same stretch that accommodated the reamer will accommodate the head tube cup when it's pressed in.

I'm not any closer to my goal of understanding what size to turn my head tubes so the cups press in without later reaming. I think I will shoot for 0.002" undersize and if the cups press in too loosely on the first one, hedge my bets with some loctite. The only way to get a general answer would be to machine many head tubes and characterize the dimensions and the pressing force and tabulate the results into recommendations, similar to many such resources you can find in the Machinery Handbook. I suspect this information exists in the world but is distributed among frame makers and bike factories.

FSA specifies an internal diameter tolerance of +0/-0.05mm and this is generally used in the mass production of bikes for press in headsets. However SOME factories don't want to comply with such a narrow tolerance window and demand that +0/-0.10mm be used instead. If you want to see the tolerances of FSA headsets and their recommended frame tolerances you can search 'fsa headset catalog pdf' on google. If you really want to get into the subject search 'engineering fits and tolerances' where you will find the three general types of fits (clearance, transition and interference) and the standards that engineers use to design for them.