Sheldon has an excellent summary of French dimemsions on his web site. The actual headtube diameter is the same for French, English and Italian, so it is only the threaded cup and the locknut, and sometimes the keyed washer, that vary. As Sheldon states, the French steerer thread that the threaded HS parts must match is 25.0 mm, compared to 1" (25.4mm) for English, so a French locknut is too small to thread onto an English steerer, whereas as an English nut would be very loose (or might even slide over) a French steerer. Ditto for the threaded cups. This isn't a direct consideration for the HS, but French steerers also have a smaller ID, 22.0 versus 22.2, so you need to use a French stem or sand down an English one. As for the keyed washer, English steerers somtimes had a keyway slotted into the back side of the steerer, whereas French steerers typicall has a flat filed into th back. Don't think I've seen a slotted French steerer, but it's pretty common to see flatted English steerers, often on bikes built in UK. Newer English steerers are often neither slotted not flatted. The point is to use a washer with either a key or a flat, depending on the steerer treatment, although a key can often be filed down to work with a flatted steerer.
Regards,
Jerry Moos Houston, TX
Ron Gurth <rononice@yahoo.com> wrote: Slightly OT: I have aquired a late 80's (?) Mercier 300 frame that needs a headset. I understand the size of the headset may or may not be French. The head tube is slightly 'ovaled' at the top, but not creased, so it appears it can be 'rounded', but I cannot get an accurate measurement of the diameter. Any idea what size would be correct? This frame looks to have been built near the end of Mercier's existence. Sticker says it's Columbus tubing. If anyone has an appropriate (ie: Stronglight?) headset (cups and bearings) for this beast, please let me know. Thanks.
Ron Gurth Carmel, IN
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