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tech talk: facing a fork crown

Posted Nov 18 2008 12:21am
This evening, Lynne brought her 1971 Gitane road bike to the shop so I could help her resolve an issue with her fork. After some initial examination it was determined that the fork was probably fine, but that there was something wrong with the crown bearing race (the part against which the headset's lower bearings roll). I pulled the fork from the bicycle, removed the crown race from the fork crown, and there it was. The crown of the fork itself hadn't been faced, or finished, before installation of the crown race, causing the race to sit crookedly on the fork crown and in turn causing poor steering response in the bike's front-end handling:

After closer examination revealed that the chroming was probably not very thick and that a small amount of facing, or cutting down the surface, wouldn't harm the tool, I proceeded to set the fork up for facing.

Cutting was a careful, methodical process, with clamping the fork's steer tube into a repair stand, and then holding the fork blades against my torso while turning the facing tool and removing the tiniest shavings of chrome and ultimately metal -- to flatten out the surface so the crown race would rest more level.

Adding cutting fluid:

Turning the facing tool:

When I was satisfied that I had improved the crown enough to make a difference (but not so much that I was taking away huge quantities of metal or risking harm to the tool -- cutting tools don't like chrome), I stopped, removed the fork and carefully cleaned everything off, reinstalled the crown race and reinstalled the fork into the bike frame.

Reinstalling loose bearings with fresh grease into the headset cup:

Test-spin: voila! Much better.

The bummer caveat: I reassembled everything and was about to take the bike for a tiny test-spin around the shop, when I remembered I hadn't tightened the stem bolt at all. I applied some pressure to tighten the stem bolt and into the second turn of the allen wrench, I heard a sickening snap! Everyone else in the mechanic's area heard it too, and we all knew what it meant. I had to explain to Lynne that her stem bolt was probably broken. I removed the stem, pulled out the broken end, and removed the head of the bolt from the stem body. The bolt was broken at the section where the allen head had been spot-welded to the bolt shaft -- a poor design choice and sometimes found on cheaper bikes of that era. Beneath that section the bolt shaft was reddish-brown with rust; the stem was well on its way to breaking anyway because of age and moisture. Still, it would've been nice for Lynne to be able to ride her bike tomorrow. She assured me she was not angry with me, and she understood what had happened. I tried to find a French-threaded replacement bolt that would fit her stem (the alternative would've been to dismantle half her handlebar setup so I could install another stem, but we didn't have one exactly the same length in the shop and there were concerns about aesthetics as well). Lynne said she had other bikes to ride in the meantime; she'd utilize her various online bicycle resources to find a replacement bolt so she could keep the original stem. I felt a little bit better. I know it wasn't my fault but still it was disappointing. I gave Lynne one of our old French stems to take with her in case she decided to swap stems later.

That's the challenge with old French bikes -- the threading is its own thing, not compatible with British or Japanese threading and therefore not interchangeable with newer bike parts. French bikes with their own oddball threading size went the way of the dodo in the 1980's, and newer bikes made by French companies adopted the industry standard (often called "British"). But those old French bikes are super-classy and can be satisfying to restore. I hope Lynne will be able to find a replacement stem bolt quickly.

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