Headsets, stems and the frame all have to fit together accurately to work well. Unfortunately, standards have changed frequently over the past 30 years, as the threadless headset was introduced, and France got with the program and adopted international standards. Another unfortunate part of this is that many french built bikes were imported into the US, and so it is very possible to come into possession of a bike using french standards. (Doubly bad because some of the french bikes are in really good condition, and are pretty nice frames, for being 30 years old and french.)
Recently, Kristina bought a tandem that happens to be french. It was in overall good condition, with a few issues that some wrench time quickly cleared up. One of the really annoying issues was that there was a ton of slop in the headset. I assumed that the problem was simply that the headset needed to be repacked and tightened. Kristina pulled the headset and repacked it, and I made sure that everything was clean (or rather really greasy) and tight, but the play remained. We rode it around for the next few days, until I noticed that the slop had gotten worse on the ride into campus. I decided that the problem was in the seating of the cups/race, the balls were too small, or that we had simply messed it up the day before. I took it into the coop, pulled the headset again, knocked out the cups, found the next size up of balls, faced the head tube, reseated the cups, and reinstalled the headset and fork. The headset was still super sloppy, but better than it was before. That's when I poked around the crown race and found that it was moving just slightly, but more than enough to cause the slop. Blegh.
Pulled the headset for the third time, and pried the crown race off with a tire iron, which should be fairly impossible, as the race should be pressed (with the setting tool, and a large hammer) onto the crown of the fork. Lo, there was a massive crack all around the crown. It seems like someone rebuilt the crown with some JB weld or had welded to the crown and machined it back down. At some point, the repair had failed, and the crown deformed and cracked, making it all broken and gross. Either way, there was no way to set a race on that mess.
The coop keeps a drawer of random headset bits, and bunch of 1 1/8" threadless headsets in stock, so I was banking on being able to find a used fork and headset that would be able to fit the tandem. And then I measured the head tube and headset bits. According to the internet and Sheldon Brown, I was looking at the rare french tandem headset. This is the black sheep of french headset standards, as it is not directly compatible with any other standards. (Note: The obsolete french 1" standard is interchangeable with other 1" headsets and forks as the head tube is the same size for all.)
After some fiddling and measuring with calipers I consulted the magic chart of headset sizes, and found that the french tandem standard is very close to 1 1/8" threadless, within a few tenths of a millimeter. The cups fit into the head tube with finger force alone and don't wobble, but they need to be shimmed up for a proper fit. (There are some specific tolerances for press fitting headset cups, and how close these need to be in size. Park Tools is a good reference, but for 1 1/8" needs 34.0mm cups, and 33.75-33.9mm head tube. According to the internet, beer cans have a sideall thickness of .09 ) I used a convenient beer can for this, using a single thickness with about a 2 cm overlap, which seemed to be about the right size of shim. I made them twice as tall as the cup, so that there would be some extra to hold it in place in the head tube. The order of operations for getting the shims in properly turned out to be super important. For your elucidation, I have included them below.
First, clean up the shims, and grease the head tube a bit. Hopefully this will prevent interactions between the aluminum and the steel, as well as making it easier to get the headset out later.
Stick the shim a little over halfway in. Slide the headset into the shim, making sure the edges of the shim are flush with the flange of the cup. The cup will be pushing the shim in, so getting everything square and aligned will make everything much easier. Press the cup in a smidgen, so that it locks in against the shim and the head tube, or else it will fall out at the worst possible moment. (Right before you get the press screwed all the way on.)
Press the cup in using the headset press.Slow and easy does it, and odds are the shim will squish out of alignment a bit. If it gets really ugly, it's best just to stop and make a new shim, as any can protruding defeats the purpose of facing the head tube.
Now do it again on the other side. Feel free to cry a little, and bandage any cuts from sharp can edges.
I was doing this at 3am after being in the shop for a good long time, so I might have been a little tired when I was working on this. But it was pretty frustrating. I think I ended up messing it up in some way twice on each cup, and having to pound them out and start again. On the upside, the old french tandem is now sporting a nifty disk brake and 700c by 1 1/8" fork. And the play is out of the headset, which makes the tandem much easier to control now. Of course, if I ever have to replace those cups, I'll be very very sad.