Hello,

Forgive me, I had never worked on a bike until a couple of weeks ago when I bought a new-used Mercier road bike. The fork was dented and I swapped it out at a DIY bike place. They had only one fork that had the 27" wheel and enough length to fit the front tube. The advice I was given was to take it! I was lucky to find anything that fit! What they did not tell me was that new forks come long and can be cut down to fit. Nor did they tell me that some forks take different sized stems...

Skip over several hours of clumsy, frustrating work and the fork is now installed with the headset from my old CCM road bike (no longer ridable due to an ovalized front tube on the frame). The stem from my old road bike does not fit (the stem from the Mercier had to be cut as it was rusted into the fork). This is because the fork, though having the same outward diameter as the CCM fork (from which I got the headset), requires a 21.1mm stem. Eventually I found a shop with 21.1 stems and purchased one.

Yesterday was my second day of really riding the bike. This morning I went to take it to the corner and nearly fell off when the stem turned a different way than the front wheel.

My question is: are there different headsets for 21.1mm stems? Did the stem loosen because the headset cap is sized for a 22.2? The headset fits on the fork, but as I said, the outside diameter of the fork is the same as your standard fork that takes a 22.2 stem... do I need to find a new headset? Just a cap? Or did my stem loosen just because of the terrible condition of my city's roads?

Thanks a lot.

if I am wrong others may correct me
but I think there was also a french standard
of 22.0 mm or something quill diameter
if you try to use a 21.1 stem in a 22.0 steerer
I bet it wont work too well

otherwise
steerer tubes are often internally butted
and if the stem is inserted too far
the butting interferes with the stems ability to hold itself in place
so make sure this is not the case
simply by raising the stem close to its min insertion mark
and seeing if it can be adequately tightened

What the dealer failed to notice or tell you, is that Merciers are French bikes, and yours therefore had a French 25mm fork. That means the thread, and stem diameter are different than the 24tpi fork he sold you. Now you'll need to replace the stem with a 22.2mm one. Unfortunately it also means that you'll likely have to change the bars to match the stem.

It's a case of the shin bone is connected to the knee bone.....

You could have avoided the hassle by sourcing a French thread fork at the time, but they're hard to find new these days. In any case you crossed that bridge long ago, and must now continue to move forward.

Thank you both for your responses.

It seems that my convoluted first post created some confusion:

The fork is no longer the original fork; whether the new fork is french or not, I do not know, but everything fits on it (including the headset pieces from my older 22.2 mm stem/24tpi fork). If at all possible I want to avoid changing the fork. The fork, though it accepts the 22.2 mm headset pieces, takes a 21.1 stem. This I know from using calipers and also from trial; I cannot replace the stem with a 22.2 as that would mean also replacing the fork! I specifically bought a 21.1 stem to fit this new-used fork (the original stem was cut with a hack saw; I'm not sure what it's diameter was).

The only thing is that the headset cap piece does not sit tight against the 21.1 stem as it was designed for a 24tpi fork and 22.2 stem. Nevertheless it screws on tightly to my mystery fork. I'm wondering how much of a role the headset cap plays in keeping the stem tight and in place. Is a makeshift fix possible (i.e. could I use hockey tape on the inside of the top cap to make up the one mm dif, so that it is flush against the stem)?

OK, this clears it up a bit. You have a thick-walled (American standard) 1" fork, and the correct stem for it. You don't have a problem at all, since the headset locknut has nothing to do with holding the stem. The stem holds by wedging into the fork. It's nice to have a closer fitting locknut to avoid the visible gap, but that's the only difference.

Perfect timing; I was just about to order a special headset: 21.1mm Threaded Headset CHROME
You saved me $20.

Thank you very much!