How this headset works
Andrew, if it weren't for people questioning my knowledge, I probably wouldn't know anything...
A seat post clamp won't work because the purpose of the part is different.
The top clamp on the Dia Compe Diatech (or ABN) headset isn't intended to secure the clamp to the steerer tube, and I think this is what confuses people in setting it up (even bike shops). Tightening it down hard thinking it is supposed to do that is what causes folks to break or strip the clamp bolt, I think.
The inside of the top clamp is beveled in its entirety at about a 60 degree angle. Underneath it is a conical ring also beveled at about 60 degrees.
As you tighten the clamp bolt, the top clamp becomes smaller in diameter. This causes the top clamp to ride up the conical ring, taking slack out of the headset bearings by making the clamp assembly taller, in essence.
This allows us not to have to use a 20-inch long aheadset cap bolt to set the headset bearing preload.
So when you install the headset, you should start out with the top clamp bolt as loose as possible without having it fall out. Then install the steering column and steering column clamp, and push them down onto the headset top clamp as far as you can. Then tighten the steering column clamp to secure the column to the head tube.
Only at this point should you start to tighten the top clamp bolt, while rotating the fork to guage the bearing tightness. When the slack is out of the headset, but the bearing still turns freely, this is the correct setting for the top clamp bolt.
This should be checked every time you un-pack and re-assemble your Quest.
It's pretty elegant and effective when you know how it works. It's pretty confusing when you've never seen one before.
I'm writing a blog post with illustrations to clarify all this.
Cheers,
Doug
