That is a good question Mark.
The primary application for teflon tape in curing bottom bracket creaks is on cartridge style BBs. You install the tape between the cartridge and the cups. The outside diameter of the cartridge is a slip fit with the inside diameter of the cups. Neither surface is tapered. Because it is a slip fit there necessarily has to be a small amount of room between the components. The cups are tightened against the ends of the cartridge and they hold it from shifting axially but that isn't doing much to keep it from moving (fretting) within the small amount of space left between the I.D. and the O.D. The manufacturers often put some rubbery stuff on the I.D. of the cup to buffer that fretting but it eventually wears out. You'll start hearing a creaking sound when you are out of the saddle and the loads on the BB are maximized. Teflon tape is a convenient replacement for the the rubbery stuff. The tape doesn't last forever either. If the creak comes back after a few years its time to refresh the tape.
Jamie Swan Centerport Cycles Inc. http://www.centerportcycles.com (mapped) http://www.limws.org http://www.liatca.org
On Jul 16, 2008, at 3:30 AM, Mark Fulton wrote:
> In the June 2 issue of VeloNews Tech Talk, Lennard suggests using
> plumber's tape on bottom bracket bracket threads. I've used
> plumber's tape on the hydraulic fittings when I was installing
> braided brake lines on my racing motorcycles, and once when I was
> installing a new gas range. But both of those applications utilize
> pipe fittings. They're tapered so initially there's a larger space
> between the threads of the two elements. That space gets smaller as
> the fittings are tightened. In these two examples, the plumber's
> tape is effective in preventing the escape of a fluid and a gas.
> Bottom bracket threads are parallel, the space between male and
> female threads is literally microscopic. I can't see how you could
> even start the the threading with plumbers tape wrapped around the
> cup threads. Tell me I'm wrong and I'll try plumber's tape on my
> next bottom bracket installation. That'll be this weekend.
>
> Mark Fulton
> Redwood City
> California
> USA