I'm putting together an old Peugeot. It had a cottered crank on a cup and cone bottom bracket, French thread of course. I went to the co-op and found a square taper spindle that fits perfectly - it is even marked "Peugeot" - so everything is back together with lots of Mobil 1 synthetic grease, spins nicely, the new BMX crank fits just fine.

Hmm.

Grease is just able to push out around the spindle where it exits the cups. That must mean that water and road grit can get in there. There is about 7 mm of exposed spindle between the cup and the crank. Maybe it will be 5-6 mm when I have the cranks tightened down for real. I wonder if I can fit a rubber ring in that gap to make things just a bit more water resistant.

Anyone do this? I've never seen it. Why not? Am I missing a "gotcha"?

(When assembling the headset, I put an o-ring in the groove of the locknut. I think it will help keep water out of the bearings.)

The rubber ring may just as easily hold water in as keeping it out. Perhaps there is a reason you haven't seen it done.

i've owned a couple of cup and ball BBs that had o-rings around the hole in the cups. can't say for certain that it was OEM, but may have been.

There are various BB's, such as thick wall Campy that actually have threads in the spindle holes to direct the grease out of the BB. Your BB is working as designed with no rubber seal. A little grease around the spindle is supposed to stop dirt and water from getting in.

Stuffed an O ring over the Campag 111 cartridge BB,
before the crank arm went on ..
squeaks a bit when not wet, but OK.
the old campag cups were thick to put that CCW rifling in.

traded away a 110 SKF Sachs BB that had lip seals in the cup, sealed bearing
cartridge behind that..

Designers wanted the axles shorter for stiffness,
rather than long enough to double seal the BB
so that Idea went nowhere.

Does the brand of BB ever matter? Jw

I make o-rings out of squishy foam like you would find in cushions. Sometime I come across a sheet of it from packing material that's just the right thickness. Cut out a circle, then cut a little hole in the middle and slide it over the axle before putting on the crankarms. They do a great job of keeping dirt out of the BB.

Years ago the various maintenance books used to recommend wraping pipe cleaners (remember those?) around the outside of hub and bottom bracket axles and spindles to keep out dirt. O-rings can do the same thing. Using LOTS of grease will let it leak out for a few rides but also forms an effective seal.

I have several BB that contain rubber seals and several that do not. To be honest I never found one or the other needing less maintenance.