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.
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.)