One could drill a hole and then put a bolt into it. Water, particularly over time, should be able to sift through the threads, but the hole would not really add an entry point. Also, there is this
Dry Valve by Action Tec.
Nope, the screw in the threading will block the water from flowing. Surface tension and all.
Originally Posted by
SJX426
The hole needs to be large enough that the water doesn't come out due to water tension.
I think you intended this to be "the hole needs to be large enough that the water doesn't get stuck due to surface tension" If not then it should have.
If an opening is too small it can hold in quite a bit of water before gravity overcomes surface tension. For this reason I'm not sure that the Dry Valve is a great idea either.
There's another good reason to just drill the customary 5mm'ish hole. Not only does it allow a free flow of water from inside but it also forms a large enough opening to encourage a flow of air and help dry away any residual moisture trapped inside.
Yes a hole of this sort will allow the odd splash to get in. But we've already seen that the collection of water from other entry points is pooling far more water in the BB shell than a drain hole would ever let in.
So by all means drill a 5mm or 3/16 hole in the shell at the lowest possible point. If you lean the bike to one side more frequently than the other drill the hole closer to that side. Ideally right where the inside edge of the cup is sitting. Be sure to chamfer away any burrs inside so there's no little dams left to hold in any more water than you can get away with.
Finally apply a smear of boat trailer wheel bearing grease to the entire inside of the shell. It's more water resistant than any others I can think of so it SHOULD help avoid any further rust formation. Use more of the same stuff for the BB threads for the same reason.
Something that would be interesting is if frame intended for serious amounts of riding in rain were to have a drain nipple on the shell to which a short 2 inch section of plastic tubing could be attached. The nipple and extension would easily allow water from inside to drain and also encourage airflow through the tubes for drying but at the same time the tubing would shield water from getting in from anything other than the worst possible sort of dunking.