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I have a hardtail frame made from 7005 aluminium. It is a few yearts old but in lovely condition. recently, due to heavy rain, the bottom bracket well was found to have "some" water in it, and it was suggested that I could drill a neat machine drilled drain hole (about 1mm dia)into the underside of the BB. Basically, a drainhole.
It sounds like a good idea, BUT has anyone else done this and then had problems due to it?
I'm wondering if there is a reason why so many mass manufacturers of frames do not drill such holes under the BB?
Greenbikerider.
Have done it on my Dialled PA, no problems and bb lasts longer
You'll need to go bigger than 1mm, it will just block up, 3mm or 1/8" is more suitable.
I do it to all my frames if they dont already have one, I normally drill a 4mm hole and then de-burr the edges.
I'll state the obvious but its best to remove the BB first.
I've done it on a few frames. Think I used a 5mm bit cos that was all I had, and used a cordless hand drill. 5mm is still pretty small.
Its easy to do a neat job.
OK, so no obvious consequences to doing so?
Neil: I have other breathe holes drilled around my frame, to allow air pressure to 'breathe'. Are you sure a 1mm hole will block? I'm a bit nervous about drilling too large.
GBR.
Underside of a BB shell, you could drill a 20mm hole and it wouldn't significantly affect the strength of the frame 😉
(Breather holes are for use when building the frame, by the way, they're of little to no benefit after the frame is built)
I drill a 1/8" hole in all my metal framed bottom brackets. Even that can block a bit. 1mm is pointless.
OK point taken :o)
Will drill much larger.
My BB is an external anyway!
GBR.
Yep ,3mm hole in all my frames .
Bottom bracket bearings last much longer.
Yes, a 3mm hole sounds good to me too.
I still don't understand why many manufacturers do not bother to drill such an important hole under the BB?! It's such a good idea yet they don't do it.
GBR.
Perhaps they come from a place without much rain.
You would think, but I'm about to drill a Five
My BB is an external anyway!
Crank spindle and the plastic sleeve between the bearings aren't though.
You can do it without removing the above but you dont want any swarf from drilling to get into the bearings or onto the face of the crank where the bearings rotate.
5mm hole in most of my bikes. Never had a problem. Just remember to paint with car touch up or nail varnish. Otherwise the paint can blister around the hole after a few years.
Unovolo: Yes, getting all swarf is essential. But this raises my nagging doubt - can crap and grit get IN these bearings via a hole?
I suppose the solution to preventing that, is to fit a small rubber plug (which you can then occassionly remove to drain out the frame? A bit like an oil sump on a car?)
GBR.
It was a regular practice on quality steel bikes once upon a time.
Way back in the day there was a neat little one way valve that you could fit into the frame to let water out, but nothing in.
Oh - and the drain holes in most frames aren't for welding gas expansion, they're to ensure the frames drain when they have been dipped before painting.
BTW I usually plug the drainhole in the BB with a blob of light grease. It stops spray getting in, but if any water does come in it gets displaced and drains.