Heavy corrosion of the bottom bracket shell, but rest of the frame is fine

Question

I have a Btwin MyBike cycle, which is now 8 years old. The frame is an steel alloy.

I want to replace the bottom bracket, chrank shafts & chain ring.

The BB was very difficult to remove, used a lot of degreasers/de-rust solutions, nothing worked, and runied a couple of BB removal tools in process, it didn't even budge.

Eventually, had to hit the BB cover rings with a hammer & chisel to get em off.

The threads on the frame look alright

When I removed the old BB, i found that all the tubes (down tube, chain stay, seat post stays,etc) drain into the BB section.

I believe that its cos of this design that the BB section is completely rusted, while the other parts of the internal frame are rust-free(almost)

I thought I should drill a hole at the bottom of the bottom bracket shell to let the water drain out.

Not sure what to do or how to remedy this issue.

Answer 1

Many bike frames have a vent/drain hole in the bottom of the BB. Drilling one is a good idea. There are also normally small vent holes on the inside of the chain and seat stays too, more for welding purposes than drainage.

Downside is that a hole is a two-way passage and road salts/dirt can enter via this vent too. The rear wheel throws gank everywhere.

You can also minimise water intrusion by looking where the water enters the frame. The seatpost clamp area is a prime suspect, and I sometimes put a "skirt" of heatshrink tubing over the whole joint to help shed water better.

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As for saving the BB, products like Rust Converter exists which helps stabilise red rust. But the threads may be slightly filled with black iron oxide. It pays to clean them out before treating rust.

The correct tool is a pair of taps to chase the thread, but they're ridiculously expensive. Instead, use a sharp right-angle pick and drive the point around every single thread valley and remove any debris.

You'll be fitting a new cartridge bottom bracket, so do grease the threads at install time. Be super-careful to not cross-thread, and remember left and right are opposite threads.