Although restoring a rusty and abandoned bike is a worthy weekend project, a stuck or stubborn bottom bracket can derail your whole show. Throw enough WD-40, leverage, and a DIY tool at the problem, however, and you can get your bicycle project back on track.
The bottom bracket is at the heart of any bicycle's drivetrain. It keeps the cranks spinning smoothly, turning circular pedalling motion into forward motion of the chain. As with other rolling surfaces on a bike – such as the wheel hubs and the headset – it commonly involves ball bearings jammed between two concave machined surfaces (a fixed "cup" and an adjustable "cone") that are swimming in grease. The precise adjustment of such mechanisms is maintained by a lock-ring. Given enough time and contamination, however, rolling friction between cups and bearings will loosen the cups, bringing the mechanism out of adjustment. Wobble will develop; nerds call this "mechanical play."
All older bikes, as well as modern department store bikes (and a few high-end holdouts), use the "cup-and-cone" type of bottom bracket I described above. The crank arms fasten to a steel axle which runs through the bottom bracket shell; a pair of cups then thread into each side of the shell. The right-side cup screws in counter-clockwise and is tightened down with great force; the adjustable left cup screws in clock-wise and is locked into adjustment with a lock-ring. It can be a pain to adjust. The only nice thing I can say about this type of mechanism is that it can be serviced many times before the rolling surfaces wear out.
Sealed cartridge bottom brackets are the modern standard. They are readily available in a variety of sizes—and they are cheap. You install them once by simply tightening down the cups on each side and then you can simply run them into the ground. No maintenance is possible. (Pro-tip: if you have a quick-release seat and are in the habit of removing it to spurn thieves, be mindful of the fact that precipitation can pour down your seat tube, corroding the bottom bracket prematurely.)
Now that you're hopefully sold on sealed bottom brackets, here's how to upgrade your old bike to one.
First, remove the cranks. Then, remove the seat and seat post, and spray WD-40 down the frame's seat tube. The solvent will flow into the bottom bracket, helping to loosen any rust that would make the cups stick to the bottom bracket shell. The left-side cup should come off easily enough. It's the fixed right-side cup–which unscrews clock-wise and may require a lot of force to get moving.
The wrench normally used to remove the right-side cup, has a circular hole with two flats on each side, and can easily slip off the cup and get gouged if not aligned properly. That's why I use a length of threaded rod to clamp the wrench over the cup and against the bottom bracket shell, using a nut and a pair of washers on each side (see top image). This will prevent the cup wrench from slipping sideways when you apply some force. You may need to gently tap the wrench with a mallet to get the cup moving, but once it's in motion you are home-free. (Alternately, if you lack the cup removal wrench, you can try clamping the two flats on the cup between the jaws of a vice, and rotate the frame clock-wise around this point. The narrow profile of the cup makes it easy to slip out of the vice, possibly damaging both cup and vice.)
You can now install the sealed cartridge unit—which will probably outlast the rest of your bike.
Bonus tip: That same above "tool," sans the cup wrench, can be used to press in the cups of a bicycle headset.
[Images by author]