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Internal bottom bracket: refitting

Check the new bottom bracket. The fitting threads are either metal on both sides, or metal one side and plastic the other. Grease the threads that will take a metal thread, but do not grease those that take plastic. All titanium threads need a generous coat of antiseize.

Generally, the cups are marked ’L’ and ’R.’ Usually, the body of the bottom bracket is the right-hand side, with a loose cup that attaches from the left. This is not universal though. If it’s the other way around, reverse the fitting order. The bottom bracket threads are very fine, so it’s important to take care when fitting that you don’t ’cross-thread’ (thread in crookedly) the cups. Start each side by hand so that you can’t force the threads to start unless they fit properly. Riding with a loose bottom bracket will damage the threads. New frames, and those that have been resprayed, may have paint stuck in the threads that prevents fitting the new bottom bracket. If the threads are damaged, get the frame to your bike shop. Unless they are really badly damaged, they can be recut with a tap (a big tool with the same size and shape thread as the bottom bracket, but with hard, sharp cutting blades instead of a plain thread).

Internal bottom bracket refitting - Step 1

Step 1: Roll the left-hand cup a couple of turns into the thread on the left-hand side. Start fitting the right-hand side – the main body of the bottom bracket – into the right-hand side of the frame, tightening anticlockwise by hand. Once you’ve got it in a couple of turns, look from the left-hand end of the bottom bracket, to check that the axle is coming out exactly in the middle of the hole in the cup you just fitted.

Internal bottom bracket refitting - Step 2

Step 2: Tighten anticlockwise. It needs to be fitted really firmly – you need about 300mm (12 inches) of leverage and a good grunt home. Once the body of the bottom bracket is fitted firmly, tighten up the left-hand cup. Take care with plastic cups – they need to be fitted fairly firmly, but won’t take as much force as the main body of the bottom bracket. Overdo it and you damage the plastic splines and make it difficult to remove next time.

Internal bottom bracket refitting - Step 3

Step 3: Refit the cranks as on pages 250–51. If you fit a new bottom bracket, watch as you fit the chainset side. Even if you’ve measured carefully, it’s worth checking that the chainset doesn’t jam on the chainstays as you tighten the crank bolt. You need at least 2mm (1/8 inch) of clearance between the chainset and the chainstay – if there’s not enough, remove the bottom bracket and fit a longer one.

Stubborn bottom brackets

Sometimes bottom brackets get wedged in hard. Usually, it means they weren’t fitted with enough grease or antiseize in the first place, or that they’ve simply been in there too long. Living by the sea in a salty atmosphere, doesn’t help either. Try the following measures:

A good dose of release agent helps in three or four applications over a couple of days – especially if, in the middle of proceedings, you put everything back together and go for a good hard ride. A light spray like WD40 will work as a basic penetrant, although you can get something tougher at your hardware or auto shop. You can also get bike-specific release agent from your bike shop – Shimano makes one called Get-A-Grip that is frighteningly effective. All release agents contain various nasty chemicals, so use sparingly in a well-ventilated space and don’t get any on your skin. Check once again that you are trying to turn the tool in the correct direction – it’s easy to get confused. Looking from the right-hand side of the bike, the right-hand cup is removed by turning it clockwise – a reverse thread. Looking from the left-hand side of the bike, the left-hand cup is removed by turning it anticlockwise – a normal thread. Once you’ve soaked the bottom bracket thoroughly, try using as long a lever as possible on the tool. Find a tube that fits over the end of your adjustable wrench, and use that to increase your leverage, bracing yourself carefully so you don’t slip and hurt yourself when the tool starts to move. Clamping the tool in place helps stop the tool slipping off and damaging you and the splines. You need to fiddle with the washers to clamp it successfully. Use the crank bolt and find a washer that stops the crank bolt from slipping into the hole in the centre of the tool without interfering with the wrench flats.

If you have a vice and assistance from a friend, take off the wheels. Drop the tool into the vice and clamp it so the bike is held horizontally over the workbench. Use the bike as a lever and turn it to undo the tool.

Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Internal bottom bracket : removal”