External bottom brackets have replaced internal (square tapers and splined) versions on higher end models. They are marketed as stiffer, lighter and easier to work on, and fit into standard frame configurations so can be fitted as an upgrade to bikes that currently have square taper or splined bottom brackets. You can’t mix and match between internal and external elements – it’s one or the other, because with external bottom brackets the axle is integrated onto the chainset as a single piece. Fitting and removing the assembly is relatively simple. You’ll need the specially shaped tool for the preload cap and bottom bracket cup, as well as a 5mm Allen key. The pinch bolts need care when refitting – they’re bolted into the soft aluminium of the cranks and will strip easily if overtightened. However, if they’re left too loose the crank will work loose and fall off. It’s tricky to judge exactly how tight is just right, which is where an investment in a torque wrench pays off. These can seem like overkill, but the payoff for our hunger for lighter bike parts is that modern components have less margin for error. They come in a variety of sizes – choose one with a range from around 2-20Nm. Most of the small nuts and bolts on your transmission, suspension and braking systems will need a torque in this range. The standard Shimano versions of external bottom brackets are not serviceable – once the bearings are worn out, the cups have to be replaced. However, manufacturers like Race Face market serviceable versions, and the tools needed to get them apart and back together. Worth considering if your mileage is high or you ride in especially harsh environments. These instructions cover Shimano versions from XT downwards – the XTR setup is different, with a more complex preload adjusting arrangement.
Step 1: Undo pinch bolts on left- hand crank using a 5mm Allen key. If the bolt heads are dirty use a small brush or screwdriver to pick out any detritus so you can insert the Allen key snugly into the bolt head. They’ll need to undo a few turns each.
Step 2: Undo and remove the preload cap. You’ll need a Shimano Hollowtech II tool for this. It should come off easily. Tuck it somewhere safe so that it doesn’t get lost.
Step 3: Use a small screwdriver to gently lift the small black plastic safety tab that sits in the slot in the crank. It’s the side furthest from the bike that should come away. The other end is held captive by the innermost bolt – leave it in place. The tab is there to stop your axle falling off if the fixing bolts fail.
Step 4: The crank arm should come away with a gentle wiggle, exposing the splined end of the axle. Use a plastic hammer to tap this gently through the frame. A wooden block is an acceptable alternative, a hammer is not. Deforming the splines would be an expensive error. Support the chainset whilst you tap, so it doesn’t pop right out. Once the drive side comes away from the frame, pull it gently free by hand.
Step 5: Remove the left-hand (non-drive) cup first, anticlockwise with the correct Hollowtech II tool. Steady the tool against the cups, as shown, so that it doesn’t slip off and damage the cup (or you). You’ll need to use the full length of the tool to get sufficient leverage. The cup will be fitted firmly in the frame – if it comes away easily, it wasn’t tight enough. Set aside any spacers.
Step 6: Repeat with the right-hand (drive-side) cup, but note that this side of the frame has a reverse thread, so releases clockwise. Note the positions of any spacers and set them aside you’ll need to replace them in the same places when you come to fit the new bottom bracket.
“It’s tricky to judge exactly how tight is just right… an investment in a torque wrench pays off”
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Internal bottom bracket: refitting”