Bike Repair – Fit Shimano external BB and cranks

Cyclist fitting Shimano external BB and cranks

Threaded external bottom brackets are surprisingly easy to remove and refit. Here’s how to service them along with their matching Shimano cranks.

At first glance fitting Shimano cranks and their matching bottom bracket might seem like a job for the professional bike shop mechanic, but it’s actually one you can do yourself – it’s both pretty straightforward and doesn’t take that long. You don’t need that many tools, but do make sure your that new cranks have the right width bottom bracket for your bike’s bottom bracket shell.

Out with the old

Remove the old cranks and bottom bracket from the frame, then give the bottom bracket shell a spray with degreaser and wipe it out with a rag. You should be left with bright, shiny threads with no muck, grit or water in them. If the threads are still contaminated, scrub with a toothbrush and yet more degreaser.

Grease the BB

With the threads clean and dry, apply a dose of grease. It’s easiest to work the grease in with a small brush – you use less grease and it’s more effective at getting into the right places. Our favorite grease is ProGold, which is £6ish for 2oz.

Measure BB shell width

Measure the width of the bottom bracket shell – it’ll be either 68 or 73mm wide. The width will dictate whether you need to use any of the spacers supplied with the bottom bracket or cranks. Generally, 68mm shells need one spacer per side and 73mm shells don’t need any. If you’re in any doubt, re-measure the bottom bracket shell and consult the fitting instructions that came with the bottom bracket; these should tell you whether you need to use spacers.

Fit external bearings

Now fit the cups into the correct sides of the shell. Most cups have clear L and R markings to indicate which side of the frame they should be fitted into; the letters correspond to the sides of the bike as you would see them when sitting on the saddle. Use a good quality bottom bracket tool to tighten the cups – we prefer Park Tools’ and Pedros’ models.

Slot axle through

With the cups installed, place the bottom bracket axle (attached to the right-hand crank, complete with chainrings) into the right-hand bottom bracket cup. To get the crank in properly you might need to give it a light tap with the flat of your hand or a soft wood or rubber mallet. It’s an exact fit, so be patient. When the crank is installed into the cups you should see the left-hand axle stub protruding from the left-hand cup.

Preload cranks

The axle stub is splined to fit the inside of the left-hand crank. Slide the crank on to the axle – it should go on with ease but might require some help with a mallet. Then preload the crank by tightening the small plastic starhead screw on the arm until snug.

Before tightening

Finally, do up the twin pinch bolts on the left-hand crank arm, giving one a half-turn with a 5mm Allen key and then the other, repeating until the arm is firmly secured. That’s it, you’re all ready to ride.

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