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Troubleshooting : bottom bracket

Internal bottom brackets

Troubleshooting bottom bracket - Photo 1

Symptom Cause Solution
One crank rocks from side to side, the other is firm Loose crank bolt Tighten crank bolt
Both cranks rock from side to side Bottom bracket unit loose in frame Remove both cranks, tighten bottom bracket in frame, replace both cranks
Bottom bracket worn out Replace bottom bracket
Cranks loosen repeatedly Crank bolt loose Tighten firmly – use a longer wrench for more leverage
Crank mating surface worn by being ridden loose Replace crank
New crank still loosens repeatedly Replace bottom bracket
Creaking noises from bottom bracket area Dry or loose interface between components Remove cranks, clean interface between cranks and axle, replace, retighten firmly
Bottom bracket loose in frame Remove both cranks, tighten bottom bracket in frame, replace both cranks
Bottom bracket works loose constantly Insufficiently tightened in frame Use a longer wrench on the bottom bracket tool for more leverage
Bottom bracket shell stretched by riding with loose bottom bracket For minor stretching, use Loctite threads to fill gaps. For major stretching, the only option is to replace frame
Front derailleur won’t shift on to largest chainring Bottom bracket too long Replace with shorter one
Chainrings rub on frame Bottom bracket too short Replace with longer one
Chainring clearance looks fine but chainrings rub under pressure More clearance needed to allow for chainring flex Fit longer bottom bracket
Chainring bolts loose or missing, allowing chainring to flex excessively Tighten or replace chainring bolts

External bottom brackets

Left-hand crank works loose Loose pinch bolts Torque to 12–14Nm
Creaking from bottom bracket area Bottom frame cups loose in frame Remove cranks and tighten cups firmly
Chainring bolts loose Remove grease threads and under bolt heads, replace firmly
Axle shifts in cups when cranks rocked from side to side Bottom bracket worn out Replace bottom bracket

Troubleshooting bottom bracket - Photo 2

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