What is a Crankcase & Sump?
The engine's outer casing and oil reservoir, holding the crankshaft, gearbox and the oil that lubricates it all.
Simplified animation — not to scale.
In plain English
The crankcases are the split alloy castings that house the crankshaft, gearbox shafts and, on most bikes, the clutch — with the sump or wet-sump reservoir underneath holding the engine oil.
“Think of it as the engine's skeleton and its lunchbox combined — structure on the outside, oil supply underneath.”
How it works
Oil collects in the sump and is drawn up by the oil pump to lubricate the crankshaft, gearbox and camshaft before draining back down by gravity. The casings themselves also form the mounting points that hold the engine rigidly in the frame.
Signs of trouble
- ⚠Oil leaks or weeping from casing joints
- ⚠Oil level dropping with no visible external leak
- ⚠Grounding damage from a low-speed drop or kerb strike
- ⚠Unusual clutch or gearbox noise from within the cases
£80–£600
Always get a written quote. Prices vary by bike, region, and parts brand.