Assess bottom end and internal engine damage

The bottom end holds the crankshaft, con rod bearings, main bearings, oil pump and (on most motorcycles) the gearbox, all in one crankcase. Faults here show as deep knocks, rumbles, metal in the oil, an oil warning light or a locked engine. Fixing them means splitting the crankcase with pullers, holders and a manual for the exact engine. This page tells you what you can check safely, what the workshop does, and what not to try.

Safety

• A knocking bottom end can break the con rod and lock the rear wheel. Stop riding as soon as you hear a new deep knock.
• Do not keep starting a seized engine with the starter or by bump starting.

What you can check safely

1. Check the oil level and look at the oil with Inspect drained oil for clues. Low oil is the most common cause of bottom-end damage.
2. With the engine cold and the plug out, put the bike in top gear and rock the rear wheel gently back and forth. A clunk you feel at the crankshaft can be con rod play. On a scooter, turn the engine by the kickstart or the variator nut.
3. Listen with a stethoscope or a long screwdriver on the crankcase near the crankshaft, then the cylinder, then the clutch cover, then the generator cover. Note where the noise is loudest and whether it changes with the clutch pulled in.
4. Check outside causes first: loose engine mounts (Check engine mounting bolts), a loose exhaust or heat shield, a dry chain, a worn clutch.
5. Do Test engine compression to know the state of the top end.

What the workshop does

• Removes the engine and drains the oil.
• Removes the top end, generator (flywheel puller), clutch (clutch holder), oil pump and gear shift parts in the order the manual gives.
• Splits the crankcase and checks: con rod big-end side and radial play, crankshaft runout, main bearings, oil pump rotor clearances, oil passages, balancer and gearbox.
• Replaces the crankshaft as an assembly if the bearings are noisy or loose (the K-Light manual treats crank bearings as part of the assembly).
• Cleans every passage and the centrifugal filter if fitted; metal from a failure spreads everywhere.
• Replaces all gaskets and seals; the K-Light manual says crankcase seals and bearings removed with pullers are not reused.

What to ask for

• A cost for repair compared with a replacement engine. For small commuters a good used or new engine can be cheaper.
• The failed parts, to see the cause (no oil, dirty oil, overheating, overrevving).

Do not attempt at home

• Splitting crankcases without the correct tools and manual.
• Mixing parts from different engines without checking they match.

Check your work

• After the repair: oil pressure light (if fitted) out, no knock, clean oil at the first early change.

If it did not fix the problem

• Noise remains after the rebuild: check mounts, primary drive and clutch with the workshop.

Reference values (K-Light 125)

Workshop limits, measured with the engine apart.