The drain plug (or on the K-Light, the large strainer cap at the bottom of the engine) seals with a crush washer or an O-ring. Leaks usually follow an oil change: the old washer was reused, the plug was not tightened, it was overtightened and the thread is damaged, or the wrong plug was removed and refitted. Oil running down from higher up can also collect on the plug and look like a plug leak. MAVCC notes that tightening harder does not cure a drain leak; find the cause.
Is it safe to ride?
No, not until you know the plug is secure. A loose plug can fall out and empty the engine in seconds.
Quick checks first
- Is the plug finger-loose?
- Is the oil level still above the lower mark?
- Was the oil changed recently? Was a new washer fitted?
- Is the area above the plug dry?
Diagnose it step by step
1Wipe the plug and the engine above it dry. Run the engine for a minute. Does fresh oil appear first at the plug, or does it run down from higher up?
- From higher up: go to Trace an oil leak and Oil leaking under the bike.
- At the plug: go to step 2.
2Engine cold. Does the plug turn by hand or with light force on a spanner?
- Yes, loose: go to step 3.
- No, it is tight: go to step 4.
3Was a new washer or O-ring fitted at the last change?
- No or unsure: drain, fit a new one and tighten to the manual torque with Replace the drain plug sealing washer. Check level with Check the engine oil level.
- Yes: tighten to the manual value and recheck. If it loosens again, go to step 4.
4Remove the plug (drain the oil first). Are the plug threads or the case threads damaged, or is the seat cracked?
- Threads damaged on the plug only: fit a new plug and washer.
- Case threads stripped or the seat cracked: do not ride. A workshop fits a thread repair insert or an oversize plug; a crack may need case repair. See Assess bottom end and internal engine damage.
- No damage: fit a new washer, check that it is the correct size, and refit. Go to step 5.
5. Was the correct plug removed? Some engines have other plugs near the drain. Check the manual picture; refitting a different plug in the wrong place, or a plug of the wrong length, can leak or damage parts.
6After refilling with Change the engine oil and clean the strainer, recheck after a short ride.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Old washer or O-ring reused | Service history | Replace the drain plug sealing washer |
| Plug not tight | Turns by hand | Replace the drain plug sealing washer |
| Damaged plug threads | Torn threads on plug | New plug and washer |
| Stripped case thread or cracked seat | Plug will not tighten | Workshop thread repair |
| Leak from above | Oil runs down to plug | Trace an oil leak |
Reference values (K-Light 125)
| Item | Value | PDF page |
|---|---|---|
| Filter spring screw cap (drain cap with strainer) | 15 to 30 N·m | 194 |