Clutch slip under two-up load: free play, engine oil specification, or worn plates?
A 125 cc commuter with a wet multiplate clutch has covered 24,700 km. With two riders on a gradient, the engine pulls cleanly to 7,000 rpm in third, but in fourth and fifth the tachometer climbs while road speed does not, and hot friction material can be smelled after a long climb. Clutch lever free play at the lever tip measures about 2 mm against a specification of 10–20 mm, and at full steering lock either way the cable is visibly tight. The engine oil was changed 1,200 km ago with 15W-40 API SJ oil intended for passenger cars, taken from a 20 litre drum; the level is correct and the oil is dark but not burnt. The owner rides with a pillion daily and wants the clutch plates replaced immediately. Service limits are: clutch spring free length 34.5 mm, friction plate thickness 2.6 mm, steel plate warpage 0.15 mm. No clutch measurement has been made yet.
1. Set out the order in which you would check this fault and explain what each result would prove. How would you answer the owner's request for immediate plate replacement?
Read the worked answer
Free play first, because it is free and reversible. 2 mm at the lever tip against 10–20 mm means the cable is under tension and the release mechanism is held partly disengaged, so the springs cannot clamp the plates fully. A cable tight at full lock confirms it, and steering movement can even pull the clutch in. Adjust at both the lever and the casing adjuster, leaving visible slack, then road-test.
Second, the oil. Car 15W-40 API SJ may carry friction modifiers and is a classic cause of wet-clutch slip; SJ says nothing about clutch friction.
Third, only if slip remains, measure: friction plate thickness (limit 2.6 mm), spring free length (limit 34.5 mm), steel plate warpage on a surface plate with a feeler gauge (limit 0.15 mm), and clutch basket tangs for notching.
Replacing plates before correcting free play and oil would repeat the failure at the customer's expense. Decline the immediate plate job, explain the sequence, and agree that plates will be measured and replaced only if slip persists after steps 1 and 2, tested two-up up a gradient in top gear.
2. The owner wants to keep using the drum of car oil because it is cheaper. Explain, in friction terms, why this oil can cause slip and what oil specification the motorcycle needs.
Read the worked answer
JASO MA and MA2 are wet-clutch friction standards: oils carrying them are tested for dynamic and static friction and for clutch lock-up, with MA2 giving the highest friction for larger wet clutches. API SJ is an engine-performance classification only and says nothing about clutch behaviour; a car-intended SJ oil may in practice behave like an MB oil.
Friction modifiers - molybdenum, boron and some ester and FM additive packages - are designed to lower metal-to-metal friction. In a wet clutch they form a film on the friction plates and reduce the coefficient of friction, so the springs cannot transmit full engine torque and the clutch slips under load. This is not a wear failure and adjustment cannot correct it.
Drain the car oil, replace the filter if fitted, refill with the viscosity and API grade in the manual that also carries JASO MA or MA2, and road-test. Point out that the saving per litre is small against a clutch job, and that continued use can glaze the friction plates, so plates should not be measured or condemned until the correct oil has been run.
3. Slip persists after free play and oil are corrected. Describe the measurements and decision criteria you would use before replacing parts.
Read the worked answer
With free play and oil correct, persistent slip under two-up load justifies measurement.
- Friction plates: measure each plate at three points with a micrometer; limit 2.6 mm. Look for glazing, burnt odour and contamination. A plate at or near limit is replaced as a set; lightly glazed plates may respond to deglazing on a flat surface with abrasive paper, but only if thickness allows.
- Steel plates: check flatness on a surface plate with a 0.15 mm feeler gauge; any plate that will not pass is replaced. Blue discolouration indicates heat from slip.
- Springs: free length limit 34.5 mm; springs below that, or noticeably shorter than the set average, are replaced as a set.
- Clutch basket and inner hub: notched tangs stop plates moving freely and cause both drag and partial engagement; file smooth or replace.
- Release mechanism: check pushrod straightness and actuator travel.
Reassemble with the specified JASO MA/MA2 oil, soak the friction plates in the same oil for 10 minutes, and test by attempting to pull away in third gear uphill: the engine should lug or stall, not flare.