Basics

JASO MA, MA2 and MB explained

JASO is printed on almost every motorcycle oil label sold in Asia. Most riders treat it as a compliance stamp and move on. But the two letters after JASO — MA, MA2, or MB — encode the single most important compatibility requirement for your engine. Get it wrong and your clutch pays the price.

This is the definitive reference for what each rating means, how it is tested, and which one your bike requires.

Why JASO exists

The global API and ILSAC oil standards were developed primarily for car engines. Car engines use dry clutches — the clutch is mechanically separate from the engine oil. Car oil formulations freely include friction modifiers, additives that reduce internal friction to improve fuel economy. In a car, this causes no problems.

Most motorcycles use a wet clutch — a clutch assembly that runs submerged in engine oil. A wet clutch depends on friction between its plates to engage correctly. If the oil contains friction modifiers, those additives reduce the friction coefficient of the clutch plates. The clutch slips. Power transfer suffers. Accelerated plate wear follows.

The Japan Automobile Standards Organization developed the JASO T 904 standard specifically to address this. It defines the friction performance requirements that motorcycle engine oils must meet, separating oils that are safe for wet-clutch use from those that are not.

How JASO oils are tested

JASO T 904 measures three friction-related properties using a standardised clutch friction test (the JASO DHTCB — Dynamic High Temperature Clutch Brake test):

Dynamic friction index (DHTCB): friction coefficient of the clutch plates during engagement under dynamic conditions. Higher values mean more clutch grip.

Static friction index (SSTCB): friction coefficient when the clutch is fully engaged and stationary. Affects how positively the clutch holds under load.

Stop time index (SSTCB-S): how quickly the clutch brings a spinning mass to a stop — a measure of clutch system performance and feel.

Each JASO classification defines minimum and maximum values for these indices. An oil must be independently certified against these criteria to carry the JASO rating.

JASO MA: the baseline for wet-clutch motorcycles

JASO MA oils meet a minimum dynamic friction index of 1.6 and static friction index of 1.7. They contain no friction modifiers, or only those at levels that do not reduce clutch friction below the threshold. MA is appropriate for most manual-transmission wet-clutch motorcycles under normal street riding conditions.

JASO MA2: the stricter standard

JASO MA2 sets higher minimum friction thresholds than MA: dynamic friction index of 1.8 minimum (vs MA's 1.6) and static friction index of 1.9 minimum (vs MA's 1.7). The higher friction coefficient provides more positive clutch engagement under sustained load — important for higher-performance engines, bikes ridden hard, or bikes used by delivery riders with constant clutch operation.

MA2 is backward compatible with MA applications — an MA2 oil can be used wherever MA is specified. The reverse is not always true for high-performance applications.

All TWIIN motorcycle oils are certified JASO MA2.

JASO MB: for automatic scooters

JASO MB is a lower friction classification. MB oils may contain friction modifiers — they are specifically formulated for automatic scooters with CVT transmissions, where the engine oil does not contact a wet friction clutch in the same way.

Using an MB oil in a wet-clutch manual motorcycle will cause clutch slip. The friction modifiers in the MB formulation do exactly what they are designed to do — reduce friction — and that is precisely the problem in a wet-clutch system.

Using an MA2 oil in an automatic scooter causes no harm but may not optimise CVT performance the way an MB formulation does.

Rating	Min. dynamic friction (DHTCB)	Friction modifiers	Clutch type	Typical application	TWIIN product
JASO MA	1.6 minimum	Not permitted above threshold	Wet clutch	Manual motorcycles, standard use	—
JASO MA2	1.8 minimum	Not permitted above threshold	Wet clutch	Performance motorcycles, hard riding, delivery use	M10W40F / M10W50F / M10W60F
JASO MB	Below 1.6	Permitted	CVT / automatic	Automatic scooters (Honda Beat, Yamaha NMax, etc.)	S10W40F