Synchromesh (Synchroniser)

Each gear in a manual gearbox has a synchroniser made of a cone-shaped brass or carbon ring and a sliding sleeve. Reverse gear often has no synchroniser, which is why it can grind if selected while moving. Multi-cone synchronisers use several friction cones to handle larger speed differences, usually on first and second gear.

As the lever moves, the ring is pressed against a cone on the gear. Friction speeds it up or slows it down until both turn at the same speed and the sleeve can slide into engagement.

Worn synchros cause grinding in one or more gears, often 2nd or 3rd. Using the clutch fully and changing gear smoothly extends their life. Shifting too quickly forces the synchroniser to work harder and wears it faster.