The Role of the Synchroniser
For a gear to engage silently, the gear and the shaft must be spinning at the same speed. That is the job of the synchroniser: a ring that slows or speeds up the gear during the fraction of a second in which you push the lever. When it wears out, the speed-matching is no longer complete and the dog teeth clash together — that is the crunch.
One Gear or All of Them? The Question That Decides It
| What you notice | Most likely cause | Concerns |
|---|---|---|
| Crunching on one gear only (often 2nd or 3rd) | Worn synchroniser or dog teeth on that ratio | The gearbox |
| Crunching or stiffness on every gear | Clutch no longer disengaging fully | The clutch |
| Mainly when cold, better once warmed up | Worn or incorrect-viscosity oil | Maintenance |
| Reverse only | Reverse gear has no synchroniser | Often normal |
When It Crunches on Every Gear
If the clutch is no longer fully disconnecting the engine, the gearbox input shaft keeps spinning and no gear goes in cleanly. The causes are often straightforward: air in the hydraulic circuit, low clutch fluid, a worn master or slave cylinder, a slack cable. These should be checked before blaming the gearbox.
Can You Keep Driving?
Yes, by going easy on the affected gear, but wear accelerates: every crunch damages the dog teeth a little more, until the gear no longer stays in and jumps out under acceleration or when you lift off. At that point, the repair becomes far more involved.
Repair or Replace the Gearbox?
Replacing a synchroniser means removing and opening the gearbox. On a high-mileage unit, the other synchros, bearings and dog teeth are rarely in any better condition. Since the labour for removal is the same either way, a reconditioned gearbox on an exchange basis — with all wear parts replaced, tested and warranted for up to 24 months — is often the more lasting choice.
Handy tip: once the gearbox is out, the clutch is right there. It is the ideal moment to check its wear and avoid a second time the vehicle is off the road.