Crunching at the moment the lever goes into gear is an unpleasant, memorable sound: a short metallic rasp, as if something were slipping over teeth. Mechanically that is exactly what happens. But the source varies, and which one it is separates replacing an inexpensive part from opening the gearbox. Here is what crunches, how to work out the cause from the way the car behaves, and how the repair is decided.
What a synchro does
Inside a manual gearbox the gears are permanently meshed, and a gear is selected by a sleeve that locks the chosen gear to the shaft. The problem is that the shaft and the gear are turning at different speeds at that moment. The synchro — a small ring with a conical face — equalises their speeds in a fraction of a second through friction, and only then does the sleeve engage.
The synchro ring wears: the cone rubs away, the teeth round off, and there is no longer enough time to equalise. The sleeve engages before the speeds have matched, and you hear the crunch. The clear marker of synchro wear is crunching in one particular gear, most often second, while the rest engage cleanly. And a telling detail: shift slowly and there is no crunch; shift quickly and there is.
When the gearbox is not to blame
| Cause | How it shows | The check |
|---|---|---|
| Worn synchro | Crunch in one gear, usually second | Slow engagement is quiet |
| Clutch dragging (not disengaging) | Several gears crunch, first is hard to select while parked | Gear engages badly with the car stationary |
| Air or a leak in the hydraulic linkage | Soft pedal, sinking to the floor | Fluid level, hose condition |
| Worn shift linkage or cables | The lever moves too freely | Play in the lever in neutral |
| Low gearbox oil | Crunching worse when cold | Check the level |
A dragging clutch is arguably more common than worn synchros, and cheaper if caught in time. The sign is simple: with the engine running and the pedal to the floor, first gear engages with resistance and a crunch even though the car is stationary. That means the input shaft is not stopping.
Checking it yourself
- The first-gear test while parked. Engine running, pedal to the floor, wait two seconds and select first. A crunch with the car standing still is a question for the clutch, not the synchros.
- Slow engagement. Repeat the problem gear while driving, moving the lever at half your usual speed. Silence when slow and a crunch when quick is classic synchro wear.
- The cold test. Check whether the crunch is there on a cold gearbox and whether it goes away after 10–15 minutes of driving. A difference points at the oil: thickened, or the wrong viscosity.
- The pedal. Notice whether the pedal travel has changed and whether new sounds have appeared as you press it. The general guide to noises under acceleration is on the symptom page knocking when accelerating.
How the repair is decided
The order of steps follows cost. A gearbox oil change is the first move when the crunch is worse cold — it is cheap, and the specification matters more than the brand: many gearboxes need oil with a friction modifier package designed for their synchros, and generic gear oil makes shifting worse rather than better.
If the clutch turns out to be dragging, the cause may be as simple as air in the hydraulic line or a worn master cylinder — both far cheaper than the clutch itself, and worth ruling out before anyone quotes for a kit.
If it really is the synchros, the deciding fact is that the gearbox has to come out. That means the clutch kit is replaced at the same time, along with the rear main seal and the gearbox oil: the labour is already paid for, and doing them separately later means paying it twice.
You can drive with a crunch, but every unsuccessful shift wears the ring further, and one day the gear will refuse to engage at all. If one gear crunches, plan the repair calmly. If the crunch has spread to several gears and shifting has become hard, have the car looked at within days.
If you want to know what is crunching before the visit, record the sound in the Pro-Stuk app twice: selecting first while parked, and the problem gear while driving. The app matches the recordings with your answers and shows the likely causes with percentages.
