We sell a lot of gearboxes, and the ones that come back under warranty almost never come back because the box was bad. They come back because something around the box was left alone to save a few hundred rand, and it took the new box out with it.
Here is the short list. All five are cheap relative to the labour of dropping a gearbox, and all five need doing while it is out.
1. The clutch kit, on a manual
If the box is out, the clutch is accessible. If the clutch is accessible and it is not brand new, replace it. A clutch that is halfway through its life will need doing within a year or two, and doing it later means paying the same labour bill twice. Fit the plate, cover and release bearing as a set from the same manufacturer.
2. The rear main seal and the input shaft seal
These are inexpensive rubber parts sitting in exactly the place you will never reach again once the box is bolted up. A weeping rear main will contaminate a new clutch plate within months and you will be told the clutch was faulty. It was not.
3. The spigot bearing or bush
The little bearing in the end of the crank that supports the nose of the input shaft. It costs almost nothing. When it fails it makes a noise that sounds exactly like a failing gearbox, and it will have you dropping the box again to find out.
4. The gearbox oil, and the right oil
A used box arrives with storage oil or drained. Fill it with the correct specification, not whatever is on the shelf. Manual boxes are fussy about friction modifiers and the wrong oil will make the synchros crunch on a perfectly good box. On automatics, replace the fluid and the filter, and flush the cooler lines.
That last point matters more than people think. If the old automatic failed, its debris is sitting in the cooler and the lines. Bolt a good box onto an unflushed cooler and you have just fed the wreckage of the old one into the new one.
5. Mountings and the propshaft centre bearing
Perished gearbox and engine mountings let the driveline move under load, which chews the new box and makes the shift feel vague. On a rear-wheel-drive vehicle, check the propshaft centre bearing and universal joints while the shaft is out. Both are cheap, both are a nightmare to do afterwards.
One more thing, on four wheel drives
Check your diff ratios front and rear before fitting anything. Mixing ratios will destroy a transfer case, and it happens more often than you would think when somebody has fitted a second-hand diff at some point in the vehicle history.
Why we bang on about it
Because our warranty and your weekend both depend on it. Fitting a used gearbox properly makes it a genuinely good buy. Fitting one in a hurry, on a worn clutch, with the wrong oil, turns a bargain into an argument.
Ask us for the correct oil specification for your box when you order and we will send it with the quote.