Gearbox rebuild kit: what is inside and when is it enough?

A gearbox rebuild kit is a package of wear parts for rebuilding a gearbox that has been taken apart: bearings, oil seals, gaskets and, for a manual gearbox, usually synchro rings. The kit replaces what wears out in normal use. It does not contain parts that failed through damage, such as broken gears or a faulty mechatronic unit.

That difference is exactly where rebuilds go wrong. You order a kit, rebuild the gearbox, and a thousand kilometres later it is whining again because the real cause was never replaced. This article explains what a rebuild kit normally contains, why the right kit depends on the gearbox type rather than the car, how to find your gearbox code, and when it is better to have the gearbox rebuilt. At ATR Transmissies in Montfoort we rebuild gearboxes for drivers and garages in the Netherlands and Flanders, and we start every job the same way: first find out what is actually wrong.

What is a gearbox rebuild kit?

A gearbox rebuild kit bundles the standard wear parts you replace during a rebuild anyway, in one package that fits one specific gearbox type. The alternative is ordering loose parts by part number, which takes more research and brings the risk that you are one oil seal short halfway through the rebuild.

So a rebuild kit is not a repair pack for a specific fault. It is meant for the moment the gearbox is already open. During a gearbox rebuild, also called a gearbox overhaul, the gearbox is fully stripped, cleaned and checked for wear. Everything that wears as standard comes out, whatever it looks like, because nobody wants to open the same gearbox again for one small bearing.

What is in a gearbox rebuild kit?

What is in a gearbox rebuild kit depends on whether it is for a manual or an automatic gearbox. A complete kit for a manual gearbox almost always contains bearings, oil seals and gaskets, plus synchro rings. Some suppliers add extra parts, such as selector forks, circlips, shims or the correct gearbox oil.

Manual gearbox rebuild kit

• Bearings: the ball and roller bearings the shafts run on. Worn gearbox bearings cause the familiar humming or whining noise that rises with road speed.
• Oil seals: they keep the oil inside where the input shaft and the drive shafts leave the casing. Leaks often start here.
• Gaskets and sealant: the seals between the casing halves and the covers.
• Synchro rings: these match the speed of the gear and the shaft as you shift, and they wear the fastest. Worn synchro rings are why a gearbox crunches when you select second gear.
• Small fitting parts: circlips, clips and sometimes shims to set the bearing preload again.

Automatic gearbox rebuild kit

For an automatic gearbox the wear parts are different, because an automatic shifts with hydraulically applied clutch packs instead of synchro rings. Suppliers sell automatic kits in tiers, and the names are not standard between brands:

• An overhaul kit usually contains gaskets, seals, O-rings and sealing rings, without clutch plates.
• A banner kit adds the friction clutch plates, and with some brands also steel plates, bands and bushings.
• A master kit is the most complete tier, typically with friction and steel plates, seals, gaskets, bushings and a filter.

Always compare the actual parts list before you order. Whether the friction plates, steel plates and bands are included makes a big difference. Parts such as the torque converter and the valve body or mechatronic unit are never part of a standard kit.

Bearing kit, gasket kit or complete rebuild kit?

Not every kit contains the same parts, and that is the most common source of wrong orders. A gearbox bearing kit contains only the bearings and usually the oil seals. A gasket or seal kit contains only gaskets and seals, and is meant for curing a leak without working on the inside of the gearbox. A complete rebuild kit combines both and adds the synchro rings.

If you are rebuilding a gearbox completely, you need the complete kit. Only fixing a leak? Then a seal kit is often enough, as long as the gearbox otherwise shifts well.

Why the right kit depends on the gearbox code, not the car

A gearbox rebuild kit is always specific to the gearbox type, not to the make of car. Manufacturers fit the same gearbox to several models and brands, and within one model they fit different gearboxes depending on engine power, drive and year. A kit that seems to match your car by name often does not fit the gearbox that is actually in it.

Bearing sizes, the number of gears, shaft dimensions and the synchromesh design all differ between gearbox types. Order by car model instead of gearbox type, and you can end up with bearings that are a few millimetres off. You only find that out when the gearbox is already in pieces on the bench.

How do you find your gearbox code?

You find the gearbox type from the gearbox code on the identification plate or the code stamped into the gearbox casing. That code, usually a combination of letters and numbers, identifies the exact version and is what counts when ordering parts.

• Look for the plate, sticker or stamped code on the gearbox casing, often near the joint with the engine or on the top.
• Check the owner’s manual or service book. Some manufacturers also print the code on a sticker in the boot or near the spare wheel.
• If that does not work, a specialist can trace the gearbox code from the chassis number (VIN). In the Netherlands the VIN is listed on your registration card (kentekencard).

Take a photo of the code and send it with your request. If you are not sure you have the right code, send the photo anyway. It takes five minutes and saves you a kit you cannot use. For automatics, our guide on which automatic gearbox you have shows where to look on the most common types.

What a gearbox rebuild kit will not fix

A rebuild kit replaces wear parts, not damaged parts. Anything that is broken, worn into or electronically faulty has to be ordered separately or repaired. These fall outside a standard kit:

• Damaged or broken gears. Pitted tooth flanks and broken teeth need separate replacement, and those gears are often the most expensive part of the whole rebuild.
• Bent or cracked shafts. Rare, but real after heavy overloading.
• A faulty mechatronic unit or control unit. In automatic and DSG gearboxes the fault often sits in the electronic and hydraulic control, not in the mechanical parts. No rebuild kit helps with that.
• The torque converter and, depending on the kit, the clutch packs of an automatic. These are rebuilt or replaced separately.
• A cracked or distorted gearbox casing. At that point a rebuild is no longer an option.

That is why the diagnosis decides what you order, not the other way round. Metal particles in the oil, for example, are almost always a sign that more is going on than normal wear. Not sure what the noise or shift fault in your car means? Find out what your symptom could mean.

Rebuilding a gearbox yourself with a kit, or having it done?

Rebuilding a gearbox yourself is realistic if you have experience with gearboxes, own a press and pullers, and can look up the factory specifications for bearing clearance and preload. Without those three you will get stuck, usually when setting the clearances again.

Where DIY rebuilds go wrong is rarely the strip-down. It is the reassembly. Bearing clearance has to be set to hundredths of a millimetre, oil seals must go in straight and to the right depth, and the tightening torque of the shaft nuts matters. A gearbox that is just slightly out of adjustment can wear out again within a few tens of thousands of kilometres.

If you have the gearbox rebuilt, you are paying for expertise, tools and a tested result. What that costs depends on the gearbox type, the damage found and the parts needed; our page on gearbox rebuild costs explains how the price is built up. If you cannot wait for your own gearbox to be rebuilt, an exchange reconditioned gearbox is often the fastest route.

Loose gearbox parts or a complete kit?

Order a complete kit when you are rebuilding the whole gearbox, and loose gearbox parts when you are solving one clearly defined problem. Replacing a single leaking oil seal does not call for a kit of dozens of parts.

In practice it is often somewhere in between. You start with a rebuild kit, open the gearbox and find that one gear is worn. That gear then comes on top as a separate part. Allow for that in your planning and your budget: only once the gearbox is open do you know what it really needs. This is also why ATR inspects and measures before replacing, instead of fitting a full kit and hoping for the best.

For garages: rebuild kits and parts from ATR

ATR Transmissies supplies rebuild kits and loose gearbox parts to garages and workshops, alongside our own rebuild work. Send us the gearbox type or gearbox code, and we will check which kit really fits and what you need on top. Prices depend completely on the gearbox type and the size of the kit, so contact us for a price indication based on the gearbox code or registration number.

Stuck on a gearbox in your own workshop? With ATR Remote we can help you with the diagnosis remotely, so a gearbox does not sit on a pallet for days while you guess.

Frequently asked questions about gearbox rebuild kits

How much does a gearbox rebuild kit cost?

The price depends entirely on the gearbox type, the number of gears and what is in the kit. A seal kit costs a fraction of a complete kit with bearings and synchro rings, and an automatic master kit is a different category again. Contact us for a price indication based on your gearbox code or registration number.

Can I rebuild my gearbox myself with a rebuild kit?

Yes, if you have experience with gearboxes, a press, pullers and the factory specifications for bearing clearance. Reassembly is the hardest part. Without the right adjustment the gearbox wears out again, and you lose both the kit and your time.

What comes in a transmission overhaul kit?

For an automatic, an overhaul kit usually contains gaskets, seals, O-rings and sealing rings, but no clutch plates. Banner and master kits add friction plates and often steel plates, bushings and a filter. The names differ by brand, so always check the parts list.

What does a full gearbox rebuild include?

A full rebuild means removing the gearbox, stripping it completely, cleaning and inspecting every part, and replacing all wear parts plus anything damaged. The bearing clearances are then set again, and the gearbox is tested. A rebuild kit covers only the wear parts in that process.

Is the mechatronic unit included in a rebuild kit?

No. A rebuild kit contains mechanical wear parts, not electronics or hydraulic control units. A faulty mechatronic unit in an automatic or DSG gearbox is repaired or replaced separately, and the fault codes need to be read first to confirm the fault.

Is a rebuilt gearbox as good as new?

A well rebuilt gearbox shifts like new again, because all wear parts are replaced and the clearances are set again. The quality depends on how carefully it was measured and assembled. Always ask what is covered after the rebuild and what was replaced.

Conclusion

A gearbox rebuild kit gets you a long way, as long as you know what you are buying: standard wear parts for one specific gearbox type, and nothing more. Find the gearbox type from the gearbox code first, check whether the damage falls within the kit, and only then decide whether to rebuild it yourself or have it done. Order on a hunch and you pay twice.

ATR Transmissies in Montfoort, in the province of Utrecht, rebuilds manual and automatic gearboxes for drivers and garages across the Netherlands and Flanders. Not sure which kit suits your gearbox, or would you rather have it rebuilt? Book your car in or call 0348 475100 (+31 348 475100 from abroad) with your gearbox code or registration number to hand, and we will work it out for you.