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gearbox repair
18 July
Bởi: Tùng Yên TNHH / Bình luận

Gearbox Introduction:

A transmission is also used on pedal bicycles and fixed machines, where rotational speed and torque needs to be adapted. A transmission often has multiple gear ratios (or simply “gears”) and offers the function of switching between them as the speed varies. Bicycles are usually provided with multiple-gear transmissions and switching  is done manually. Using chains and chain wheels that are smaller than the tires, many more revolutions can be attained with much less exertion from the cyclist. For example, the chain wheels might have 20 teeth, compared to the tires which are 25-inches in diameter. The rear gear might have 28 teeth. This creates a ratio that allows the tires to turn much more often, than would be possible with a single turn of the pedal.

A similar principle applies in case of an automotive manual transmission. Thanks to the proper arrangement of cogs, five or six gears and a reverse gear are available. The driver may also make use of the idle gear, when the car is not moving,  although the engine is operating.

The drive train includes the gearbox, clutch, prop shaft (for rear-wheel drive), differential and final drive shafts. In case of a car, you need a clutch because the engine spins all the time.  However, the car’s wheels do not spin. That is why in order to stop a car without killing the engine, the wheels need to be somehow disconnected from the engine. The clutch allows us to smoothly engage a spinning engine to a non-spinning transmission, by controlling the slippage between them. The clutch is the mechanism for transmitting rotation, which can be engaged and disengaged.

Causes of Gearbox Wear

There are a lot of causes but the most common those involve:

Aggressive driving and improper shifting

Transmission component wear is very often caused by driver’s mistakes, including improper shifting. This results in the striking of cogs against each other with a force that causes their premature wear.

Because they are vulnerable, synchronizers suffer the most. It may also result in premature teeth abrasive wear or gear tooth pitting.

Gearbox repair

Gear rack with synchronizer.

Old oil or insufficient gearbox oil level

One of the most overlooked parts of car maintenance is keeping your gearbox oil fresh and filled. The more the clutch and transmission works, the more the oil is subject to “shear”. It loses its viscosity and results in the overheating of some components and their premature wear. Dirty or poor quality oil first results in synchronizer wear that initially very often goes unnoticed.

As heat destroys the fluid’s lubricating qualities and friction characteristics, the “worn out” oil does not transfer heat away from the working components.  This is particularly evident in the winter, when you need to use more force during shifting. The old oil is quickly subjected to solidification at low temperatures and  thins excessively at high temperatures.

How often should the oil be changed with a manual transmission? The oil should be changed at least as often as recommended by the manufacturer. For some cars, the oil change should be changed after reaching 60 thousand km. This type of maintenance is very simple. Some vehicles are provided oil that is intended for multi-year operation.   The manual for these vehicles does not recommend any more frequent change.  However, in practice, it is worthwhile to change the oil more often. This is especially true for vehicles which are between ten and twenty years old. The more frequent oil changes help to enhance transmission life.

Insufficient oil level in the gearbox is another reason for its premature wear. It is most often caused by gearbox leakage. Gearbox capacity is on average ~ 2l, and even a minor leakage can cause the gearbox to totally empty.

So please remember checked Gearbox oil level frequently.

Driver errors –shifting into the wrong gear for the rate of speed and engine load.

Shifting into the wrong gear for the rate of speed and engine load, also results in the premature wear of the gearbox bearings, due to their operation under extreme loads.
As a result, there is wearing of the metal balls and bearing cones that causes excessive play on the gearbox shaft and at the same time noisy operation. Continual operation of such a gearbox, may cause poor teeth alignment and gearbox damage.

Gearbox Anticipation

Some symptoms or minor defects that go overlooked for a long time, may cause serious failure and put the car out of operation. It is possible for you to personally notice or hear some gearbox irregularities, if extra caution is given.

The main irregularities that may indicate improper gearbox operation include:

Oil leakage

gearbox repair

Visible gearbox oil leakage.

At the beginning of the diagnosis, it is worthwhile to check the oil level. Even minor leakage, such as with the gearbox housing, half-shafts sealing or drain plug, may result in gearbox wear or put the car out of operation while traveling, which can cause expensive problems. Check for any leakage near the gearbox and engine, as well as half-shafts.

Regularly check your parking space for leaks when you pull awayIf you see any leaks, get it fixed before lack of oil or fluids can cause expensive problems.

Noisy Gearbox operation

Another symptom that requires your attention is noisy gearbox operation.

Noisy gearbox operation is mainly caused by worn out bearings. In this case, noise is evident and noticeable when driving in any gear.

Sometimes noise or irregularities evident during driving in any gear, may indicate wear of the differential gear of the rear axle (rear drive and four-wheel-drive vehicles have a rear axle).

If noise follows at that particular gear, it may be caused by gear tooth pitting.

Sometimes noise may be evident when pressing on the clutch. This may be caused by worn clutch bearings. The bearing operation actually follows after pressing down on the clutch pedal.  This is why the noise does not follow after the release of the clutch.

Grinding noise and resistance during shifting

A grinding noise or resistance during shifting, very often indicates worn gear synchronizers. They are used to match up the speed of the two moving cogs inside your box. When you move the shift, it pushes/slides a cog onto another cog, and the gears engage. To make this a smooth transition, one cog which is usually cone shaped (tapered) and the other will slide on to it, like putting the cap on a pen, with both of them rotating.

During quick shifting (in particular at high revolution speeds), the worn gear synchronizer fails to synchronize the shaft and cog rotation. Grinding follows as a result.
Since each gear is provided with own synchronizer, a synchronizer failure usually relates to the particular gear ( such as grinding during shifting from one to two gear indicates two gear synchronizer failure).

gearbox repair

Worn gear synchronizer, which often results in grinding noise

Gearbox repair – WHAT IS INVOLVED?

Gearbox repair is complicated and may vary significantly for each gearbox (due to different engineering solutions utilized by manufacturers). That is why this guide will not focus on this process (there is further information in the car owner’s manual).

Repair of the gearbox includes the following activities:

  1. Identification of faulty gearbox components on the basis of shifting and gearbox operation.
  2. Removal of the gearbox.
  3. Disassembly of the gearbox (required for the repair).
  4. Determination of particular component wear.
  5. Replacement of damaged and non-repairable parts, or their regeneration.
  6. Assembly of all the components using new gaskets and sealing.
  7. Installation of the gearbox on the vehicle and filling it with fresh oil.

In general, all of the worn parts of the gearbox shall be replaced.  This refers primarily to synchronizers and bearings. During repair, it is recommended that you consider the replacement of all bearings.  It does not significantly increase  the total repair costs (usually disassembly, repair and installation of the gearbox account for the majority of the costs).

Gear cogs, differential gears, gearbox bodies, selector forks or shafts (driven and driving ones)  also sometime require repair.

If the gearbox has been disassembled, it is recommended to also replace the gearbox sealing, as a half-shaft seal or jackshaft seal.


As with any car gearbox repair, you may always choose the DIY method, provided that you possess the required skills and know-how.

If you do not have required skills, knowledge and necessary tools or devices, the best option will be to have the repairs made in the repair shop. Transmission mechanisms have been precisely synchronized.  Therefore, unskilful handling (any error during assembly) may result in serious gearbox damage.

That is why it is recommended to visit the repair shop to have any repairs and adjustments made by professionals.

It is worthwhile to find a gearbox repair shop or professional with expertise and experience in this field. Some websites and social networks, as well as references of your acquaintances may be very helpful. The best solution is to have the repair or adjustment made by the authorized service station or the repair shop having expertise for specific car type ( such as repairs of Japanese or BMW cars only).

This approach ensures professional gearbox repair and prevent any extra unplanned expenditures resulting from services performed by incompetent and inexperienced mechanics. Some service stations have the required spare parts available in stock.  This allows the gearbox repair to be made relatively quickly and saves you time and trouble.

There still are some maintenance operations that you can perform on your own,  such as checking the gearbox oil level and refilling it, if necessary. If you have the required facilities for these maintenance operations available,  for manual transmissions, you can change the oil on your own. It is actually very simple to change the gearbox oil for those who do not have any expertise in this field, if it is done with care and precision. It is essential to fill the gearbox with the recommended oil (preferably a new and synthetic one).

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