How to top up Gearbox Oil on a Classic Car

How to top up gearbox oil on a classic car

This guide captures ‘bygone’ advice on how to check and top up gearbox and back axle oil on a Classic Car. It is part of a series I am building that describes what to do on a Classic Car Basic Service.

Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.





Why lubrication is essential

Regular thorough and correct lubrication is essential for preserving the life expectancy and efficiency of nearly all the mechanical components of your car.

Inspecting oil levels (engine oil aside), topping-up procedures and general lubrication can be conveniently dealt with as part of the 10,000 km (6,000 mile) service.

The gearbox and rear axle require particular attention and though these and many other components need lubricating far less regularly than was once the case there are still many parts of the car that must be attended to.


What oil to use in a Gearbox

Most gearboxes and rear axle differentials use specially developed lubricants which are designed to cope with the extremely high pressures that are created between meshing gears. These oils can be identified by the designation EP.

EP stands for extreme pressure.

They are usually supplied in soft plastic bottles with extended nozzles which enable you to squirt the oil into difficult to reach openings in the gearbox and axle.

The grade you require may be anything from SAE 80 to 140, though EP 90 is the most common. The gearbox and rear axle may use different grades and its worth checking in a workshop manual.


EP Classic Oils on eBay


Some cars, for example the Chrysler Arrow, have gearboxes which run on ordinary multigrade oil. But these still demand an EP lubricant in their rear axles.

On certain transverse engined front wheel drive (fwd) cars, such as the Leyland fwd models, the gearbox and final drive share a common oil supply with the engine. Multigrade engine oil is used and the gearbox is automatically topped-up and the oil is renewed at every engine oil change.

Automatic transmissions use a special type of fluid which varies from one gearbox to another. Be sure to use the correct type and grade and on no account substitute a grade other than that recommended by the manufacturer.

On some cars the transmissions are designed to be sealed for life and the oil in them will last for as long as the life span of the components. No drain or filler plugs are built into the gearbox or rear axle and they do not require any attention. The majority of transmissions however do have to be checked at regular intervals for correct oil level and replenished if necessary.


Topping up the Gearbox and Axle

Find the filler/level plug on the side of the gearbox casing. It is usually bolt-shaped and hexagon-headed
Find the filler/level plug on the side of the gearbox casing. It is usually bolt-shaped and hexagon-headed
Use the correct grade of EP oil to top up the level. Oil is usually supplied in a bottle with a nozzle to facilitate filling
Use the correct grade of EP oil to top up the level. Oil is usually supplied in a bottle with a nozzle to facilitate filling
  1. Check the specifications for the gearbox and rear axle before starting work.
  2. Wait until the engine is cold so that the oil will have run down the inside of the casing to show its true level.
  3. It is important to get the car as level as possible so that the oil levels can be judged accurately. This may prove difficult if you have to raise the car on ramps or axle stands in order to create sufficient room to work underneath. In this case a slightly different approach is necessary.
  4. Raise the car, remove the filler plug in the way described below, overfill the component with oil and then replace the plug loosely (one or two turns of the thread is enough).
  5. Lower the car to a level position and reach underneath to remove the plug with your fingers. Let the surplus oil drain out of the hole then replace the plug, Raise the car again so that you can get underneath to tighten up the plug properly with a spanner.
  6. Most cars have a filler/level plug on the side of the gearbox. This is usually a hexagon-headed bolt­-shaped object that can be removed with a spanner. A similar plug may also be found on the bottom of the casing. This is the gearbox drain-plug. Do not undo this plug as it is not necessary to drain off the oil at this 6000 mile service interval.
  7. Clean away all the dirt surrounding the plug before you remove it.
  8. Remove the level plug. Fill the gearbox from the squeeze bottle until the oil level comes up to the bottom of the level-hole and just begins to run out.
  9. If the filler plug is on the top of the gearbox then use the dipstick to check the level. Pour the oil in slowly, a little at a time. Wait for a few moments so that the oil has a chance to run down and settle and then check the level with the dipstick. Do this until the level comes up to the right mark, just as you would when topping-up the engine oil, but do not overfill.
  10. The filler/level plug on the rear axle is easily identified. It will be found on the side of the bulbous-shaped differen­tial housing in the centre of the axle. Top-up the axle with the correct grade of oil in exactly the same manner as described for the gearbox.

Useful Oil Pumps on eBay


Topping up Automatic Gearboxes

Some vehicles, especially those with automatic trans­mission, have a combined filler cap and dipstick somewhere on top of the gearbox casing.

This may be located just in front of the bulkhead in the engine compartment or through an access cover on the transmission tunnel inside the car.

Checking and topping-up automatic gearboxes involves a similar procedure to that above but there are some extra points that must be noted.

  1. Manufacturers give different instructions for topping-up automatics depending on the type. Some specify that the job must be performed with the engine running, others specify a warm or cold engine.
  2. You may also have to select ‘drive’, ‘park’ or ‘neutral’ when you are noting the oil level. So check with the makers if you are unsure of the details for your car. Remember that cleanliness and the correct type of fluid are vital

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