Gearbox Maintenance

The only scheduled regular maintenance task relating to the gearbox is to check the level of or replace its lubricating oil. Check the level every 1,000 miles and change it every 6,000 miles. If the box is losing discernible amounts of oil over a 1,000 mile distance, this suggests it is leaking, since it is not consumed by combustion like the engine oil. With modern oils, the 6,000 mile change interval is probably also conservative.

Naturally, the car has to be on the level to get an accurate dipstick reading. The dipstick to check the level is accessed via the transmission tunnel right-hand side below the fascia and is concealed under the carpet. The carpet is folded back for access and will need protection from inevitable drips and spills. There is a rubber plug in the tunnel that has to be popped out to get to the dipstick below. The dipstick hole is also the filling hole, so most owners acquire a suitable funnel and devise their own favourite way of overcoming the fact that access is a bit of a fiddle. Funnels with a flexible spout are useful because they allow you fully to engage the spout in the hole, whilst presenting an open top tilted towards the pouring oil can. Most owners also develop their own favourite curse for the genius that designed this lay-out, although it does beat crawling under the car.

Draining the gearbox oil does require getting underneath to undo the drain plug. The plug has a square socket that takes a special tool to unscrew it. Unlike the engine sump, it has no bronze sealing washer. Have a look at the oil that drains out. It will be much cleaner than used engine oil. If it contains any metal fragments, that suggests damage to internal components like gear teeth.

The gearbox takes the same oil as the engine. Do not be tempted to use EP type oil as it attacks the bronze components.

Apart from lubrication maintenance, the only other work is likely to be a full rebuild after many miles of use. The need for a re-build is normally revealed by symptoms like jumping out of gear on the over-run, crunching gears or noises while running. This gearbox also has a deserved reputation for poor synchromesh on second gear, though most drivers develop strategies to overcome this, like double declutching. It is wise to do a systematic analysis while the car is still running and Barney Gaylord’s MGA Guru website contains a useful article about this and the rebuilding process: . This is probably one of the most intricate jobs undertaken by the home mechanic and many shy away from it as “a job too far”. However, there are numerous owners that have taken up the challenge and completed it with flying colours. The keys to success are a methodical approach and clean, tidy surroundings. Photographs will help remind you how things looked before you took them apart. It is possible to remove the gearbox without removing the engine. See the Clutch Maintenance section for more details.

If you prefer to hand the work to someone else, then it is normally possible to find a local transmission specialist. If they are uncertain about taking on classic car work, you can explain that the gearbox is internally almost identical to the early MGB gearbox, with which they will almost certainly be more familiar. Alternatively, MG specialists normally offer a re-build service even if they don’t have exchange units on the shelf.

Be warned: gearbox components are generally expensive. If you need to replace the laygear, you will be saying goodbye to about £140 (2010 price including UK value added tax) or a first motion shaft is about £200. Bearings, oil seals etc are not so expensive, so if your major components are still serviceable, you may get away with a low budget job.  If you do not do many miles, then you have more latitude to decide that a part-worn component will soldier on for years. But missing teeth or other serious wear cannot be ignored.

It is possible to replace some internal components with the stronger equivalents from an early MGB gearbox but this requires reaming out the casing to accept larger diameters. Another alternative would be to swap your tired original gearbox for the Hi-Gear Engineering 5-speed conversion that uses a Ford Sierra box. (See Modifications section for information on both options).

You have no rights to post comments