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  BMC 'A' Series differential
  BMC 'A' Series differential

The BMC 'A' series axle was fitted to a wide range of vehicles for more than twenty-five years. The axle became a very popular fit into kit cars and small volume specialist cars. As the differential is light in weight and was available in a variety of ratios it was fitted to many front engined racing cars. Rebuilding the differential is a specialist job but it is 30 years since they were last produced and with the passing of time people with the experience, knowledge, skill and the right equipment have mostly gone. It is not surprising that we have found there are those who think rebuilding a differential is just a parts changing job and are not aware of the requirements to achieve a successful rebuild. Over the years we have seen numerous differentials that were "rebuilt" with either a good condition or new reproduction crown wheel and pinion that has rapidly become noisy and worn because it was not built correctly.

We have a very long experience of classic BMC 'A' Series differential rebuilds, a large range of components to draw from and an established rebuild process. First a casing is selected of the correct type, there are four variations. It is then chemically cleaned and inspected for damage, cracks and corrosion and that the pinion and side bearing housing are undamaged and the correct size. Many fail these checks but if the casing is satisfactory then the internal casting and stress raising irregularities are then ground out. Also the casing flange is remachined to ensure it is flat. The casing has a final chemical clean.

The drive flange is the next part worked on. Many of these are beyond further use due to wear, damage and corrosion. A suitable flange is selected and machined on the front face and the end face to ensure the surfaces are flat.

Next a crown wheel carrier is selected and chemically cleaned and inspected for damage, wear and cracks. Two planet gears and a shaft and two side gears are selected and inspected for damage wear and cracks. If any of these parts are excessively worn or damaged they are replaced with parts in good condition. The parts are then rebuilt using new selective fit planet gear and side gear thrust washers. The planet gear shaft retaining pin is welded in to prevent it falling out and causing damage.

The most important aspect of the rebuild is achieving correct meshing of the crown wheel and pinion teeth. Also the correct rotational torque on the pinion and the correct preload on the carrier bearings. To help achieve this we use special service tools and specifications and a lot of experience that is especially useful with some reproduction crown wheel and pinions that can be difficult and time consuming to set up.

To start with, new pinion bearings are fitted to the casing and measurements are taken using special tooling to determine the size of the pinion bearing spacer required. A spacer is then made. Also, measurements are taken in the carrier bearing housings in the casing and the crown wheel carrier to determine the size of shims required to obtain the correct bearing pre-load.

A trial assembly is done with a crown wheel and pinion together with new pinion bearings and new carrier bearings and everything is set up to obtain a gear tooth meshing marking. Depending on the result achieved a change of size of the pinion spacer may be required, then another spacer is made in a different size based on the meshing marking indicated. This process is repeated until the gear meshing is satisfactory. As an indication of the accuracy needed to achieve this there were 26 different pinion spacers in 2 types and sizes and 5 sizes of carrier bearing shims used when the differentials were originally produced. In the final build a specially made pinion bearing sleeve, a new seal and pinion nut are fitted and the pinion bearing rotational torque is set using special tooling. The crown wheel and carrier assembly are then fitted.

The plug in the differential casing used for oil filling and level checking can be a problem to remove as the countersunk square hole in the plug is usually badly worn. This is replaced by a specially made plug with hexagonal head.

A new flange gasket and a set of all steel locknuts that do not loosen with high temperatures are provided with the completed assembly.

To enable customers to easily send their differential to us, we can supply a purpose made box for the unit to fit into and, if required, arrange for collection.

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