Objective and background

In the past the importance of ensuring that the rear gearbox mounting is kept in good condition, and the symptoms that may be experienced with poor mountings was described in the article “The rear gearbox mounting”.

A number of owners have found it difficult to understand some of the relationships between the actual parts, and also the fitting and adjustment. The individual owners are not helped because the parts come under a number of headings and are in different locations in the parts manuals. A list of parts has been complied and included here to show those who wish to look just where they are listed.

In no place in the manuals will all the components that make up the front and rear mounting assemblies be found illustrated together in an understandable fashion. For the benefit of the owner mechanic this situation has been rectified and the separate front and rear assemblies will be found illustrated within the body of this article under Fig 5 and Fig 13.

It is intended that this article will assist in replacing both front and rear mounting assemblies, including tie rods and buffers, and also may encompass other useful data for the owner mechanic.

Engine and Gearbox general mounting design and fittings

The main front mounting is positioned high up, under the water pump, directly on the front of the timing case. In comparison the main rear mounting is positioned within a channel shaped crossbearer, position to the rear and below the gearbox. The mountings themselves and distances between their centres are identical on both the manual and automatic transmissions, on all EPW cars prior to the Silver Cloud I / Bentley S1 range, except for some, but not all, 4.9 ltr Silver Wraith engines.

These two main mountings take all the circa 800 lbs weight of the engine and transmission, but provide little resistance to any torque, or fore and aft movement. The high front and low rear positions allow a roll centre, or a line drawn through each of the mounting centres to approximately intersect the centre of the engine flywheel. This design is not unique to R-R and was used on US produced cars before WW2. It does however have the advantage of the coupling line passing through one of the heaviest rotating masses at the flywheel and results in the rotating mass being very insulated from the driver.

A disadvantage is that the engine movement, or roll, tends to react around the flywheel centre. To provide a reaction to both power and overrun torque, or twisting, a crossmember is attached to the gearbox centre line and terminates higher up on each side above the chassis line. This connection takes the form of just compressing rubber blocks or buffers between the gearbox crossmember and a chassis bracket on each side. That is without directly coupling the mountings in the normal way. A common name for this gearbox torque reaction crossmember is the “cowshorn” crossmember and this is a reflection of its shape.

This main mounting arrangement and the extreme rear highly mounted torque reaction arrangement also has another major disadvantage, as well as advantages. This position can be seen clearly in an image of a chassis in Fig 14. Under certain circumstances of power demand the engine and transmission move in a corkscrew or yaw path. A path that is quickly reversed when the brakes are used and overrun torque is transmitted in the opposite direction through the mounting. In normal circumstances this undesirable movement is restricted but when the rear main mounting in particular has decayed, usually through contact with chassis lubrication oil, the degree of movement increases. At that point running problems become apparent to the driver, brakes poor, exhaust fouling the chassis, clutch action erratic, a non-compliant gearchange and sometimes erratic throttle action. This action also overloads the tie rod arrangement, discussed below, when the engine corkscrew path can cause bending and breakage of the tie rod itself encouraged by a side to side movement on the rear mounting.

All main fore and aft forces are controlled through a longitudinal small bar, or tie rod, that is mounted between the rear of the gearbox and chassis cruciform. At each end of the tie rod a limited flexible connection is formed by the compression of four circular rubber blocks. The rod arrangement is designed to retain the engine and transmission in its rested or neutral position even under circumstances when high fore and aft loads are applied. The adjustment of the tie rod takes the form of threaded ends and nuts that in turn clamp circular heavy washers and the rubbers to the attachment points.

Part numbers and location

The following list shows the parts book section location and the part numbers.

Section G4.
Rear cows horn bearer and Rear mounting shims part number GB 5266, normally three (3) per chassis. Manual suggests 4 are required.

Section G10.
Tie rod components and RG 3428 rubber spacers, four (4) needed per assembly.

Section U2. Part 9
Gearbox rear cross member shown as part number RF 5222. In fact this is an assembly number for welding 2 x RE 3624 threaded strip to the actual RF 5336 Gearbox crossbearer. Not normally needed.

Section U2. Part 10
Torque reaction chassis bracket, part FB4865, two (2) per car. Not normally needed.

Section U7. Part 3
Front Engine mounting arrangement with RF 727 rubbers, two (2) needed per assembly.

Section U7. Part 5
Main gearbox rear mounting part number FW 1970, one (1) per car, see note about threads.

Section U7. Part 6
Torque reaction rubber buffers part number RE 9001, two (2) per car.

Section U7. Part 6
Buffer holders and threaded adjuster part number FB4537, two (2) per car.

Points to Observe when fitting replacement mountings

There are a number of different ways of attacking these jobs but if the front mounting is being changed it is much better to complete that change before attempting the rear mounts. In the case of the rear mounting no attempt is made here to change the mounting with the cross bearer in the chassis.

Also the description follows the path of changing the mountings with the absolute minimum of component disconnection, for example the exhaust is left untouched. This means that any alteration of the engine and gearbox positions, by jacking up or down, needs doing slowly and carefully, but the job is well within the range of the owner mechanic.

Whenever jacking takes place, which can be on the rear of the engine oil sump, the rear part of the main manual gearbox case, or sump pan of the automatic, a degree of care needs exercising. It is necessary to pack or support under the component being lifted, so that given any slip that the drop or lift of the part is kept very limited, and less than 0.50 inch. To protect the alloy engine sump or automatic transmission pan two thick flat pieces of ply board about 0.50 inch thick, placed between any jack and the jacking point, spreads the weight. Between the actual contact areas it will be found that very thick cardboard provides a useful buffer. Whatever method is selected the importance of inserting packing to keep pace with the jacking cannot be over emphasised. I find it preferable to lift the rear end by jacking under the gearbox and then safety packing at the rear of the engine oil sump.

Front engine mounting access

If you happen to be looking to renew the front engine mounting, examine Fig 1 noting where the blue arrow points. This arrow shows roughly the direction to look for the mount, under the water pump and mounted off the timing case. Accessibility, well it is enough to give you a bad day!

A camera pointed in the right direction may provide some light relief and it could produce the result shown in Fig 2. This clearly shows the mounting rubber in good shape. Provided this mounting has been replaced at engine rebuild time or whenever the radiator has been removed, all should be well. It suffers much less than rear mountings, which get contaminated with oil.

In extreme cases, on early cars with single central steering idlers, it becomes impossible to remove the engine oil sump without disturbing the steering idler because the engine has dropped so much on the mount. On later cars with twin idlers it is not so noticeable unless work needs completing on the lubrication pipes on top of the idlers.

To gain access to the front mounting it is necessary to remove the shell, shutters and radiator. In turn by implication this requires removal of the front bumper, spot lamps, horns, bonnet and of course draining the cooling system at the very least. It is assumed for our purpose that access has been gained because removal of these items will differ slightly on each model.

However a few notes may be useful.

Earth Bonding

The insulated earth return systems for the dynamo was incorporated to deter cooling system silting, there are two main types and these can be identified as shown below. All the earlier type non-insulated return dynamos should have also been modified and the earth strap system should be identical to the later type.

In the early type the dynamo is insulated from its mounting bracket by rubber bobbin ferrules. Although the earth strap connects on the dynamo to a non-insulated terminal the earth strap should be insulated or covered along its length.

In the later insulated system both the output and earth wires egress from the dynamo rear end from insulted terminals and also the earth strap is fully sheathed or insulated. The dynamo earth strap connects to the bottom lower right hand corner of the radiator yoke frame, before finally terminating at a chassis connection adjacent to the right front brake hose. That is your left hand side if you are working on the car at the front end.

It is probable that some previous knife and fork mechanic has altered the connections to the detriment of the cooling system, because they could not gain easy access to the lower right hand earth cable radiator coupling bolt. Owners may find that the earth cable incorrectly couples directly between the dynamo and the chassis, or in some cases no connection exists. In the later case, even on a basis that it does not hurt, it is preferable to drill a 0.250 inch hole in the lower right corner of the radiator frame and form an earthing point.

When the radiator is out, take advantage to correct the situation by cleaning up the area around the radiator earth attachment point and fit a long bolt into this hole onto which are positioned copper washers. Tighten the bolt with the nut facing to the front of the car, the copper washers trapped against each side of the radiator frame with at least 1.25 inch (30mm) of thread protruding through the nut. When the time comes to refit the radiator, pass a length of string through the loose end of the earth wire and tie the string off at a forward point near the anti roll bar mounting.

Refit the radiator, pull the end of the earth wire forward between the radiator and side panel with the aid of the string. Fit a copper washer to the protruding bolt thread, position the earth wire on the bolt, and place on another copper washer followed by a plain washer then spring washer and finally a nut. Tightening the nut will be easy because the bolt will already be held firmly with the initial nut from turning, in short the job can be done with one hand. . This arrangement will aid the removal and fitting of the earth and through copper washers provide first class earth bonding to the radiator. When the job is done, smear grease on the end of any protruding bolt thread.

Radiator Flushing and heater system bleeding

As the radiator must be removed to change the front engine mounting rubbers it is the ideal opportunity to comprehensively flush the cooling system. On no account be tempted to replace the thermostat with a different type, or leave it out of this engine.

The radiator should be reversed flushed to clear water passages at a minimum, taking particular attention to ensure the bottom tank adjacent to the bottom hose outlet is clear of debris. Cooling system silt will gravitate to this point by water pump suction.

Attention to the coolant flow system will show that coolant both enters and exits the heater matrix at the bottom, any air naturally rising to the matrix top. Once the coolant system is refilled it is sensible to bleed out this air to prevent an air lock by running the engine with the return heater hose to the water pump inlet detached. The engine should be run at a fast idle of about 1200 rpm until a full flow of coolant flows from the heater return pipe. Beware of the revolving fan blades, especially if you are using a fluorescent hand lamp.

It is suggested that on no account should the engine operated with either the heater taps, if fitted, turned off, or the heater detached. The coolant flow through the heater matrix provides a very helpful flow through the rear of the engine and cylinder head, and helps prevent the normal situation whereby the coolant flow is stagnant at the rear end.

Bonnet (Hood) Storage

The bonnet (hood) will need handling with care, it can “gull wing” suddenly with no warning and cause a lot of damage, it is better removed with two strong people, making sure thick protection is placed on the top of the radiator shell. To aid storage of the bonnet without damage it is possible to construct two plywood bucks, which are each cut in a half moon shape, see Fig 3. One panel with a large radius to suit the rear of the bonnet radius and one small radius to suit the front end radius. These two panels are flat on their base to represent a line rough below the bonnet line and they are joined together by two length of timber.

An explanation of how the bonnet rests will answer outstanding questions. The bonnet is rested over the two radius panels in exactly the same way as it would rest in the closed position on the car. In essence one could say it rests on the two bucks like a saddle on a horse. The edge of the bucks can be taped to prevent under bonnet scratching. This arrangement was used on production to store bonnets once the two individual bonnet panels had been assembled.