N.W.Geeson   I.Eng.   M.I.R.T.E   M.S.O.E    (VER 2.JAN 2010)

History, background and the problems arising

Since developing the rear wheel bearing conversion for the earlier MkVI and R Type and similar R-R models some years ago, many requests have been received to develop a similar conversion for the Bentley S type or Silver Cloud Models.

Only time has delayed such a successful conversion being tried. I have always been aware that the actual bearing measurements are identical on all cars from the C series Bentley MKVI through to the Bentley S3 cars, except for the Silver Wraith LWB and the Phantom IV, V, VI. In each case the bearing was of Metric dimensions.

The wheel bearings of the S type / Silver Cloud chassis range differed from the earlier cars in being fitted with grease seals and seal carriers on each side of the bearings. These seal carriers not only support the oil seal “O” ring in an external groove but they also have an Acme scroll grease return thread on the inner diameter. Unfortunately from a spare parts point of view the seal carriers have “handed Acme” threads, that is, left and right hand threading. These seal carriers can be removed easily if they are left in place whilst the bearing is pressed off the half shaft. If, on the other hand, they are removed to regrease the bearings, which was their original intention they are liable to break unless the correct removal tool is used. In any event they are extremely expensive, as are the bearings.

Original this seal design was to be incorporated on all cars from C series MkVI onwards retrospectively on spare parts, and introduced on S types from new. The scheme was drawn up as (Park Lees) PL 7072 in February 1952. To retrospectively fit this design to early cars would have required an alteration to the existing spacer that fits onto the half shaft before the bearing, the addition of the spacers and finally slight alteration to the old bearing. In the end there was no retrospective action.

The original wheel bearings had a special inbuilt end float and the service manual warned against renewing the bearings just because the end float was present. Unfortunately this bearing end float allows the half shaft splines to end float in the splined bevel wheels. Readers who are in the automotive engineering field will realise that splined shafts resist end float when under driving conditions, but when the splines do slide, they wear the half shaft splines. When the shafts do not slide, they transfer the end load directly onto the axle bevel gear thrust washers.

The rear wheel bearings are about the last components that need renewal on these cars, long after the engines have worn out, however after some 50 years the bearing grease is almost non existent and what remains is not a good lubricant. Even attempting to re-grease the bearing provides no guarantees as the tracks, balls and cages are worn and they fail with little or no warning. Re-greasing a bearing after a considerable period of time, without doubt, will appear to make the bearing revolve smoother. It does not however replace metal that has worn off the bearings and which is captured in its granulated state within the sealed bearing. Metal, which given time will guarantee a collapsed bearing but no guarantee is given as to whether the affected car stays on its four wheels. The only decision you have to make is whether your life and others are worth the cost of the bearings.

Persistent running with slack and partially failed rear wheel bearings will eventually cause the bearing to turn on the half shaft and within the bearing housing. A faulty rear wheel bearing is one cause of failure of the main axle bearings and in fact scrapping, or the need to insert the main axle case and the inner differential gear case. Turning the subject on its head, the failure of the axle side bearings will, and does, fail the rear wheel bearings. Each set of bearings support the half shaft at either end, indirectly through a bevel gear at the inner end, so a complete bearing failure at one end affects the other end.
The initial failure of the wheel bearing is often followed by a clicking noise, which sometimes disappears temporarily when the car is reversed. Unless the bearings are really badly worn it can be difficult to detect actual wear when the car is jacked up with the load relieved from the bearing and the wheels are spun. In fact it is much easier to detect the wear when the wheel is removed as any imperfect movement of the bearing is hidden by the heavy flywheel effect of the road wheel. Once the wheel is removed it becomes necessary to have a good ear and light touch, rotating the hub back and forth feeling and listening for noise and roughness. Along with this back and forth movement the half shaft needs continually indexing around to cover the full 360 degrees of the bearing.

As the tooling and spares are not easily obtainable when out on the road, it is advisable to renew the rear wheel bearings unless the owner knows for sure that the bearings have been changed. Failing that, at least check the rear wheel bearings most thoroughly for any signs of wear.

I can almost hear the average enthusiast saying that he / she is bound to hear the trouble arising before disaster strikes. I am afraid that does not bear out on the facts. As I mentioned initial failure is often fairly quick. At 50 / 60 mph the rest is history!

Conversion Description and detail

The conversion mentioned below is suitable for use with all S type car half shafts, including S1, S2,S3 and their Continental derivatives.

The original rear wheel bearings are still available but very expensive. This original bearing is capable of taking side thrust loads and any replacement type must be a thrust bearing, a normal roller bearing will quickly fail. As the original bearing was metric it makes an alternative scheme fairly easy to achieve. Erring on the side of good engineering principles it makes sense, and has advantages, to fit a dual row bearing.


The owner must realise the usual disclaimers apply when a none original bearing is utilised. In the case of the original, the bearing was a single row thrust bearing which was wider in the centre section than the outer.

The various workshop manuals describe the removal of the half shaft assemblies, which are removed complete with bearings and bearing housings. Fig 1 shows a very old unit to illustrate what the assembly looks like after its removal from the axle.

Replacement Bearing Type This is a F.A.G 3309B 2RSR or 5309 bearing. Fig 2, Fig 3 and Fig 4 show comparisons of the original and new bearings in various states of dress. Note in Fig 2 how the two original side oil seals and carriers make up the width of the old assembly to fit in the wheel bearing housing.

These new 3309 bearings can be sourced with a double-sided neoprene seal, there is also an equivalent 5309 numbered bearing. Both bearings are available with steel seal shields, however due to slight construction differences on the side walls these steel type shielded bearings may foul the bearing retaining collar, unless the collar is relieved locally to the outer diameter of the steel shield. The neoprene seal types are therefore recommended only to overcome the possible fouling problem, but both types are shown here for illustration purposes.

The 3309 bearing is nominally 100 mm x 45mm x 40 mm wide and unlike the original, it is a massive double row thrust ball bearing, the inner and outer tracks being of the same width. The conversion described is based on this 3309 neoprene sealed type bearing. This particular heavy bearing was chosen because it has far greater load carrying capacity than the original bearing and the wider outer track enables the wheel loading to be evenly spread along the wheel bearing housing. In addition the width of the bearing is the same as the housing depth and enables a straightforward replacement when the original grease seal carriers are discarded.

Tolerances

For the record the following tolerances apply:-

Axle shaft BEARING bore diameter = 1.7712 / 1.7717 inch ( 44.998 / 45.011 mm)
Axle shaft BEARING JOURNAL diameter= 1.7723 / 1.7726 inch ( 45.017 / 45.023 mm)
Axle shaft COLLAR BORE = 1.768 / 1.7685 inch ( 44.906 / 44.919 mm)
Interference fit, axle shaft to BEARING = 0.0006 / 0.0014 inch ( 0.016 / 0.035 mm)
Interference fit, axle shaft to COLLAR = 0.0038 / 0.0046 Inch (0.098 / 0.117 mm)


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