Rear Axles….What Fails and Why (part 3)
R-R Silver Dawn, Silver Wraith. BENTLEY MKVI, R type.
N.W.Geeson ver 1. 2007

As in part 1 and 2, the writer declares quite openly an interest in Bentley and R-R post war axles up to R-R Shadow I, as it is a main line of business. Although the article is directed towards the early post war cars most of the points highlighted may be useful when working on other models.

References to the Silver Wraith in this article apply strictly to the Short Wheel Base (SWB) model and do not include the Long Wheel Base (LWB) car, unless notated specifically.

References to Bentley MKVI normally include Bentley R type, Bentley R type Continental and R-R Silver Dawn models.

Auxiliary Parts

Spring Saddle Rubbers

Once the axle is removed it will be quickly established that the saddle rubbers, which are positioned between the springs, support plates and axle will need renewing. It is almost unknown to find the rubbers good enough to refit and it is also almost a secret just how new saddle rubbers alter the ride on these cars. Fig 1 shows one half of the wider 2.25 inch rubber saddle for the R type and Silver Wraith on the left, with one half of the narrower 2.00 inch Bentley Mk VI type on the right. Each spring requires two rubbers, top and bottom, each being slightly different originally. These rubbers are fitted in steel shield support plates. The shield plates vary in width depending on the model and they are apt to rust away, luckily their construction is simple and they can be fabricated from steel sheet by the average owner who possess a vice, hammer and hacksaw.

Shock Absorber Silent Bloc bushes

You will undoubtedly find that the shock absorber silentbloc bushes are also unserviceable. At the very least it is advisable to replace the bottom ones on each side, two in total, at a minimum. The reason is that these bottom bushes are position in the spring U bolt plate, which has to be removed when changing the axle. In service changing of these bottom bushes is to say the least extremely tedious unless the spring plate is removed again. One example bush is shown in lower of Fig 1 and they can be extremely expensive if sourced from the wrong supplier.

The Bentley R type and Bentley MKVI silentbloc bushes are slightly different, the former are longer on their inner tube by around 0.0625 inch. It is feasible to use the narrower Bentley MKVI bushes on all cars and adjust the bush inner tube length by shimming with steel washers. This shimming method has been used officially for some years when the longer bushes have not been available.

A number of different rear shock absorber linkages have been used. As far as bushes are concerned early types have three bushes, whilst all later MKVI and all R types have two. The bushes themselves are constructed from three separate parts and the rubber is not bonded. During manufacture the rubber is propelled down a tapered mandrel under air pressure and comes to rest between the steel tubes. These bushes are an extension of the road springs and shock absorbers and faulty bushes will degrade the ride.

Brake Equaliser Hanging Link and Bushes

This link supports the rear brake linkage off the rear axle though the medium of two rubber bushes. There is another single large bush at the lower end of this link that connects the link to the equaliser bar itself. The position of the bushes are shown in Fig 2, they are subject to degrading by oil from the chassis Bijur lube system and from time to time they need renewing.

If you really value your life, then the large bush that is mounted on the equaliser bar and the bottom bush should not look like those shown in Fig 3 or Fig 4. Here the large bush has all but reverted back to its former chemical parts and the small bushes are not far behind. At this stage of rubber degrading the potential dangers of having a main link of the braking system in this state should not need describing.

When the large bush is replaced it is necessary to slide the complete equaliser link complete with the bush, as shown in Fig 2, all the way down the equaliser bar from the front end. The bush cannot be fitted from the rear end of the bar. It is advisable to clean and actually paint the bar and let it dry before attempting to fit a bush. Prior to fitting, the bar is then liberally coated in WD 40 and the bush assembly slid smartly down the bar without stopping. It is a brave man who stops when fitting one of these bushes, once stationary the grip of the bush needs seeing to believe and moving the bush from the resting point is not a good option. Fig 5 shows clearly the final positioning of the equaliser bush. This position is important in order to allow for rear road spring deflections and axle movement without such movements applying the rear brakes. The complete equaliser bar, equaliser and the linkage assembly are hung off the inner end of the right hand axle tube and should look as shown in Fig 6.

The position of the trunnion, which embraces the actual equaliser bar, must be above and not below the bottom end of the equaliser hanging link. The trunnion is shown correctly positioned in Fig 2, Fig 5 and Fig 6. Failure to correctly position the trunnion on the hanging link will result in the brake cross rods running uphill to the wheel brake actuators, instead of the rods being absolutely horizontal.

The two smaller bushes on this assembly are mounted on the equaliser link and the bushes are captured on the link by the ends of each link leg being riveted over. In the past these link assemblies were sold as a service assembly and more recently the links have been re-riveted. This past service history is gradually causing a problem, as the old links cannot continue to be re-riveted.

The modification shown in Fig 7 has been designed by the writer to overcome the longer term problem. This link can be reused and changing the small rubbers can be done on the car if required. In this instance the topmost threaded section is cut off short and level with the end of the securing nut, and each end is drilled to accept a split pin, before the link is fitted. Cutting one of the hanger threads short is required to prevent a foul with one of the inner axle tube retaining nuts.

At any time when the equaliser link top end mounting is being removed from the axle tube very special care must be taken. This fixing point comprises a long square headed bolt inserted from inside of the right hand axle tube and retained by a special long hexogen nut, which is only partially threaded. Onto this stub the equaliser top link end is placed and the stub end passed through the top link mounting. A large diameter bevel edged washer followed by a castellated nut and split pin then secures the assembly. Any major damage to the threaded end of the stub square headed bolt will mean that the right hand axle tube will need removing and the square head bolt then replaced.

Axle Breather

The most important part of any of these axles is the drain plug, and its intended use is clear. Providing the axle receives annual oil changes regardless of mileage, these units have an extremely long life. These annual oil changes are vital to remove dirt that has been ingested through the breather…… and mileage is of no consequence.

It is not generally realised that dirt ingress is somewhat proportional to the number of times an axle is warmed up and cooled down. Mileage is almost a secondary factor. Each warm up period results in expanding hot air being expelled from the axle casing, only to be ingested again when the axle cools down. Hopefully during the cool down stage the incoming air will travel by way of the chimney pot capped breather and not through a worn pinion seal or the axle shaft seals. For these reasons breathers need removing and cleaning out periodically and the axle oil certainly needs annual changes.

The breather is mounted at the top of the axle, except on the Silver Wraith LWB axle when it is positioned at the front end of the main case between the top and drive shaft. The breather can be unscrewed from the axle case and the parts are riveted together, it is unwise to attempt disassembly. Cleaning can be accomplished by immersing and soaking in degreaser and blowing through with an air line

The axle breathers changed with the introduction of the Silver Cloud series of cars and Silver Wraith LWB cars. Silver Wraith SWB cars retain the earlier breather.

The early breather had 1 inch diameter threads of Whitworth form at 16 tpi and an internal breather bore of approximately 0.180 inch. The later Cloud type breather was 1 inch diameter with threads of UNS form at 16 tpi and 0.200 inch breather bore. Unless the breather bore is checked the wrong breather can be fitted and because the thread angles are very similar but different, alloy will be shaved off the axle casing straight between the internal axle gear set. Fig 8 shows these two breather types. Notice the smaller diameter breather tube on the left hand earlier RG 6117 Bentley MKVI breather, whilst the later type UG 566 Bentley S1 breather is slightly larger. Both types become blocked solid with dirt over a period of time.

The early breather takes a hexagon socket of 11/16th Whitworth, effectively a 32 mm socket will do the job or a socket 1.200 inch across the flats. The later breather will also accept these socket sizes except that the across the flat socket needs to be slightly larger than 1.200 inch.

Cars that are operated in dusty climates can be modified so that the breather is replaced by a stainless steel hose or similar between the axle and boot floor. The breathing system extending into the cleaner environment of the boot space, where a filter is located. It is important to ensure that any hose used for this modification will not be degraded by mineral oil from the axle otherwise debris from the internal structure of the hose will fall back into the axle space. The hose must have sufficient length to reach the axle during maximum rebound of the axle beam.

Half Shafts

Spline wear

All the half shaft splines on Bentley MKVI, Bentley R type and R-R variants were 0.100 inch wide when new. Worn wheel and axle bearings take their toll on the splines. On these cars very little spline wear is caused by actual driving conditions, only by bearing wear, and providing the bearings are in good condition, the half shaft splines will last 100’s of thousands of miles. Fig 9 shows a typical example of wear on a R-R Silver Dawn half shaft, which has been operated with excessive bearing wear the spline thickness can then deteriorate from 0.100 inch wide to 0.080 inch wide. The axle bevel gears splines into which the half shaft splines are engaged can wear but in general the wear is very much less than the half shafts. This can only be because the bevel gear material and hardening process is superior to the EN19 steel and tempering of the half shafts.

Occasionally one is faced with the horror of a Silver Wraith axle that has been run with the left hand RG 4345 Bentley MKVI half shaft instead of the correct left hand GB 4635. As the Silver Wraith shafts are both longer than the Bentley MKVI / R-R Silver Dawn shafts, there is a shortfall of spline penetration into the axle bevel gear. The results on spline wear are totally destructive, and Fig 10 shows a Bentley half shaft on the left that has been used in a Silver Wraith axle. Note how the splines have only entered the axle bevel gear a short distance, compared with the same shaft on the right that has been used in a Bentley axle.

Fig 11 illustrates the differences between Silver Wraith half shafts and the Bentley MKVI type. The two outside shafts, with masking tape around their centre section, are Silver Wraith half shafts, the two inner shafts fit Bentley MKVI, Bentley R type and R-R Silver Dawn. Silver Wraith shafts are also thicker all the way down the shank compared to the Bentley MKVI type, see Fig 12, the Silver Wraith shaft is on the left side. Both shafts are shown in this view with their bearing retaining collars removed.

Half Shaft Changes and Bearings

Bentley MKVI from series ‘A’ and ‘B’ had shafts that accepted a smaller wheel bearing inner diameter than the later cars, although the outer bearing diameter was identical. These shafts RG 3040 for left hand and RG 3041 for the right hand later were part numbered RG 6977 and RG 6976 respectfully. The Rolls-Royce part numbers for the smaller bearings to fit the early shafts was RG 6974 or RG 6981 and were manufacturers part number 88128. They consisted of an outer track 3.937 inch diameter and an inner track of approximately 1.5205 inch bore.

Later Bentley MKVI from ‘C’ series used shafts with larger bearing shanks; the shafts were RG 4345 left hand and RG 4346 right hand. The bearing RG 5365 or RG 6986 was 1.7717 inch bore, 3.937 inch outer diameter and 1.378 inch width, the manufacturers number being D 88609. Although the dimensions of these late type bearings are always given in imperial, they are in fact, fully metric size bearings. All Silver Dawns, long or short boot and Bentley R types had the later shafts and bearings. Silver Wraith SWB were fitted with GB 4635 left hand shafts and RG 3192 right hand shafts, which always had the larger heavier RG 5365 / RG 6986 bearings.

Fig 13 shows both bearing types, and their identical RW177-2 outer track numbers can be clearly seen, not so evident is that both bearings are inscribed with their R-R parts numbers. More than likely they had both been through internal R-R bearing strip inspection at R-R Derby and then had their part numbers engraved afterwards.

Still evident are the current rear wheel bearing failures; just ten weeks catch Fig 14. Some of these bearings are now some 50 years and the grease has turned to soap. Should anyone suggest to you that these bearings do not fail, walk away, thirty years ago that would have been true, but it is now 2007 as I write, not 1977. The bearings shown in Fig 14 have not been failed by the writer, they were failed by owners and specialists long before I saw them, I just agree they have failed!

All half shafts are stamped with their part number on the flat section of the outer end of the shaft, which makes it possible to identify half shafts before their removal from the car. This is important because a number of RG 3040 / RG 3041 early shafts were converted by fitting adapters to accept the later bearings and in fact some early shafts have been fitted to later cars.
Much later models in the Silver Cloud series and Bentley S1 had at least six major axle alterations during the production lifetime. Taking a visual assessment of the main axle case together with the half shaft part numbers can help to identify certain internal components before committing to strip the axle, and save hours of work.

Fig 15 to Fig 18 show an adaptor and bearing retaining collar that allows a non standard double row 3309 bearing to be used with the earlier RG 3040 / RG 3041. This modified double row bearing can be fitted providing the wide nested type bearing housing RG7021 is fitted to the car. The substitution of the original single row type 88609 bearing with a 3309 double row was discussed in J section ‘Rear Axle and Springs’ under ‘Replacing and Modifying rear wheel bearings’. The earlier bearing housing RG 3176, which has a narrower nesting for the bearing was only used on ‘A’ and ‘B’ series Bentley MkVI cars and most have already had housing swapped for the later RG 7021 type. This late type wide nested housing is actually shown in Fig 18.

Fig 15 shows the adaptor for the early half shafts on the left, with a copy of a RG 588 bearing retaining collar on the right. This RG 588 collar is a smaller internal diameter than the later collars in order to fit the small half shaft bearing shank of the early shafts. As this adaptor is intended to be used only with the modified 3309 bearing it has a flange some 0.205 inch wide Fig 16, which acts as a spacer between the bearing and the existing half shaft spacer that is retained. Fig 17 shows a set of bearings with the adaptors already pressed in, the bearing on the right illustrates the adaptor flange. The adaptors should be fitted to the bearings before the bearings are pressed into the housing or onto the half shafts. Then the bearing assembly is pressed into the bearing housing as show in Fig 18 before being pressed onto the shaft. Fig 16 shows the length of the adaptor from the flange to the end to be 1.530 inches (38.86mm). This is shorter than the bearing internal width and allows the adaptor to fall short of the end of the bearing Fig 17, thus ensuring that the adaptor does not prevent the finally fitted bearing retaining collar, from abutting the bearing directly.

With the appropriate alteration to its length and the removal of the flange this type of adaptor can be used to fit the original later genuine bearings to earlier shafts. The observerant will already have noticed the different internal diameters Fig 15 of the adaptor and retaining collar.

Axle Tubes and Corrosion

Fig 19 Illustrates the outer end of an axle tube that has been sawn off to show the extreme corrosion that can occur in these units. The end flange to which the half shaft and brake assembly are bolted can be clearly seen, as can the spring saddle face to which the road spring is clamped by the ‘U’ bolts. Fig 20 on the other hand shows a spring saddle in closer detail and one in acceptable condition, the hole inside the saddle area is used to locate the head of the spring centre bolt. Compare this good saddle area with Fig 21 that shows a close up view of the axle tube shown in Fig 19. In Fig 22 we can see some notes super imposed on the image of the good tube.

The rectangular hole in the front of the axle tube in Fig 21 was made by extremely light hand force on a large screwdriver. This area had already rusted through and the half shaft could be seen on the inside of the axle tube. Many of these axle tubes are now suffering from very serious corrosion it is caused by the bath tube shape of the saddle bracket. When water enters there is no way out, and if the water contains salt the corrosion is accelerated. It is normal for the bath tube shape dish to fill with dirt. If this wet debris line is allowed to reach high enough to contact the main axle tube, the tube will rust through as in Fig 21. If the dirt line is lower or even none existence it is normal to still detect damage to the lower spring saddle area, as shown in Fig 23.

The joint between the axle tube and saddle bracket is a point of high stress and flexing, it is extremely important that the weld profile is not altered. Any attempt to fully weld around these brackets will result in the weld cracking and the bracket tearing away from the axle, perhaps with fatal results. The original welds were only one inch long and on the inside of the bracket at four points only. This welding profile encourages the area to flex freely without undue strain, and the tubes were originally stress relieved after welding. Owners who locate cracks in the axle tube welds are advised to seek the services of a Lloyd’s coded welder to rectify the problem. The task is beyond the facilities of a normal repair garage.

The moral here is to examine this spring saddle area of the axle tube at each service and at the very least blow out the depression in the bracket with air pressure. Fig 22 shows some pointers to examine. A better solution is to clean out the area and fill the depression with Bee’s Wax, this is available from any Bee keeper, see your local Bee keepers association in the telephone directory. It is preferable to remove the axle tube to conduct this service operation, it is not beyond the expertise of most owners, and it could save your life. The area within the bathtub depression can be cleaned out with the axle intact on the car but access is extremely difficult and the rusted areas really need cleaning out correctly. Providing some care is taken the rusted areas can be sand blasted but a light blasting medium applied with a water jet is the preferable option. Initially the heaviest deposits will require removing by careful hand chiselling.

Fig 24 shows how Bee’s wax can be tapered off by inclining an axle tube during application of the wax, this obviously assists in repelling water, but can only be achieved when the axle tube is off the car. Once the saddle area is clean the steel plate that normally fits between the axle tube and spring saddle rubbers requires clamping in place to reduce the potential loss of Bee’s wax through the bottom hole. The operation of melting Bee’s wax with a blowlamp and allowing it to run into the spring saddle bracket should be undertaken with extreme care. On no account let the wax drip or contact any area, which could become a wick to promote a fire. To be safe, treat this product in the same way as cooking fat, and be prepared for spillage, the wax will almost certainly run out through the small gaps formed by the axle tube and steel plate. Once the wax has been allowed to run into the depressed area it will quickly cool and set, if a tapered finish is required the axle tube must be inclined at an angle until the wax has set. Cooling shrinkage will cause slight cracks and this requires the wax to be reheated to fill these areas.

Bee’s wax is a natural product that tends to melt under the influence of heat from the axle, but only enough to repair any small cracks caused by movement in the axle tube. It is extremely efficient in repelling water and rust, and for that reason R-R used it in the internal body structure of the standard steel saloon. Some owners have used Silicon to achieve similar results and for some enthusiasts it may prove a better option than Bee’s wax given the safety issues. In cases where the axle tubes have been dismounted from the car the wax treatment most likely wins the day for long lasting effective prevention. Wax has a tremendous ability to bleed through and fill gaps, Fig 25, not only initially but it creeps on a continual basis.

Axle tube types

Anyone unfortunate enough to find their axle tubes corroded very badly, could have a difficult time trying to obtain replacement second hand tubes. New tubes generally have been unavailable for some time. Availability is made worse because these axle tubes come in three types. The moral here is, clean out and protect the axle tube saddle brackets now!

Silver Wraith tubes are the longest, and the extra length can be gauged by looking back at Fig 11, which showed the length differences between Silver Wraith and Bentley MKVI type half shafts.

Owners of Bentley MkVI and early short boot Silver Dawn cars should also be aware that axle tubes for their cars are 0.125 inch (3mm) shorter between the main axle case and the centre of the saddle bracket than the later R type and long boot Silver Dawn axle tubes. The 0.125 inch length difference is due to the fact that later cars have road springs that are 0.250 inch wider than the early cars, the spring centre bolt is therefore 0.125 inch further away from the axle casing. Due to the spring width variance the welded saddle bracket itself is also wider on later cars. This width differential also carries through to items such as the spring ‘U’ bolt plate. Fitting the wrong axle tube will induce undue side strain on the spring threaded shackle pins and inherent accelerated wear. The width of the saddle bracket on late cars like the Bentley R type is approx 2.50 inch (63mm) and on the earlier Bentley MKVI type this is 2.25 inch (57mm), the longer Silver Wraith tubes always had the wide brackets.

Unfortunately the axle tubes are handed and cannot be swapped side to side. Each tube has a bottom drain hole at the inner end and this drain hole must be in alignment with the axle drain plug when the tube is fitted. Any misalignment will require the tubes to be repositioned again as this placing eventually governs the final polar location and alignment of the rear brake adjuster and actuator.

The right hand tube only has an extra hole near the top at the inner end to accept the bolt fitting for the brake equaliser link fixture. In addition each tube is drilled in a mirror image fashion at both inner and outer ends and the spring saddle brackets are also welded to the tubes with a 2 degree inclination. This latter point needs to be taken into consideration if ever replacement brackets are fabricated and welded onto the tubes.

The inside of each axle tube and indeed each corresponding half shaft should be lightly covered in high melting point grease to avoid internal rusting and the egress of rusty deposits from the tube drain holes.

A detailed description of the rear springs and relevant data together with information on wrapping of the springs will eventually be included in Rear Axles….What Fails and Why (part 4).