Main Damper Tools

Special Collar used to extract the damper from the crankshaft

The special extractors used for removing these dampers are quite rare. In the absence of an extractor use can be made of the starter dog nut, providing provision is made to utilise a spacer inside the nose of the crankshaft. Within the crankshaft nose is the specially shaped head of a long through bolt that at its opposite end retains the bell shaped oil blanking plate in the front main bearing. Refer to Fig 6 and note the head of the bolt recessed in the top of the crankshaft nose. It is necessary to exert a thrust on the inside core of the crankshaft nose but without placing any load on the specially shaped bolt head. If this bolt head is disturbed there is a serious risk of the oil blanking plate coming loose in service and rectification would involve a strip down of the timing case, a problem to be avoided.

The solution is to make a small mild steel collar with an O.D of 1.24 inch an I.D of 0.925 inch and 0.3125 inch long. This collar can be inserted in the nose of the crankshaft once the serrated damper retaining nut has been removed the hub refitted and the starter dog screwed in again and tightened to withdraw the damper from the taper. A little heat applied to the inside of the damper hub before screwing in the starter dog will aid extraction.

Serrated Nut Tool

The usual way that one sees these tools constructed is by hand filing or grinding castellations onto the end of a known diameter size of tube. This type of construction leads to a weak tool, which is also inaccurate. The type of tool shown in Fig 13 and Fig 14 is strong, robust and importantly very accurate, meaning it will not damage the fixtures that it is meant to undo. Furthermore the tool can be constructed using the actual in situ serrated damper nut, as a pattern for the tool.

The tool was made in the following way. The eight serrations of the EB 3227 nut are 0.312 inch wide. A length of suitable 0.312 inch x 0.312 inch square key steel was obtained and eight lengths of 1.20 inches were cut off. It was established that the base diameter of the serrations on the EB 3227 nut was 1.650 inch and a section of steel bar some 9.0 inch long was turned to this diameter. As the serrated nut that we wish to undo is positioned part way down from the crankshaft nose, the tool must pass over the end of the crankshaft. In order to allow this positioning our 9.0 inch long bar was counter bored at one end to a diameter of 1.425 inch for 2.0 inches deep and at the other end a hole needs drilling through the bar at 90 deg to allow a tommy bar to be used as a lever. At this point in construction the bar, which in effect forms a tube at one end, is placed over the crankshaft nose end and up to the face of the serrated nut. Whilst holding the bar in this position the eight pieces of key steel are positioned in the serrations of the nut and secured to the shank of the tool with a hose clip. The assembly is then withdrawn and the eight individual pieces of key steel are tack welded to the tube. It will be noted that the steel fingers must overhang the body of the tool by at least 0.750 inch to clear the crankshaft nose.


Damper Poundage Lever Tool

An example of this tool is shown in Fig 15, in this instance it is shown with five locating holes which provide extra angular positions, only three of which will be needed. In service it is permissible to drill a number of alternative holes to suit the operator. The crucial dimension being the 17.5 inches between the centre lines as shown. The lever needs to be used together with a spring balance with a scale of at least 0 - 25 lbs.

A couple of tubes threaded appropriately either BSF or UNF according to the damper can be made to extend the crankshaft gear studs as shown in Fig 16. If required the holes in the lever tool can be enlarged so that the lever can be slipped over the tubes, the tubes being threaded or having nuts welded on one of their ends.

Although the damper poundage slippage rate is critical it is also just as important to obtain a situation whereby the position of slip and stick positions are almost imperceptible. The positions are referred to as slip and stiction. A spring gauge is important as the applied load needs not only checking with a pulling effort but also at one point needs tensioning on the gauge while the damper is held steady. The method of damper loading was considered so important that during one set of R-R Ferodo tests the weight was applied in stages by dripping water into a receptacle position at the end of a lever. It should be obvious that the limited rotational movement available for testing and the requirements of feel make the use of a torque wrench a none starter. It is vital that an absolute known load can be held, slackened off and re-applied smoothly during poundage testing. The difference between an engine damper set in the correct way and one that has been rough set with a torque wrench is turbine smoothness compared with roughness.

Spring Balance

A spring balance of at least 0-25lbs or preferably 0-30 lbs is required to be used with the Lever tool. A simple balance is shown in Fig 17. These gauges are simply spring operated and it is wise to make at least two checks of the gauge against a known quantity and weight of water. Normally the gauges have an adjustment for calibration purposes.

Damper Mounting Mandrel

The crankshaft can be used as a mounting and setting jig for the damper, but the shaft is heavy and large to hang in a vice. It is feasible to utilise the crank in its normal horizontal position, but it is rather awkward.

The alternative is to use a mandrel that can be mounted vertically in a bench vice and which in essence is a copy of the crankshaft front nose damper mounting point. A simple mandrel is shown in Fig 18, in this case having only one locating key made from some 0.187 x 0.187 inch square section key steel. The single key is adequate as it only needs to prevent the friction drum from rotating against a 25 lbs pull on the spring balance.

Note that the damper mounting tracks may be a different diameter to the standard measurements that are shown. In that instance the mandrel will need to be sized accordingly, in short to replicate the actual crankshaft nose measurements.


Crankshaft damage and associated problems

Anyone having read about crankshaft dampers cannot have failed to notice the references to broken or damaged crankshafts, enough to put fear into the average enthusiast. You would be hard pressed to find a broken post war six cylinder where the breakage was caused by a faulty damper, not impossible, but not common. It is not surprising that some specialists have never experienced a broken post war crankshaft. The company themselves did not face such an experience until November 1953.

Over the years, the majority of owners have been conditioned into avoiding high engine speeds but warnings regarding sudden throttling back from medium to high speeds, which have astounding consequences in immediately reversing the stresses, are few and far between. There is no doubt in my mind that most of the crankshaft damage found today is the result of suddenly releasing the throttle and hence gas loads, allowing a wound up crankshaft to immediately and uncontrollably unwind. This condition is exaggerated by the overrun driving inertia of the car, and not helped if the engine is suffering maladies such as a misfire, when the gas loads will be erratic. Office based engineers unfortunately tend to ignore or forget this practical aspect and base calculations purely on torsional vibrations.

Many Bentley owners will have experienced the slackening of carburettor banjo's and the resultant fuel leakage, drivers of all models may have experienced loose sump and dynamo bolts, all a result of suspect crankshaft dampers. In engine rebuild circumstances the enthusiast will notice the effect of undamped torsion vibrations after inspecting the timing gears when the heavy wear on the alloy cam gear will be very evident. It goes without saying that the damper will be found to be unserviceable.

It is difficult for the average owner to differentiate between torsional vibration and an inherent unbalanced shaft and neither problem helps the other. It is normal for the most crankshaft vibration to be felt around 2500 rpm but company engineers had identified particular unbalances during other RPM ranges.

In spite of beliefs to this day, the first engine crankshafts were not fully balanced until late in 1953, and even then it is doubtful if this was done as a matter of course. Under the heading, crankshafts, in the Bentley S1 / R-R Silver Cloud workshop manuals, the term dynamically balanced is used for the first time. Formerly, during the 4.5 ltr engine production, the crankshafts were only generally statically balanced, and this term is used in the appropriate manuals. Only in December 1953 was the finalised dynamic balancing rig instructed for use, initially with all engines fitted with manual gearboxes although the balancing of automatic gearbox dressed engines was also becoming a problem. The glitch here was ensuring that the torus cover was filled with oil and the air was completely expelled.

It is imperative that these crankshafts should be checked for truth and straightened before regrinding. Any shaft bow can only have a detrimental effect on the timing gears, which are dependant on running concentric with the centre line of the crankshaft. Shaft bow will also cause the damper to 'wobble'. The crankshaft position is also affected by main bearing wear and this situation is made worse on the earlier engines prior to the introduction of the 4.9 ltr. Upon introduction of this last R-R six cylinder car engine the maximum main bearing clearance equalled the former minimum clearance, a substantial tightening up of shaft float. Currently available main bearing shells show a tendency to result in even wider clearances and it maybe sensible to regrind the crankshafts slightly undersize to regain the tighter limits. This can be accomplished after checking the dimensions of new shells in a nipped down position in the main bearing caps.


A few known Crankshaft and Damper problems

In the post 1953 period it was known after tests that the then current 4.5 ltr shaft concertinaed at certain RPM and that the end-to-end movement was not in phase. It was also found that the damper pressure plate could be proved to have lifted off the friction surfaces at the normal poundage settings. Of course if the pressure plate lifts off, the friction washers will be partially free to rotate and eventually could easily be envisaged to take up the rather square shape shown in Fig 11. The very important fact that the pressure plate does actually lift off and the clamping load is temporarily removed is taken into account during this description of rebuilding.

The cotton duck washers were known to stick to the friction surfaces and the damper sludged so bad they would become solid. Rigid Ferodo washers were tried together with slotted dampers and some 94 late 4.5 ltr engines left the factory with these dampers. In fact a number of materials were tested. The slots were an attempt to allow oil to centrifuge out of the damper so increasing oil flow across the friction surfaces in the hope of flushing out sludge. The London service centre is recorded as having radially drilled all pre-war Bentley dampers to achieve this aim and also had rebuilt “hundreds of post war dampers”. They also increased the oil flow from the crankshaft to the friction faces by grinding small flats adjacent to the crankshaft nose oil supply as shown at C and D in Fig 6. Although it was suggested the friction surfaces may run dry with drilled dampers the writer found that not to be the case, as did the London service station. On one of the writers tests using drilled dampers with Tufnol washers the oil suction was so great between the faces that by gripping the edges of the Tufnol washer the heavy rear drum could be lifted from the bench. Using an analogy no driver every experienced an oiled up clutch plate shedding itself of oil at any engine RPM and allowing the clutch to continue oil free.

The company engineers reported finding sludge deposits in every chassis engine instance on the inner bores of damper friction washers. Undoubtedly it was the presence of a film of sludge across the friction surfaces and the number of friction faces in action, which caused a stable friction characteristic to elude them. Although it is not mentioned specifically in the archives, these later damper tests were being conducted just after the major oil companies started to introduce ever increasing oil additive packages from around June 1953. It could be that the constantly changing additive packages were also another reason for erratic test results being recorded. It seems equally obvious in today's climate that if synthetic oils are as good as we are to believe, and they are, then their use in one of these engines can do no other than alter the damper slip off poundage.

Full flow oil filtration, together with possibly a modern screw on filter with a finer filter element, and frequent oil and filter changes are definitely to be recommended. Modern oil has a high level of detergency and dispersant ability, sludge deposits are removed and carried in the oil, only to be centrifuged with remarkable efficiency into the interior of the damper. Even if the pressure plate did not move axially and ease pressure from the friction surfaces, sludge would nevertheless find its way onto the surfaces. Wherever oil goes, sludge will follow. Once a film of sludge has been established no material, whatever the wonderful make up, will retain its friction characteristic. A drilled damper and frequent engine oil and filter changes seems to keep the friction faces and the damper very clean, admittedly at some loss of viscous damping, which otherwise might be present with an undrilled damper.

One point I knew could be avoided was the eventual sludging of the damper. I came across my first drilled damper in a post war engine in about 1962, whilst working with ex R-R engineers. They all insisted on drilling dampers across the joint faces of the front and rear damper wheels. On every occasion that the dampers were dismantled for test, their cleanliness was remarkable. I should add that at that time all these vehicles were operated on MIL L 2104 C specification oil, which was, by the standards of the day, very highly detergent oil and used in turbocharged heavy diesels.


Condensation is always present in these engines and eventually water will build up in the bottom of a damper, corroding the pressure plate spoke ends away with remarkable efficiency. This corrosive action takes place on each spoke as they come to rest near the lower position each time the engine stops in a different position. Even if the spokes have not suffered the ultimate damage cause by axial vibration, the ravages of corrosion will finish the job, unless some means of expelling the water is provided. In a seized damper the torque at critical engine speeds will be excessive and even if the friction material has some resilience the pressure plate spokes are unlikely to withstand the sudden torsional shock. A drilled damper will allow sludge and water to be expelled.

Two types of 'Bench' damper were used. These were identical except for the fact that all thread and studs on 4.9 ltr Bentley S1 engine types were UNF threads and earlier Bentley R types were BSF. In addition the rear drum recess diameter that allows the square heads of the six dampers through bolts to locate is slightly different. This allows the different sized square bolt heads of UNF and BSF bolts to locate. Bolts must not be exchanged between the two damper types and due to the vibrations the damper will experience, both square headed bolts and new lock tabs on the nuts must be used. Normal hexagon headed bolts can come loose in this location. Dampers can be exchanged as complete units between engines with 'Bench' dampers. As the crankshaft gear forms part of the working damper and this gear and the camshaft mating gear are likely to be fitted together, take note of the potential timing case foul when using a RE 22149 camshaft gear.

One of the last R-R Experimental Engine Reports was EER 1162 dated 30th November 1955. This report was raised specifically to record the most recent history, at that time, and what steps may be taken in the future. (At that time every Engineer was engaged on SCII V8 work). A few of these comments are listed below, as they appear on the report, and they provide some insight into the minds of engineers at the time.

(a) A slotted damper with the present annular type of friction washer replaced by a number of friction pads of the Ferodo type of material bonded onto either side of the friction drum surfaces. This would allow free oil circulation and avoid the edge filtering action.

(b) Stronger pressure springs in conjunction with annular type washers in a slotted damper with oil channels in the metal friction surfaces.

(c ) Axial vibration stops and the wider spoked spring plate in all slotted dampers.

(d) Alternative friction materials, in particular a sintered metal which is actually being rig tested by Ferodo”

It was felt that the different in poundage between the static stiction and slipping was more important than the sheer poundage setting, in addition that a poundage up to 30 lbs at 17.5 inches was felt to be acceptable providing the static and slip poundage differentials were low. It was reinforced that the poundage must not be below 14 lbs and in fact consideration was given to re-wording the workshop manual at one point. This and previous reports provided some fuel towards my decision to modify and experiment with the dampers.

Much is made of the 14 lb friction damper load applied at 17.5 inch centres by enthusiasts who have plucked this figure from the workshop manual. Of course, if the damper friction surface rotates by whatever small amount, then the pre-loaded radial spring drive must exert an opposite and equal reaction. The force of this reaction is ignored and yet it is governed by springs, which have a tolerance of no less than 25 lbs for a total of four pairs of inner and outer springs. This reaction is at a radius of 1.45 inches, compared with the friction washer reaction at 2.65 inch, nevertheless all these loads add up and few re-builders check spring poundage's let alone match the opposing loads.

We have already established that no damper will handle 3rd order frequency in this case at 5000 rpm plus, but we need to damp vibration at other engine speeds notably around 2500 rpm. The rebuild discussed in this article is aimed at this lower speed target.