The 6 1/2 Litre Bentley

This authoritative article on the 6 1/2-litre Bentley, both standard “Big Six” and “Speed Six” versions, has been compiled for us with great care and no little research by the Bentley Drivers’ Club, under the guidance of its President, Stanley Sedgwick. It follows similar very popular articles on other Bentley cars, the 3-litre having been covered in February, 1947, and the 4 1/2-litre in February 1948.

The “Speed Six” Bentley upheld British prestige in the nineteen-thirties at Le Mans and elsewhere in a truly noble manner, and, to-day, something like an eighth of the total cars of this type produced are owned by members of the B.D.C. Consequently, we are delighted to be able to reveal hitherto unpublished facts relating to the origin and evolution of these fine cars, and to give useful technical and servicing data. We know Mr. Sedgwick would wish us to say that Mr. R. A. Clarke, of the old Bentley Company, provided the basis for this article, which also has the blessing of Mr. H. Kensington Moir, L. C. McKenzie, and of W.O. Bentley himself. It certainly requires not further recommendations! — Ed.

So much publicity has been attracted by both the famous four-cylinder Bentleys, by reason of the niche which they carved for themselves in the annals of motoring sport during the 1920s, that it is not easy to write an article capable of doing justice to that rather more dignified big brother, the 6 1/2-litre “Big Six,” which was outstanding among the cars of nearly twenty years ago.

Dignified — yes — but let us not forget that it was one of these models which, in the bands of those immortal drivers “Babe” Barnato and “Tim” Birkin, set the final seals on the racing career of the Bentley in those memorable years at Le Mans in 1929 and 1930.

Readers of this journal will not need to be reminded of the more recent achievements of the 6 1/2-litre in the hands of enthusiasts, and future successes will doubtless be chronicled as they arise.

This article was planned three years ago as the third in a series on Bentleys, and it is hoped that it will prove a worthy successor to those on the 3-litre and 4 1/2-litre which have already appeared in Motor Sport.

As early as 1925 it became apparent to the designer of the, by then, world-famous 3-litre, that an entirely different type of car was required, to meet the needs of a different class of motorist. Such a car should have the attributes of a high-speed touring chassis, should be capable of carrying the enclosed coachwork of the time, and should handle like a dignified town-carriage. The development of such a car was no mean task and “W.O.”, ably assisted by the redoubtable “K.M.”, set about designing a prototype based on their experience with the 3-litre. The six-cylinder evolved closely followed the well-tried layout of the 3-litre, but incorporated several new features, of which the more important were: —

Six-cylinder engine, 80-mm. bore by 140-mm. stroke.
Coupling-rod-driven overhead camshaft.
Redesigned frame to carry coachwork of more generous proportions.
Redesigned rear axle gear-case to take a new range of rear axle ratios.
Gearbox with different ratios to suit this class of car.
Redesigned steering assembly.
As far as is known it was the first motor-car engine to be flexibly mounted on rubber blocks.

(The mathematically-minded will have observed that this first “6 1/2-litre” was indeed a 4 1/2-litre!)

Experimental work proved the necessity for various alterations and culminated in the adoption of a 100-mm. bore 6 1/2-litre engine as the standard power unit. The increase in engine size was not entirely unconnected with an unpremeditated “dice,” in France, between the prototype and the first experimental “Phantom I” Rolls-Royce, in which “The Sun ” — for that is the name under which the first Bentley Six was registered — had too little in hand for the liking of “W.O.”, who was driving at the time. The 6 1/2-litre engine developed 140 b.h.p. at 3,500 r.p.m., and its excellent power output at low r.p.m. met the demands likely to be made upon a chassis designed for the dual role of town carriage and high-speed touring car. The specification of the first production models was as follows: —

Engine. — Six-cylinder, 100-mm. bore by 140-mm. stroke, 6,597 c.c.
Four overhead valves per cylinder. Coupling-rod-driven overhead camshaft.
Compression ratio: 4.4 to 1.
Duralumin rockers. Ball-end tappet screws.
Dual ignition by two magnetos.
Thermostatically-controlled water circulation.
Celeron reduction gears, 80 by 60T. Autovac fuel feed. Single Smith Type 50 BVS./C. carburetter.
Clutch.Gearbox. — B.S. type. Indirect ratios: 3rd, 1.278; 2nd, 1.828; 1st and reverse, 3.364.
Steering. — Worm and sector type.
Rear axle. — Spiral bevel gears, ratio 4.16 to 1.
General. — Wheelbase 11 ft. and 12 ft. 3:3 in. by 6.75-in. tyres; 21-in. rims. 19-gallon petrol tank. “Telegauge” petrol gauge. Smith double-pole lighting and starting.
Road speed at 3,500 r.p.m. = 84m.p.h.
Chassis price, £1,450.

The first models had a half engine-speed dynamo, driven from the camshaft and located on the aluminium bulkhead as in the 3-litre, but the majority of these chassis were later modified to the engine-speed dynamo driven from the nose of the crankshaft, the radiator shell being altered to suit. Few, if any, of the original radiator shells are in existence today.

At this point it is convenient to deal with some aspects of the operation of that somewhat complicated, but nevertheless reliable type of camshaft drive, the coupling-rod crank-drive — frequently referred to incorrectly as the “eccentric drive.”

Broadly, the system consists of a helical gear-driven, three-throw crankshaft, having the crank throws at 120 deg., to which are coupled three specially-designed connecting-rods, which in turn are connected to a driven crankshaft of similar dimensions direct coupled to the over-head camshaft. The upper big-end bearings of these connecting rods are fitted with an expansion-compensating device to counteract changes in crankpin centres due to temperature variations, and it is this device at the camshaft end of the coupling rods which appears so complicated to the uninitiated. In the early production models the device comprised four heavy, square-section coil springs per coupling-rod, two on either side of each big-end bearing, so adjusted, by means of suitable spacing-washers and spring pressures, as to allow automatic self-adjustment of the centres of the connecting-rod bearings to suit the alterations in the centres of the driving and driven crankshafts, as the direct result of any expansion or contraction caused by temperature variations in the engine unit. The actual dimensional centre variations are comparatively small, being of the order of from 0.016 in. to 0.018 in.

It was soon found that these heavy coil springs were prone to fatigue fracture under certain engine running-conditions (periodicity) and, although the number of failures was small, they were replaced by a novel substitute known as the “washer drive.” This washer-drive substitute for the spring drive consisted of a tubular steel spool upon which were assembled forty-nine 25-s.w.g. (0.020 in.) spring steel washers, each assembly being dimensionally the same as the coil spring it replaced. The successful operation of this washer assembly depended upon the slight “dish” in the thin spring-steel washers, plus the oil-film between each of the washers, for the necessary spring pressures to compensate for the centre variations. In practice these washer-drive units, once correctly adjusted, remained constant dimensionally for practically the life of the car.

The setting of these drive unit coupling-rods is a simple operation requiring two ground mandrels 4 in. in length, the diameters of which are ground parallel to suit the bores of the connecting-rods (i.e., the driving and driven crankshaft diameters), and a V-footed vernier measuring-rig for measuring between the top diameters of the centre driving crank-pin and the underside of the centre driven crank-pin with both crankshafts on top centre.

The Celeron reduction gear having been correctly meshed, the centres of the crank-pins of the driving and driven crankshafts are measured with the vernier. The two mandrels are now inserted into one of the coupling-rod big-end bearing assemblies and the centres for the crank-pins checked, and so adjusted by means of spacing washers under the bottom pair of washer-drive units as to give the crankshaft centre dimension plus 0.018 in. with the assembly “tightened down” on to a 0.022-in. erecting shim under each washer-drive. The bearing seeming-studs are then filed flush with the top of each securing-nut and are stamped with an “0,” half of which is on the stud and half of which is on the nut, to ensure correct assembly. The connecting-rod assembly is then stripped down and the 0.022-in, erecting shim removed, thus giving the necessary controlled float (0.022 in.) to the top big-end bearing to allow for crank-pin centre variations when the final assembly of the drive is made. The remaining two connecting-rods are adjusted in a similar manner.

Due allowance was made in the design of the reduction gear assembly for any gear-meshing adjustments by the incorporation of eccentrically-machined bearing bushes in the driving crankshaft bearing design. These bushes are flanged; the flanges are slotted and the slots are numbered for reference purposes and are locked by a steel-tab extension from the bearing-cap housing. The movement of these bearings from one slot position to the next moves the camshaft driving crank approximately 0.005 in. into or out of mesh, according to the direction of rotation of the bushings. The total slot movements are:

From to 1, zero; from 1 to 2, 0.004 in.: from 1 to 3, 0.009 in.; from 1 to 4, 0.014 in.; from 1 to 5, 0.020 in.; from 1 to 6, 0.026 in.; from 1 to 7,0.031 in.; from 1 to 8,0.035 in.; from 1 to 9,0.039 in.; or one millimetre travel from minimum to maximum.

Another development introduced with the advent of the 6 1/2-litre was the ball-ended tappet screw, designed to give 100 per cent. valve-tip contact with the tappet-adjuster screw, despite the use of overhead rockers, thus eliminating the centre-punch effect of the orthodox tappet-screw on the valve stem face, and, by so doing, reducing the need for tappet adjustment to very infrequent intervals. These ball-ended tappet-adjuster screws have, however, one vice which presents little difficulty to those with the “know how.” If used in an inadequately vented closed valve-chest they sometimes develop a squeak or “stick” slightly when the car has stood idle for a week or so. This trouble is due to the formation of rust between the ball and socket and can be eliminated by the introduction of a small quantity of paraffin into the offending hollow tappet-screw.

Another refinement used for the first time as standard equipment was the crankshaft torsional damper of the conventional multi-disc type. Fitted to the front end of the crankshaft, this self-contained unit, when adjusted to slip at 60 to 80 foot-pounds, required attention only at infrequent intervals.

A thermostatically-controlled cooling circuit of unconventional design completed the layout of this very efficient power unit. It consisted of two distinct water circulation circuits regulated by a thermostatically-controlled valve of ample proportions. In the “cold-engine” circuit the thermostat by-passed the radiator except for a small leakage to prevent freezing-up. With the engine hot, the valve in the open position allowed the coolant access to the radiator. The whole system of cylinder block circuits was concealed within the cylinder block and the front cylinder-block jacket-plate.

The single Smith 5-jet Type 50 BVS/C carburetter supplied the mixture to a water-jacketed induction pipe of the “Ram’s Horn” balanced-flow type. In view of the frequent queries raised concerning correct jet sizes and positions in the jet-platform, perhaps a few words on this subject would not be out of place. The 5-jet Smith carburetter consists of an orthodox float chamber and float mechanism feeding a jet platform, in which are drilled and tapped five holes to take the five screw-in “pedestal” jets; i.e., four power jets and one slow-running well-jet. This jet carrier is secured to the base of the carburetter, the four power-jets projecting into the port block choke or ports. The port block, cylindrical in form, projects into the body of the carburetter proper, its flanged base being secured to the carburetter base, forming the joint cover of the slow-running annulus machined in the carburetter base fed by the slowrunning tube and the fifth, or well, jet. Mounted on a cylindrical bronze guide, a streamlined air valve slides over the machined cylindrical extension of the port block. This air valve is suction-operated by the depression in the induction pipe, and governs the mixture supply and strength according to engine demands by opening and closing the port openings in the port block leading from the chokes in the base of the port block.

As these chokes or ports are of varying sizes, the jets are of necessity of various sizes, and it is of paramount importance that the correct size jet is fitted to the correctly numbered jet orifice in the jet platform. The jet sizes are: Well, 40/45; No. 1, 50/65; No. 2, 140; No. 3, 120/130; No. 4, 85/115.

A starting device or strangler and a mixture control is incorporated in the design and consists of a cam-operated sleeve sliding over the well jet which, in the “full rich” position, closes the air supply to the well jet, and in the “full weak” position, opens a series of holes in the base of the port block. This carburetter is very reliable and, apart from choked jets, the only troubles likely to occur are: (a) air valve inclined to stick or become sluggish in action, and (b) slow-running annulus choked or orifice masked by new joint.

The steering-box, of the orthodox semi-reversible worm and segment type, was of entirely new design incorporating a meshing arrangement consisting of an eccentrically machined, slotted sleeve bearing for the segment shaft. After removing the securing tab and slackening off the sleeve pinch-bolt, the rotation of this sleeve moved the segment into or out of mesh, according to the direction of rotation. End float was adjusted by the method common to all Bentley chassis, viz., the steel sleeve with inclined slots secured by two pinch-bolts at the base of the box casting.

As in the 3-litre, the brakes were fully mechanically operated, but the front brakes were “push-rod” operated in order to utilize the considerable self-energisation developed by the torsional effect of the brakes on the front axle assembly. The method was a phase in the development of the “reversed action” front brakes used so effectively at Le Mans.

The first 6 1/2-litre chassis (WB 2551) took the road in March, 1926. In frontal appearance it differed slightly from later models by reason of the absence of the casing carrying the engine-speed dynamo driven from the crankshaft, as the dynamo was camshaft-driven at the rear end of the engine.

One of the first modifications was the introduction of the long-range E.R.6 magneto to cope with the extra flexibility demanded from the engine by town-running conditions.

Clutch judder evidenced itself in those cars used chiefly for town work and at first the use of first engagement cork inserts was tried effectively, until the advent of the spring-loaded pressure plate at chassis No. DH 2204 in February, 1927.

The drain on batteries resulting from starting an engine of this capacity coupled with the difficulty in keeping batteries in a fully-charged condition on cars used solely for town work led to the fitting of Ki-gass injectors to all chassis and the development of the five-brush, crankshaft-driven dynamo referred to earlier. The first chassis with the re-designed radiator allowing for this dynamo, the casing of which was secured to the front engine-bearer, appeared at the 1927 Motor Show. The radiator with its fuller profile and deeper (100 mm. section) matrix greatly enhanced the frontal aspect and was to remain a distinguishing feature throughout the 6 1/2-ltre’s career.

Other modifications incorporated in the 1927 Show model, and introduced as standard from chassis No. KD 2121, included a torsional camshaft damper to replace the damping effect of the camshaft-driven dynamo; coil ignition for the first time as a standard fitment to Bentleys; the enclosed-joint, balanced propeller shaft (soon to become known as the Hardy-Spicer shaft) in place of the open shaft and plunging joint used hitherto; and Dewandre servo brakes. These chassis had an enthusiastic reception from discerning motorists and development work proceeded apace. A magneto anti-vibrator was added at chassis No. MD 2649 and single-pole wiring at chassis No. FA 2514. At the same time the camshaft oilbath was introduced to prevent “rocker roller pick-up,” a modification which proved to be the most effective as yet produced to overcome this spasmodic trouble.

About this time (September, 1928) rumours were afoot that there was every possibility that a “Speed Model” of this chassis had been scheduled for development and early production. Much development work was, in fact, proceeding behind the scenes and culminated in the production of an entirely new type of chassis to be known as the “Speed Six.”

The first of these chassis to be laid down was chassis No. WT 2265 and the principal alterations in design were as follows: — High-compression pistons, giving 5.3 to 1 compression-ratio.
Twin S.U. carburetters. BM 7032 camshaft. 0.019 in. tappet clearance.
“C”-type gearbox with indirect ratios — 3rd, 1.357 to 1; 2nd, 1.823 to 1; 1st and reverse, 3.364 to 1.
3.84 to 1 rear axle ratio.
(BM 7055 camshaft with 0.006 in. tappet clearance was available as an alternative to BM 7032 for use with closed coachwork.)

The radiator was redesigned — the sides were parallel whereas the “standard” 6 1/2-litre radiator had a pronounced taper inwards at the bottom — and the “winged B” had a green label.

From a commercial standpoint the “Speed Six” development had to include exploration of the probabilities and possibilities of this car superseding the now hard-pressed 4 1/2-litre in the competition field. Intensive development work was carried out unobtrusively. Air flow tests were made, the cylinder block was redesigned, port areas were altered, and brake endurance tests were carried out.

Eventually the first Le Mans-type “Speed Six,” chassis No. LB 2332, took the road and as its preliminary try-out ran in the “Double-Twelve” race at Brooklands in May, 1929. Although ill-luck dogged the chassis premiere, the dynamo coupling disintegrating when victory seemed assured, the general performance exceeded all expectations. The brief specification of the first “Le Mans Speed Six” chassis was as follows: —

Engine — Hour-glass pistons, 5.8 to 1 compression-ratio. BM 7032 camshaft. Single-port cylinder block. Flat type inlet valves. Five-gallon sump. Large capacity oil pump. Increased oil feed to main and big-end bearings. Heavy-section, direct-metalled connecting rods. “Mintex” crankshaft torsional damper. Twin S.U. carburetters, Type HVG5. Straight-toothed metal reduction gears.
Clutch — Single plate. Steel pressure plate. Reinforced clutch stop.
Gearbox — “D”type. Indirect ratios — 3rd, 1.33 to 1; 2nd, 1.63 to 1; 1st and reverse, 2.64 to 1.
Rear axle — Straight-toothed bevels. (16/48 = 3 to 1 ratio.)
Brakes — Standard.
General — Wheelbase 11 ft. 6 in. 32-in. by 6 in. road-racing tyres. 45gallon petrol tank. Autopulse petrol feed. Duplex fuel lines. Smith five-brush dynamo. 4LSA starter motor. Lucas lamps. Young 84-amp. hour-capacity battery.

The “Speed Six” entered the lists of competition in 1929 and immediately combined with its four-cylinder stable companions to set England’s star higher in the firmament of international motor racing than ever before or, alas, since.

On May 10th, the car which was afterwards to be dubbed “Old No. 1” came to the starting line for the “DoubleTwelve” at Brooklands. It was driven by “Babe” Barnato and J. D. Benjafield, bore the number 2 and, after an excellent performance during which several laps in the region of 92 m.p.h. were completed, retired owing to a fracture in the dynamo-drive. The 24-hour race at Le Mans that year needs little recapitulation to any enthusiast, for Bentleys filled the first four places and nobody else had a look in. No. 1 “Speed Six” more than made amends for her failure in the “DoubleTwelve” by winning the race, in the experienced hands of Woolf Barnato and “Tim” Birkin. She averaged 78.68 m.p.h., covered 1,767 miles in the process and, just for good measure, gained the Rudge Cup as well. It was the first occasion upon which the winner of the Grand Prix d’Endurance had also carried off this cup.

It is most interesting to read the contemporary report of Mr. Clarke on this chassis: —

“(a) During practice: Slight steering instability reported and rectified by balancing the practice wheels and adjusting shock-dampers. Oil pressure-60 lbs.
(b) During practice: Brake adjustment used up at the 20th hour.
(c) After race: (strip report)
Engine: Nothing to report. Exhaust valves and valve springs changed as a precautionary measure only.
Clutch: Nothing to report. Clutch-stop locating ears fractured.
Gearbox: Nothing to report. Mainshaft, first motion shaft and journal bearings changed as a precautionary measure.
Rear axle: Crown-wheel and pinion — slight signs of pitting, otherwise O.K. Pinion thrust-race disintegrated. Otherwise O.K.
Brakes: Relined: Two rear drums changed as a precautionary measure (local hot spot).
Frame: Small fracture through front engine bearer engine securing bolt hole. Signs of fracture where front wing stay palms connected to neutral section of frame channel due to ‘fidgeting.’ “

Truly a remarkable strip report after a gruelling race of this calibre.

On the 29th of the same month, “Old No. 1” was back again at Brooklands for the Six-Hour Race, still driven by “Babe,” but this time with Jack Dunfee as co-driver. It bore the number 3 and again won, averaging 75.88 m.p.h. for the race.

On July 18th, Glen Kidston took the big Bentley over to Phoenix Park for the Irish Grand Prix and came in 2nd behind Ivanovski’s Alfa-Romeo. Its speed was 79.80 m.p.h.

The T.T. that August broke, temporarily, the big car’s run of success, for, in company with Glen Kidston, it ran out of road at Bradshaw’s Brae and was too badly damaged to continue. Its race number, incidentally, was 73.

The final event in “Old No. 1’s” 1929 season was the classic 500-Mile Race at Brooklands. It had a special two-seater body with a short, stubby tail, was driven by Sammy Davis and Clive Dunfee, and came 2nd, averaging 109.40 m.p.h.

Following the racing successes of 1929, the cars at the 1929 Show incorporated the following modifications (in the KR-series chassis): —

Single-port cylinder block. 5.3 to 1 compression-ratio. BM 7055 camshaft. Bosch magnetos. Shell-type connecting-rods. 38.4 to 1 rear axle ratio. Electron steering box and rear axle casing.

At chassis No. LR 2783 the three-quarter engine speed magneto and coil-ignition became standard.

Le Mans in 1930 was to see the final appearance of the “works” team of Bentleys and the cars were, for all practical purposes, identical mechanically with those of the previous year with the following exceptions: —

Engine — 6.1 to 1 compression-ratio. Three-quarter engine speed magneto and coil ignition. Shell-type connecting-rods.
Clutch — Reinforced clutch stop.
Rear axle — 15/47 gears = 3.13 to 1.

In this last season of the Bentley team the “Big Sixes” acquitted themselves gloriously indeed.

Two of them were entered for the “Double-Twelve” on May 9th and 10th, being numbered 2 (Barnato and Clement) and 3 (Davis and C. Dunfee). In shocking weather conditions these Bentleys came in 1st and 2nd, respectively, at 86.68 im.p.h. and 85.68 m.p.h. No. 3 gained its place despite a certain amount of trouble with a seized crankshaft-damper and some (probably consequent) valve-spring breakage.

Three of the big Bentleys went to the line for the last Le Mans of all in June, 1930. They were numbered 2, 3 and 4, being driven by Clement and Watney, “Sammy” Davis and Clive Dunfee, “Babe” Barnato and Glen Kidston, and these last two roared past the chequered flag, 24 hours later, having won the race without in any way extending their car, at an average of 75.87 m.p.h. This was “Babe’s” third consecutive win at Le Mans. Clement and Watney were 2nd, at 73.33 m.p.h., whilst Dunfee shortly after taking over No. 3 from Davis, had the misfortune to charge the sandbags on his first bend, sustaining damage which enforced retirement.

The team, nevertheless, again won the Rudge Cup, in addition to the entire race.

Such was the way of the closing of the greatest chapter which any single marque has ever added to the annals of British motor racing history — and the departure of the Bentley Team from racing left a gap which has not since been filled.

Altogether 544 6 1/2-litre Bentleys were made, of which 171 were “Speed Sixes” and, of these, more than 70 are on the roads to-day in the hands of Bentley Drivers’ Club members, including two of the team cars. CF 8507 (chassis No. HM 2868) is owned by J. D. Percy and is in its original form and beautifully kept. It was this car in which “Babe” Barnato won Le Mans for the third time and, fittingly, it led his funeral cortege bearing floral tributes, driven by “Babe’s” chauffeur. GF 8511 has recently come to light in the north of England and the ravages of time are being removed. MT 3464, the “Old No. 1,” winner of Le Mans in 1929, 2nd in the 1929 “500,” and 1st in that year’s Six-Hour Race, continued her wonderful career until 1931 when she took that fated plunge over the top of the banking at Brooklands killing Clive Dunfee. The engine of this car was used for a time in the Barnato-Hassan before the 8-litre was installed, and other bits were used in a special 8-litre road car built for “Babe” Barnato. Its identity has thus been lost.

The modified “Speed Six” used in post-war competition motoring sport with signal success by Pierre Marechal is one of the very few short-chassis cars (11 ft. wheelbase) and is thought to have been a spare car for the “works” team.

So concludes the story of truly one of the giants of the road, which never fails to impress wherever it appears. The majesty of the “Speed Six” will continue for many years to dwarf motor cars of younger vintage, and the rear view, if not as imposing as the front end, will frequently impinge itself upon the vision of the driver of many a modern car who thought that he himself wasn’t exactly hanging about.