The 3-Litre Bentley

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[To many people the 3-litre which was evolved by W. O. Bentley just after the Kaiser War and which remained in production for six years, represents the finest sports car in the world. Amongst members of the Bentley Drivers’ Club there are still 209 of these fine cars in use, practically all of them maintained in a condition such as should gladden the eyes even of Mobile Policemen. In spite of this remarkable popularity of a car which has been out of production for some twenty years, confusion still exists over matters of detail. Consequently, we are delighted to be able to publish this article, written by members of the Bentley Drivers’ Club at our request, in which the development of this classic car is explained, the different models sorted out and some of the makes racing successes outlined. Not only will this exclusive data provide pleasurable reading to those who own or otherwise advocate the 3-litre Bentley, but it will also be of interest to those who wish to enhance their academic knowledge of vintage types. More particularly, this carefully compiled article will be of inestimable value to persons who are about to purchase these cars, and who wish to check on facts presented by vendors, and to those seeking information about converting their existing cars in various ways. We were indeed fortunate in being able to draw on the pooled resources of one of the most vigorous and successful one-make Clubs, in order to provide our readers with comprehensive and accurate facts about a very real motor-car. — Ed.]

It was with great pleasure that the Bentley Drivers’ Club received a request from the Editor of Motor Sport for an article on the 3-litre Bentley, and no effort has been spared to ensure that these notes are accurate and correct in detail.

The general conception of the 3-litre Bentley was forming in the mind of its creator before the end of World War I. “W.O.’s” idea was to produce a very fast motor-car, on the principle of a racing-. car detuned as it were, by building into it enough strength and weight to ensure reliability and durability in the hands of private owners, as opposed to “hotting-up” an ordinary car to the point where it became unreliable.

The original experimental 3-litre chassis was erected in the autumn of 1919 in the stable of a Baker Street mews. The engine was assembled upstairs and the chassis downstairs. A nursing-home stood next door and a few minutes after the starting-up of the engine for the first time on the test-bed, the Matron arrived upon the scene. It appeared that a patient was about to shuffle off this mortal coil and she thought that the engine should be switched-off forthwith, that his departure might be more peaceful. This the three operators manifestly dared not do, and it is recorded that the inmate died to the thunder of the first Bentley exhaust. The more rabid enthusiast might acclaim it a fine death.

The engine had to be partially dismantled to allow its removal from the loft to the floor below and, when the chassis was completed, it was fitted with an open-touring body, and road tests commenced during December, 1919.

The experimental chassis, although basically the same, differed from the production model in certain details, the main ones being: The camshaft-housing was a one-piece aluminium casting, the bearings being of the split-bush type, requiring assembly upon the camshaft. This assembly was inserted in the bearing-housings from the front of the casing. Two tappet-covers were situated at 45° to the standard half-cover and the rockers were threaded on to the ground-and-hardened steel rods forming rocker bearings. These were inserted from the rear end of the casing and were held in position by an end-cover, in the centre of which was located the rev.-counter-drive coupling. The camshaft ran in the opposite direction to that of the production model and had two cams per cylinder, as against the three of the production model. The rockers were of steel and one exhaust rocker operated two valves. The magneto cross-shaft drove one magneto on the off-side, and the water-pump on the near side, but provsion was made in the cylinder-block for the subsequent use of two plugs per cylinder, it being foreseen that developments would inevitably lead in that direction. In fact, a double-spark magneto was used to develop this system prior to the adoption of twin magnetos as standard equipment. Dry-sump lubrication was used on the experimental cars, necessitating a scavenge pump and an oil-return tank, the latter being secured beneath the scuttle. Excess oil from the overhead-valve gear was drained back to the sump by external oil drains leading into two large breathers in the crankcase. Pistons were of hour-glass type, in aluminium alloy to Specification L.8.

The cone clutch was a 9-degree plain-ring type and the gearbox the prototype “A”-type with five-pitch gears. The rear-axle ratio, using 820 by 120 tyres, was 3.53 to 1 (53/15). Brakes were on the rear wheels only, but were fully compensated, employing an ingenious double-bevel-type compensator for hand and foot brakes, the cross-shafts of which were fitted concentrically and mounted at each end in self-aligning bearings, so as to nullify the effect of chassis distortion. Several different types of Claudel-Hobson carburetters were used, usually type-G.Z.P., but 48-R.A. Zeniths were also tried. The “80-bore” 4-cylinder engine was, of course, of 80 by 149 mm. (2996. c.c.). 65 h.p. was claimed at 2,500 r.p.m.

Further experimental chassis were constructed at Cricklewood in the early months of 1920, one of which, in the skilful hands of Frank Clement, gained the first of the Bentley’s many victories at Brooklands and elsewhere.

Development work proceeded apace during 1920 and 1921 and as a result one of the first elements of the early design was discarded, wet-sump lubrication superseding the original dry-sump system. A redesigned sump with cast-in air-flow cooling tubes was used, as it had been found that excessive “frothing” of the scavenged oil gave rise to various lubrication problems.

On September 15th, 1921, production chassis No. 1, engine No. 3, was completed and delivered to the public, the brief specification of which was as follows: 9 ft. 9 1/2 in. wheelbase. “A” type gearbox, 3.92 to 1 rear-axle ratio, 820 by 120 tyres, Smith 5-jet carburetter, BM 1800 camshaft (.019-in, tappet clearance). This represented a considerable improvement upon the prototype described earlier. The two-cam per cylinder shaft was replaced by the now familiar triple-cam shaft with separate exhaust-rockers. The external oil drain tubes had given place to internal ones, leading through the water-jackets. Two M.L. magnetos were standardised, the water-pump being driven by a skew-gear from the magneto crossshaft and occupying its existing position in front of the engine. Smith double-pole lighting and starting was fitted and the dynamo was gear-driven from the rear end of the camshaft, geared-up in speed and smaller in size than in later models. The air-flow cooled sump had given way to a unit with a separate oil-tank, practice having shown that the original air-ducts became clogged up with mud and road-grit after a time and were thus rendered useless. This particular 3-litre is still on the road and is in the loving care of R. W. T. Shaw, a member of the Club.

In 1922 Bentleys entered the lists of International Competition, when Douglas Hawkes took one out to Indianapolis for the “Five-Century Grind.” The race took place on May 30th and Hawkes finished 20th, averaging 80 m.p.h. On June 22nd three cars competed in the Tourish Trophy race in the Isle of Man, numbered 3, 6, and 9, driven by Clement, Hawkes and W. O. Bentley, respectively, they finished 2nd, 5th and 4th. Clement’s average speed was 55.21 m.p.h. (No. 6 was the Indianapolis car.) These Bentleys are notable for having been equipped with flat-fronted radiators, giving reduced frontal area and greatly altered appearance. On 27/28th September, at Brooklands, a 3-litre, driven by Duff, took the following International Class D records: 3, 6 and 12 hours, at 88.74, 88.79 and 87.42 m.p.h. respectively. It was not possible to run continuously for 24 hours at Brooklands, but the Bentley covered 2,083 miles in a “Double-Twelve” (86.79 m.p.h.). The 500 and 1,000, mile and kilometre, records and many others were also taken on this occasion. So ended what must be considered a pretty good first season’s racing.

Approximately the first 160 chassis were short, i.e., 9 ft. 9 1/2 in. wheelbase, with BM 1800 camshafts and .019 tappet clearances later closed down to .015 in. for silence (enthusiasts please note). The 10 ft. 10 in, chassis came into being in the autumn of 1922 as a direct result of the demand for a chassis for general-purpose use. A different camshaft, BM 2391, was used, with clearances of .004-in. inlet and .006-in. exhaust. Straight-sided pot-type pistons replaced the hourglass type, and the b.h.p. at 3,500 r.p.m. was 70-72. A 4.23 rear axle was used with closed coachwork and 3.92 with open touring bodies. These ratios were used in conjunction with a new gearbox, the “B “-type, with more widely-spaced ratios than the sporting, or “A”-type box.

A “T.T. Replica” chassis was also produced. It was the forerunner of the Speed Model, popularly known as the “Red Label.” In passing, it may be noted that the Bentley company never adopted the colour of the label (i.e., the oval background to the “B” in the badge) as a means of identifying different models. The specification of the “T.T. Replica” was briefly thus: 9 ft. 9 1/2 in.-wheelbase, “A”-type box, 3.78 rear axle, 5.3 -to-1 compression ratio, straight-sided pot-type pistons, single Smith carburetter, BM 2391 camshaft. The b.h.p. at 3,500 r.p.m. was 78-80. It is interesting to note that the early “T.T. Replica” chassis and Speed Models used the BM 2891 camshaft and that, it was not until the 1926 season that the BM 1800 camshaft was reintroduced.

At this time the manufacturers guaranteed that the long, short and T.T. chassis would attain the following speeds at Brooklands-75, 80 and 90 m.p.h., respectively, and that every chassis would do 25 m.p.g. at 30 m.p.h.

June 1st, 1923 saw Clement and Duff at Le Mans with the 1922 record-breaking 3-litre. It was a private entry assisted by the Works and the Bentley finished 4th, averaging 49.90 m.p.h.

Three 3-litre cars entered for the Georges Boillot Cup race that year on September 7th, driven by Clement, Duff and Kensington Moir. Clement retired with a burnt-out piston; Duff failed to start, having smitten a cow in practice; and Moir managed to finish in spite of carburetter trouble. This trouble was most elusive. In that race a 48-R.A. straight-through Zenith carburetter was used and the mixture gradually became weaker and weaker until at anything below 3,000 r.p.m. the car would not pick up at all. When the engine was switched-off after crossing the finishing line it immediately caught fire — this was soon put out. The interesting part of it was that the screw which went through the outside of the body into a groove on the outside of the choke tube in the top of the carburetter, to locate it, was not long enough and did not grip the choke tube properly and, although the choke tube was quite tight, it is supposed that the heat of the race gradually expanded the body of the carburetter and the choke tube then gradually climbed inside the carburetter and took the whole of the venturi away from the neck of the jet which protruded into the smallest portion of the choke tube, thereby weakening the mixture considerably.

Four-wheel brakes were introduced in 1924, and it is not generally known that, even at that time, Bentleys were experimenting with an hydraulic system. Hydraulics were ultimately abandoned in favour of the existing rod-operated layout. A point worthy of note in favour of the new braking system was the equalising mechanism, which made use of the balanced beam-type of gearless differential, which, when fitted with a damping device in the form of spring-washer loaded bolts, proved eminently satisfactory. The steering was also modified to cope with the new braking system and a shorter drop-arm used to compensate for the additional weight of the extra components. The only other changes in 1924 were the introduction of twin S.U. carburetters in the place of the Smith on the Speed Model, and B.H.B. pistons were standardised, giving a 5.3 to I compression ratio. On the Standard chassis the 45 B.V.S. Smith carburetter superseded the 45 V.S.

Clement and Duff were at Le Mans again on June 20th — and this time they won the race at an average speed of 53.78 m.p.h. for the 24 hours.

Very little alteration in design occurred in 1925. The radiator was raised 1 in. and a header tank of increased size fitted. During this period, the 9 ft.-wheelbase, 100-m.p.h. model was produced in small numbers. It had a raised compression ratio of 6 to 1, the b.h.p. being 82-84 at 3,500 r.p.m. Pressure feed to the twin S.U.s replaced autovac feed and a rather distinctive radiator shell was evolved, reminiscent of that of the Standard “Big Six,” which tapered-in at the bottom. It had a green label. One of these cars was driven by Gallop and Thistlethwayte at Le Mans in 1926. It was chassis No. 1179 and now belongs to E. A. Paxton, a member of the Club, who is busy restoring it to its original condition. This year also saw three World’s Records and thirteen International Class “D” records fall. At Montlhèry, on September 9th and 10th, Benjafield and Duff took the 1,000 kms. at a speed of 97.11 m.p.h., and the 1,000-miles record at 97.40 m.p.h. Then, on the 21st and 22nd, partnered by the Club’s. President, Woolf Barnato, they gained the 24-hour record at 95.03 m.p.h.

In 1926 the Speed Model received further attention. The compression ratio was raised; the BM 1800 camshaft was introduced as standard; duralurnin rockers superseded the steel rockers; b.h.p. at 3,500 r.p.m. was now 86-88 (the Le Mans cars used racing pistons and developed 88-92 b.h.p. at this engine speed); the sump was redesigned, and 5.25 in. by 21 in. tyres became standard, the steering gear being altered to suit. A chassis known as the “Light Tourer” was introduced. With the exception of wheelbase this was almost identical with the Standard chassis. In fact, it might well be described as a “short-chassis Standard “model for use with open coachwork.

A team of three 3-litres went to Le Mans in 1926. They were numbered 7, 8 and 9. The first of these (which was to retain its number for the rest of its life, always being known as “Old Number Seven”), driven by Benjafield and Davis, crashed in the last hour of the race and ultimately finished sixth. No. 8, driven by Clement and Duller, retired with stretched valves and No. 9 (the 100-m.p.h. model referred to above) also retired with a broken valve. In the Spring more records fell to the 3-litre Bentley at Montlhèry, the most noteworthy being World’s Records for 2,000 kms. and 12 Hours, at 100.23 m.p.h. and 100.96 m.p.h., respectively.

For 1927, the “Speed Weyrnann” chassis was introduced. Virtually a Speed Model chassis, it was fitted with a 4.23 rear axle, and a “C”-type gearbox, which ultimately became standardised on the Speed Model. It was designed, as the name implies, for use with a light-fabric saloon body of Weymann construction and was equipped with a double silencer. About this time the valve gear was redesigned, the rockers being fitted to separate rocker boxes for each cylinder. The whole assembly was much more robust than its predecessor, principally as a result of the replacement of the long rocker rod by a shorter rod of larger diameter for each cylinder.

A team of 3-litres competed in the Six-Hour Race at Brooklands on May 13th. Clement and Benjafield, driving Nos. 10 and 12, retired with rocker trouble, but Tim Birkin came in a well-deserved 3rd, at 59.8 m.p.h., having lost the use of top gear after four hours running.

Le Mans, in 1927, provided one of the most glorious events in all Britain’s motor-racing history, and its classic details are too well-known to need recapitulation. Suffice it to recount that, after the entire Bentley team had been involved in a terrible multiple crash at White House Corner just after nightfall, “Old Number Seven” (now bearing No. 3), with shattered wings and lamps, deranged steering and brakes, and a twisted chassis, continued at unabated speed and, coaxed by Davis and Benjafield, roared past the chequered flag perhaps the most splendid First of all time. It had averaged 61.36 m.p.h. for the race.

Little of note in the development of the 3-litre took place after 1927, as work was then in progress on the 4 1/2-litre Bentley, which was to supersede it and worthily carry on the Bentley tradition. During 1926/7 the blocks of Speed-Model engines were machined to allow the use of a 6.1 to 1 compression ratio with hour-glass racing pistons, although the majority continued to have B.H.B. pistons and a 3-mm. compression plate for general-purpose work.

In all, 1,620 3-litre Bentleys were manufactured and this total was made up as follows: –
Short Standard … 205
Long Standard … 779
T.T. Replica … 74
Light Tourer … 41
Speed Model … 507
100-m.p.h. model … 14

Four further chassis were built in the thirties by Bentley Motors (1931), Ltd. from parts taken over from the old Company.

Nearly 200 3-litres, the youngest of which is 18 years old, are still on the road in the hands of Club Members, and, considering that there must be many more outside our ken, this is, in itself, a fine tribute to “W. O.” The 3-litre was the first production of a make of motor-car of whose achievements Britain is justly proud and for which there still exists an enthusiasm the like of which is not to be found for any other marque.

Most of the illustrations for this informative article were kindly loaned by the Bentley Drivers’ Club. Those without captions depict the following (1) Duff’s 3-litre at Le Mans, 1924; (4) An early 3-litre driven by Dr. Benjafield, seen on the line at Kop Hill, and finally, another view of Duff, cornering at Le Mans in 1924, when he won at on average speed of 53.78 m.p.h.. for 1,290 3/4 miles — Ed.