When the article dealing with the 3-litre Bentley, contributed by the Bentley Drivers Club, appeared in the February, 1947, issue of this journal, it was visualised that it would be followed by another article similar in character covering the development of its big brother, the 4 1/2 litre. Much research has resulted in the compilation of the following information, which may be regarded as authoritative.

The principal reasons for the entry of the world-famous 4 1/2-Iitre Bentley into the motoring world of 192T were twofold:

1. An ever-increasing demand by the "Bentley-minded" motoring public for just that extra reserve of everything which they had come to value so highly when driving the three-litre.

2. It was considered that the ever-increasing pressure experienced in the field of International Competition would extend the three-litre beyond its capacity and render it less capable of dealing with its opponents ; or that, in attempting to do so the great reserve and reliability for which the marque was even at that time world-renowned, would of necessity be sacrificed.

The first 4 1/2-litre car to be produced was the famous old No. 1 "team" chassis, ST 3001, immortalised in motoring history as a participant in the epic crash at White House Corner on the Sarthe Circuit in the course of the Grand Prix d'Endurance at Le Mans in 1927. It will be remembered that hopes of a runaway victory, on this occasion, were dashed to the ground by the unfortunate triple crash involving the entire Bentley team.

Incidentally, some confusion has arisen due to the similarity of the radiator of this particular 4 1/2 to that of the standard three-litre and, indeed, the radiator-shell from trunnions to filler-cap was, for all practical purposes, identical in shape and contour to its predecessor. The difference was to be found in the overall length of the shell, the design of the mounting, and the capacity of the matrix. The first nine chassis produced had radiators the filler-caps of which were smaller in diameter than those later standardised, although nearly all of them were subsequently modified.

Development of the 41-litre followed the development of the new six-cylinder 6 1/2 litre model, the latter then being in full production, and as might be expected, the two chassis have many features in common, with the result that such components as pistons, connecting-rods, valve-rockers, valves, valve-stem collars, valve-springs, etc were interchangeable.

A noteworthy point, is that pistons which gave a 4.4 : 1 compression ratio as employed in the 61-litre resulted in a compression ratio of 4.8 : 1. if fitted to a 4 1/2-litre. This increase was due to a difference of 8 mm. in the dimension combustion head to cylinder foot of the two cylinder-blocks concerned (6 1/2 litre : 270 mm., 4 1/2-litre : 262 mm.). A word of timely advice to enthusiasts with plans for machining large quantities of metal from the block in search of extra horses. A reduction of 3 mm., i.e., a dimension of 259 mm., is the absolute safety limit to which the mean height of the dimension combustion head to cylinder foot should be reduced. The effect of this reduction is that the cylinder foot thickness is reduced to an average thickness of 9 nun., below which it is inadvisable to go.

From a general design point of view, the 41-litre was similar in character to the 3-litre. Valve gear, magneto and water pump drive, also were basically the same, in dimensional detail.

A brief specification of standard 4 1/2-Iitre chassis may, prove of interest:

Bore 100 mm. Stroke 140 mm. Cu. cap. 4398.24-c.e. Direct metalled con. rods type G.5. Twin S.U. sloping carburetters. Duralumin rockers. Belt-driven fan. Rear axle ratio 3.53:1. Wheelbase 10 ft. 10 ins. Hobson " Telegauge " petrol level indicator. Calculated road speed at 3,500 r.p.m. with 3.53 axle and 5.25 by 21 ins. tyres 92 m.p.h. B.H.B. pistons. Compression ratio 4.8 : 1. Ball-end tappet adjusters. Cone clutch. Well-base rims ; 16 gallon petrol tank. Smith double-pole lighting and starting system. " C " type gearbox. Tyres 5.25 by 21 ins. Low-geared worm and worm wheel steering. Autovac fuel feed.

Incidentally, although the 30ft. 10 ins. wheelbase was adopted as standard, eight chassis were built with a wheelbase of 9 ft. 9i ins., these being to special order.

The first modification of any importance made to the chassis was the incorporation of thermostatic water temperature control in the late autumn of 1927 (at chassis No. SL 3071). This was followed by the change over from the rigid spoke type steering wheel to the spring wheel which had proved so effective under rigorous competitions (at chassis No. RN 3035).

1927 saw the 4 1/2-litre Bentley entering international competition in no uncertain manner. "Old Number1" was entered, as previously mentioned, at Le Mans in 1927 and shortly before the crash gained, the lap record at 73.40 m.p.h.

It was driven on that occasion by (lenient and Callingham. It appeared again at the Grand Prix de Paris on August 19th and despite extremely bad weather conditions, won the race at 52.50 m.p.h.. handled on this occasion by Clement and Duller.

The winter of 1927-28 saw the adoption of the Ki-gass injector and the long range type E.R.4 M.L. magneto as standard equipment. These two improvements considerably reduced the bogey of difficult starting and the consequent drain on batteries, usually experienced with large bore engines. These changes in specification became standard at the end of February, 1928 (at chassis No. HF 3192).

In an endeavour to reduce the lubrication problems of owner-drivers and, at the same time, to reduce the wear and tear experienced with the "plunging joint" type of cardan or propellor shaft with its blocks and slippers, much development work - the direct result of high speed track experience-resulted in the adoption of the enclosed type, balanced splined propellor shaft as standard for all models. All chassis from No. NT 3133 were fitted with the Hardy Spicer propellor shafts.

The next real step in the development of the 4 1/2-litre chassis was the design, development, and production of the now famous "D" type close ratio gearbox. Up to this time all chassis had been fitted with the "C" type general purpose gearbox.

In view of the popularity of the close ratio "Aj" type box as fitted to the 3-litre speed model and the absolute necessity for these ratios in competition work, an altogether new design of gearbox made its appearance This, of course, was the "D" type and although completely re-designed to cope with the increased torque of the 4 1/2-litre engine, it retained the well-tried indirects of the "A" type box, whilst eliminating those danger points which had given trouble from time to time in the form of rapid wear in gears, main shaft keys, speedometer drives, etc.

It is a noteworthy tribute to the foresight and skill of the designers of the first truly sporting car to be produced after the 1914-18 war that the ratios calculated to be correct for the first 3-litres were re-introduced in 1928.

In the "D"-type gearbox the 7 pitch gear superseded the 7/9 stub-toothed gear of its prototype-the "A" type box : 5 pitch gears may be found on a few "A " type boxes and 5 pitch gears were also used in the Birkin Blower 4 1/2-litres. These special purpose gears were few in number, the standard 7 pitch gear being used in the majority of cars for competition work and for all standard chassis incorporating this box. Other features of the " D " type box were a ribbed casing of sturdy construction, multi-splined main and lay shafts of reduced overall length to avoid distortion under maximum loads, a speedometer drive re-designed and located similarly to the "C" type box, on the near side of the casing, and an external oil filler.

The whole design proved itself in practice to be most reliable and, for a "crash type" or non-synchromesh box, retained its silence in operation under competition conditions for extremely long-periods, being absolutely trouble-free as a unit throughout its history. Chassis No. HF 3196 inaugurated the "D" type gearbox as a standard unit for all 4 1/2-litre chassis.

Around this period several minor alterations in design were introduced, amongst which were the adoption of the .44 carbon steel brake drums in place of the hitherto standard low carbon steel units, in an endeavour to increase the wear-resisting properties of the drum against the loads imposed upon them by braking from high speeds.

Gudgeon-pins of the floating type were superseded by cirelip-secured pins, but in view of the trouble experienced with scored cylinder-bores by reason of the clips coming adrift, an early return was made to the original design of floating type pins. Incidentally, the whole secret of success with the type of gudgeon-pin that employs the conical aluminium alloy pad method of location-apart from the use of the correct alloy for the pad-lies in the maintenance of point-contact with the cylinder wall by the conical pad. As soon as the area of contact with the cylinder wall exceeds for all practical purposes "point-contact", there is a tendency for the pad to rotate in the gudgeon-pin bore with disastrous results to the cylinder walls-as many amateurs experienced to their cost. It is, therefore, of paramount importance in design that the angle of the conical buffer or rubbing pad is such that it precludes the possibility of the outer edges of the cone coming into contact with the cylinder walls, until the wear-factor on the tip of the cone is such that replacement of the pad becomes essential. Further points worthy of note, if readers fit replacements, are:

1. A small air-release hole should he drilled in one pad to prevent pressure-generation in the tubular gudgeon-pin and a consequent forcing from position of the pad.
2. The bore of the gudgeon-pin should be most carefully finished to ensure the correct interference lit between pad diameter and gudgeon-pin bore.
3. The interference fit should not be excessive if distortion and local swelling of the ends of the gudgeon-pins is to be avoided.
4. The length of the plain portion of the pad should not be less than I5 mm. for this diameter gudgeon-pin.

Le Mans 1928 heralded the reappearance of the old veteran chassis No. ST 3001. victim of the White House crash in the previous year, this time driven by Itarnato and Rubin, hut supported by two new 4 1/2-litre chassis Nos. KM 3077 and KM 3088. These were fitted with distinctive Vanden Plas bodies designed to conform to the stringent Le Mans body regulations in force at the time. They were easily recognisable by the position of the tank and spare wheel carrier at the rear of the body and were to become international celebrities during the course of their racing careers. It was No. KM 3077 in which Tim Birkin. in partnership with the veteran Jean Chassagne. experienced the burst tyre episode which relegated them to the fifth place in this event ; there was nevertheless a degree of consolation in the fact that they broke the lap record with the creditable speed of 79.73 m.p.h.

KM 3088, driven by Clement and Benjafield, retired through a water leak. It is not generally known, however, that the chassis frame was broken and this, if not the direct cause, was a contributory factor in the elimination of the car from the race because of a split or displaced hose joint.

ST 3001 won the race at an average speed of 69.11 m.p.h. This ear, too, had a broken frame and covered the last lap without water, a hose having pulled off! Heavier frames were incorporated the following year.

A brief specification of these cars will accordingly be of interest and was as follows:

Compression ratio 6.1:1, Hour-glass pistons. 3m.m. removed from foot of block. BM-3480 camshaft. .019 tappet clearance. Twin S.U. carburetters, Cone clutches in two chassis, plate-clutch in KM 3088. Close ratio gearboxes. Direct-metalled, heavy-section con-rods: rear axle ratio. 3.3:1. Straight-tooth gears. 31 ins. by 5.35 Dunlop road racing covers and tubes. Road-speed at 3.500 r.p.m. with 3.3 : 1 rear end ratio and 31 ins. by 5.35 tyres. 100 m.p.h.

The following important changes in design received their baptism of fire during the 1928 racing season:

(a) The self-energising front-brake shoes, commonly known as "servo-shoes."
(b) The single-plate clutch.
(c) The vertical type S.U. carburetter. Type HV G.5.

These units were the outcome of development over a considerable period. carried out in order to cope with the exacting conditions of international competition.

It was during the 1928 season, too, that the reinforced H-section front axle-beam was introduced. The sectional thickness between spring-pad and swivel-pin eye. in this axle, was increased in order to counteract the increased brake-torque occasioned by the new servo-shoes. These re-designed axles did not. however, become standardised until May 1028. at chassis No. KM 3092.

Three 4 1/2-litre Bentleys were entered for the Six Hours Race at Brooklands on May 18th that year. Their race-numbers were 4, 5 and 6 and they took eighth, third ami sixth places respectively, carrying off the Team Prize in the process. No. 4 (Old No. 1) was driven by Barnato and Clement and. despite some brake trouble, finished at 70.29 m.p.h.. No. 5 by Tim Birkin (72.27 m.p.h.) and No. 6 by Benjafield and Rubin (71.26 m.p.h.). Some aspects of Le Mans 1928 have already been discussed and it will additionally be recalled that Barnato and Rubin brought "Old No. 1" over the line in first place at 4 p.m. on June 18th.

Up to this period the compression ratio of the standard engine had been set at 4.8 : 1 but. with the introduction of the BM 3622 piston at engine No. KM 3081 this was raised to 5.3 : 1. By employing a 13 S.W.G. compression plate, however, this ratio could he reduced to 5 : 1 thus providing readily available alternative ratios.

The plate clutch, servo-type front brakes and vertical S.U's were not standardised until the advent of the XR series chassis at No. XR 3332 during the autumn of 1928.

The policy of regarding the race-track as the most suitable test-bench possible was pursued with greater intensity than ever and two 4 1/2s in the hands of Tim Birkin and H. Cook, finished 5th and 7th in the T.T. on August 23rd. Birkin appeared again with a 4 1/2 in the Georges Boillot Cup in September and finished 5th, after beating all records for the race with a speed of 73.16 m.p.h. He even took one to Nurburg, a course to which it was by no means suited, and yet managed to achieve 8th place after a drive which drew much applause and favourable comment from those present.

Another modification introduced about this time, was the use of stainless steel in place of aluminium sheet for the cylinder water-jacket plate. This modification, coupled with the use of a corrosion-resisting aluminium alloy for the water pump body, reduced to a very great extent the sludge content of the radiator matrix, which consisted, for the greater part, of the products of decomposed aluminium. The hitherto prevalent bogey of the perforated water-jacket plate was thus eliminated.

In the early autumn of 1928, it was evident that the demand would persist for a 4 1/2-litre chassis fitted with closed coachwork and it was therefore necessary to revert to the "C" type or general purpose gearbox and to fit an "anti-boom" type double silencer to these chassis. Another feature was also inaugurated with the installation of a single-pole wiring layout and this was duly standardised in time for the Motor Show.

Early in 1929, the magnesium alloy known as "Electron" made its debut and replaced, at intervals through the year, components formerly manufactured with the standard aluminium alloys.

Throughout the spring of 1929 various minor modifications were standardised, culminating in the preparation for Le Mans, wherein heavier section frames made their initial appearance.

This preparation followed very much the same line as in the previous year. The engines used flat-topped inlet valves in conjunction with stronger valve-springs.

The remainder of the specification was substantially the same as for 1928.

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 11ft. 6 in. 32-in. by 6 in. road-racing tyres. 45-gallon 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 "Double-Twelve" 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 "Double-Twelve" by winning the race, in the experienced hands of Woolf Barnato and "Tim" Birkin. She averaged 73.63 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 race: 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. Main-shaft, 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 13th, 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. l'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 Bent-leys 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 m.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 6J-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. GF 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.