The 8-Litre Bentley

This article on the 8-litre Bentley, surely the most representative example of the luxury vintage car, has been written specially for Motor Sport by “Nobby” Clark at the request of the Bentley Drivers’ Club. It has been “vetted” by various experts and is comprehensive and authentic.

This is the fourth of these special articles on vintage Bentleys written for us by the Bentley Drivers’ Club. In the February, 1947, issue the 3-litre Bentley was dealt with. A year later, in February, 1948, the 4½-litre was covered and in the February, 1949, issue, the 6½-litre article appeared. Reprints of the articles were issued to Club members but unfortunately these back numbers of Motor Sport are now out of print.

The present article is the last of this valuable and absorbing series and is in some ways topical, for more than forty 8-litre Bentley cars are still in use.—Ed.

For several years, the readers of Motor Sport have been accustomed to expect the annual appearance of an article from the Bentley Drivers’ Club devoted entirely to one of the models of that world-famous marque, the Bentley. In this article, the last of the present series, it is therefore proposed to cover the development of that extremely successful example to which, perhaps, the adjective “formidable,” in its fullest Gallia sense, is better suited. We refer, of course, to the 8-litre Bentley.

Although only one hundred of these chassis were produced and sold to the public during their short production life, they nevertheless created a mild furore in the motoring world of twenty years ago and the reasons for all this are not hard to comprehend. Let it be remembered that we are discussing design developments of two decades ago, yet here was a normally-aspirated engine, installed in a chassis of orthodox design, bearing the luxurious coachwork of those days at all speeds between ten and one hundred miles per hour with a degree of comfort, silence and stability which one had to experience in order to believe.

Whilst the power unit followed normal Bentley design, by now so well known as to render further description superfluous, the chassis layout was in some respects a complete departure from what had been regarded as standard practice in previous models. Here is a brief description of these alterations:—

In the production of this chassis, the designers catered especially for the heavy, unrestricted coachwork usually associated with the enclosed-limousine type of body and paid due regard to insulation from noise and vibration, together with the provision of adequate leg-room and as unrestricted a floor space as practicable. In fact, the new frame-design can be fairly said to have been developed entirely with a view to attaining these main considerations. Two lengths of wheelbase were adopted as standard, these being 12 ft. and 13 ft., respectively. In parallel with the new frame development, a centralised lubrication system superseded the point lubrication, which had for so long remained is cardinal feature of orthodox Bentley chassis design. This system, in fact, became an absolute essential in view of the extensive alterations to suspension design which were to constitute a feature of the new chassis.

As regards suspension, the designers had for some years been aware that, with the gradual evolution of the luxury carriage from the sports chassis of the early twenties (which had been its prototype), their sphere of activity was extremely limited, while they adhered to orthodox Bentley frame design. With the advent of the redesigned frame, therefore, he was able to increase road-spring lengths without jeopardising the roadholding qualities for which the marque had by now become world famous. The alteration in design, aided by admirable and efficient hydraulic shock-dampers of ample proportions, produced a suspension which, though stable, nevertheless possessed a road comfort range which surpassed all expectations. The power unit, meanwhile, of orthodox Bentley design—and developing approximately 220 b.h.p. at 3,500 r.p.m.—provided an ample power-reserve under ill running conditions. Naturally, however, with an increase in b.h.p. of approximately 45 per cent., it was requisite that the transmission should be modified accordingly. This, then, was duly done and resulted in the production of the last of the famous “lettered” series of gearboxes, namely the F-type box, and this unit, together with a redesigned hypoid rear axle of ample proportions, catered adequately for the extra loads imposed by the additional power.

Brakes of characteristic Bentley size, entirely new design and assisted by a vacuum servo-motor, rounded off a chassis-layout which, even twenty years later, could in truth be said to follow conventional modern practice, except, of course, for the lack of independent front suspension. This, incidentally, would have followed as a matter of course in a relatively short space of time and would have been quite in accordance with Bentley technique and design, had not the original company perished in the economic blizzard which swept the world in the early ‘thirties.

A general specification of the chassis is given below:—

Engine:
110-mm. bore by 140-mm. stroke.
Cubic capacity: 7,982.81 .c.c.
R.A.C. rating: 44.99 h.p.
Six-cylinder monobloc, four valves per cylinder.
Overhead camshaft, connecting-rod driven.
5.5 to 1 compression ratio.
5.1 to 1 compression ratio, using a 3-mm. compression plate.
H-section, machined all over, shell-type connect big-rods.
Tappet clearance: 0.008 in. (hot).
Valve timing:
I.O., 10 deg. b.t.c.
E.C., 12 deg. a.t.c.
Five-gallon capacity sump; oil consumption 1,000 to 1,500 m.p.g.
Gear-type oil pump, 35 lb. per sq. in. at 40 m.p.h.
Self-contained crankshaft torsional damper (Mintex).
Self-contained camshaft torsional damper.
Three-quarter engine-speed magneto (Bosch) and coil (Delco).
Separate slow-running carburetter.
Twin S.U. carburetters, average consumption 11.6 m.p.g.

Clutch:
Single-plate clutch of the 6½-litre type.
Lined centre plate (Halo linings).
Clutch stop.

Gearbox:
Split casing, dog-change for third and top speed.
Insulated mountings.
3rd speed ratio 1.345 to 1
2nd speed ratio 1.792 to 1
1st speed ratio 3.243 to 1
Reverse speed ratio 2.925 to 1
Constant mesh ratio 1.692 to 1

Overall ratios were as follows:—

15/53
Rear Axle
Top .. 3.533
3rd .. 4.752
2nd .. 6.331
1st .. 11.457
Reverse .. 10.334

14/53
Rear Axle
Top .. 3.785
3rd .. 5.091
2nd .. 6.783
1st .. 12.275
Reverse .. 11.071

14/57
Rear Axle
Top .. 4.071
3rd .. 5.475
2nd .. 7.295
1st .. 13.202
Reverse .. 11.908

Oil capacity 9 pints—oil recommended, Castrol “R.”

Front axle:
Conventional Bentley design.
3 to 4 deg. pivot lean (trail or castor).
Wheel inclination 1 deg. 30 min.
Toe-in, 1/8 to 3/16 in.

Steering:
Conventional Bentley. Worm and sector type.

Rear axle:
Dypoid gears.
Single rear brake-shoes.

General details:
Petrol tank capacity 20 gallons.
Autovac petrol feed system.
Tyres 7 in. by 21 in.; road revs. per mile, 575.
Thermostatically-controlled radiator shutters.
12 ft. by 13 ft. wheelbase frames.

Electrical equipment:
Dynamo: Smith five-brush constant-current, Type 2 DAC 5.
Starter: Bosch, Type B.N.E.2/12RS2.
Magneto: Bosch Type F.U.6, GF6A and RS/6. Gap: 0.012 in.
Coil: Delco Type M.R.S.12. Gap: 0.026 in.
Headlamps: Lucas P.100, DB.
Single-pole wiring, negative earth.
Sparking plugs: K.L.G. K.1, M.30.
Gaps: Magneto, 0.020 in.
Coil, 0.025 in.
Ignition timing: Magneto, 45 deg. b.t.c.; Coil, 47 deg. 30 sec.

Operational instructions for the 6½-litre engine were, for all practical purposes, identical with those for the 8-litre engine, with the slow-running carburetter as the one exception of any note.

This device, consisting of a single diffuser unit mounted centrally on the induction-pipe between the twin S.U. carburetters, feeds mixture through feed-pipes to each unit, entering the induction-pipe just above the top of the butterfly-throttles. Petrol is fed from the rear carburetter to a small well, situated under the diffuser and thence through a calibrated jet. Here, in conjunction with a correctly adjusted air-shroud surrounding a miniature choke, the mixture reaches the induction-pipe via the connecting mixture-supply lines. The correct operation of this slow-running device renders superfluous any interference with the main mixture, once this has been properly adjusted.

Another device, not previously encountered in the range of Bentley models already discussed, was the thermo-statically-controlled radiator shutters. In previous models, a thermo-operated valve in the coolant system isolated the radiator from the cylinder-jackets (except for a predetermined by-pass to prevent radiator freeze-up, and, by so doing, maintained a 75 to 90 deg. C. temperature control.) The obvious disadvantages inherent in this system were entirely overcome by the introduction of the thermo-controlled shutter system, the one fault here being the tendency of the shutters to jam, as a result of the entry of flying stones into the operating mechanism. That trouble was, for all practical purposes, eliminated by the designer, principally by due attention to the form of shutter-pivot anchorage. These shutters, incidentally, were balanced, so as to counteract the effect of wind pressure at high road speeds.

In earlier 6½-litre engine design, the crankshaft torsional damper was of the metal plate, multi-disc type and was lubricated from the engine. In view, however, of the difficulty experienced in maintaining the correct damper-loading, this type was superseded by the “Mintex” version, so called since its drive-plates were made from the well-known frictional linings bearing that name. These maintained constant loading and required a minimum, of attention. Together with this modification, the dynamo drive from the crankshaft, complete with its mounting, were redesigned in the light of racing experience, gained the previous year.

This drive, coupled to a Smith constant-current dynamo, provided a source of power-generation which ultimately proved not only completely reliable, but fully capable of withstanding the duties imposed upon it.

The road speed of the standard, saloon-bodied chassis, using a 3.53-to-1 rear axle ratio and 7 in. by 21 in. tyres, was 104 m.p.h. at 3,500 r.p.m., and, in truth, these chassis could rightly have been dubbed “The 100-m.p.h. 8-litre chassis.” Considering that one hundred of these superb motor cars ultimately reached the hands of the motoring public and that it is quite usual to expect certain alterations to design in the first few machines delivered, the number of actual modifications was exceedingly small and these were relatively unimportant, apart from the introduction, at engine number YR 5099, of the strengthened electron crankcase. This modification was introduced to eliminate the tendency for “creep fracture” generation in the crankcase cylinder-foot face. The percentage of these “creep fracture” developments was extremely low and has received more publicity than the fault actually deserved. The strengthening took the form of extra internal ribbing in the affected region. Of interest to enthusiasts is the fact that the front axle beds on the earlier models were machined on the spring-palms at 90 deg. to the pivot-pin centre line, the requisite castor-angle being obtained by appropriate packing between axle and spring. On the later models, the 3-deg. castor-angle (or pivot-lean) was machined into the spring-palms. This point should be carefully checked in the event of an axle change, in order to avoid steering complications. A further point is the correct assembly of the rear joint of the steering drag-link. This joint is permitted 0.010 in. to 0.015 in. end-float, under the pressure exerted by the loading springs. This reduces the tendency for road shocks being transmitted to the steering-wheel via the worm segment and worm. Back-lash in the latter assembly should be reduced to the absolute minimum of the adjustment provided.

The rear hydraulic shock-dampers were the most efficient of their type available at that period and, apart from their robust nature, provision was made for both internal and external adjustment to suit all conditions.

The recommended maximum engine revolutions per minute on indirect gears are as given below:—

2.533 ratio:
3rd speed: 3,500 r.p.m. — 77 m.p.h.
2nd speed: 3,000 r.p.m. — 49.5 m.p.h.
1st speed: 3,000 r.p.m. — 27.34 m.p.h.

3.785 ratio:
3rd speed: 3,500 r.p.m. — 72 m.p.h.
2nd speed: 3,000 r.p.m. — 46.2 m.p.h.
1st speed: 3,000 r.p.m. — 25.5 m.p.h.

4.071 ratio:
3rd speed: 3,500 r.p.m. — 67 m.p.h.
2nd speed: 3,000 r.p.m. — 43 m.p.h.
1st speed: 3,000 r.p.m. — 23.7 m.p.h.

Enthusiasts should note that the following components are fabricated from the magnesium alloy “Electron” in place of the aluminium alloys previously standardised.

Engine:
Camshaft casing, top and side covers, bearing caps, drive bearing caps, crankcase, bearing caps, oil pump body, sump, dynamo drive housing.

Gearbox:
Casing and cover.

Steering:
Casing, covers and top bearing.

Rear axle:
Rear cover and fillet casting.

Frame:
Dashboard.

Finally, one can only conclude this short story of the 8-litre by saying that, if the development of this chassis had proceeded normally and according to accepted Bentley practice, its record, both in the field of international sport and national motoring would have easily surpassed the brilliant records of all its predecessors.

As with its smaller brothers, perhaps the best testimonial to the design and manufacture of the 8-litre is the large number of cars still in every-day use. One-third of the total production is in the hands of club members alone, and a list of some 40 cars and the latest information as to their whereabouts is set out below:—

Chassis No. Reg. No.
YF 5000 GK 672 *
YF 5002 GK 706 O. A. Batten.
YF 5004 GK 8447 *
YF 5006 GN 199 *
YF 5010 GO 4010 J. B. Hilton/C. Ree.
YF 5011 JK 2 H. E. F. Parkinson.
YF 5013 GT 8778 *
YF 5015 ? D. Cameron Peck (U.S.A.)
YF 5018 GN 5182 Major W. T. Pitt.
YF 5019 GO 1567 K. J. G. Reimers.
YF 5021 JN 1333 D. G. Bird.
YF 5023 GP 31 R. E. Carr.
YF 5024 MG 4200 H. L. Wolfe.
YM 5027 GY 7850 R. H. Markes.
YM 5029 GI 8777 S. H. Richardson.
YM 5032 GN 4110 *
YM 5033 GX 6162 R. Dutt.
YM 5036 GX 2338 *
YM 5037 GP 8271 A. A. Stuart Marshall.
YM 5039 WD 4499 (In Canada).
YM 5040 CD 7679 C. R. Townsend.
YM 5042 KJ 3154 J. Ellis.
YM 5045 GX 2423 (Now fitted with Diesel engine by Leyland Motors).
YM 5047 GY 6838 G. Courtney.
YM 5050 UL 7 O. H. Goodwin.
YR 5085 ? G. Rivas (Mexico).
YR 5087 GP 5 M. C. Crowley-Milling.
YR 5088 SM 8794 R. A. Henderson.
YR 5093 DV 9354 C. M. P. Burgess.
YR 5095 GP 401 R. C. Presland.
YX 5112 — Dismantled.
YX 5117 GY 88 A. E. Waller.
YX 5118 GX 6815 J. C. Badcock.
YX 5120 — Dismantled.
YX 5121 GW 2926 F. Lycett.
YX 5122 DS 2270 J. E. Foden.
YX 5123 GW 1559 G. Ratcliffe.
YX 5124 GX 8867 C. R. Townsend.
YX 5125 Unregistered Is now in a London Garage (not having run under its own power) and is in the same state as when it left the works nearly twenty years ago.
? JNE 901 G. D. A. Price.
? ? Briggs Cunningham (U.S.A.)
? MH 1030 Major Stewart (possibly first experimental 8-litre).
* Known to be in existence since the war, but present owner and whereabouts unknown.

We can only therefore, in conclusion, state that this country in general and the motoring world in particular, were the losers by its untimely passing in those by now far away days.

No article on the 8-litre Bentley would be complete without reference to Forrest Lycett’s famous car. Most readers know something about this car and it is hoped that a complete history of its development and performance will form the subject of a separate article at a later date. Another car which is of interest is the ex-Barnato-Hassan, now owned by Gerry Crozier, and we add the following notes thereon:

The Barnato-Hassan
Though this famous Outer Circuit car first appeared in the 1934 500-Mile Race at Brooklands, then propelled by the old 1929 “Le Mans” 6½-litre engine, it is in its 8-litre guise, as it was raced the following year, that our interest lies.

As prepared by Walter Hassan, and driven by Oliver Bertram to take the lap record at Brooklands August meeting 1935, at 142.6 m.p.h., it had the same specially designed frame and 8-litre Bentley engine as now used in its present road form.

When last-raced as a Track car in 1938 it had special tubular connecting-rods, as employed now, and Martlet pistons giving an 8.7-to-1 compression ratio. Three S.U. carburetters were employed and a slide-type throttle, which for her present road work has been discarded in favour of the normal Bentley method of operation.

The cylinder head and ports were polished, and a second Bosch magneto replaced the coil half of the ignition system. The engine sump was supplemented by an additional container underneath with two extra gallons.

In this form it scaled, with its single-seater body, without fuel, 20 cwt., compared with 32½ cwt. in its present-day two-seater (almost) road-equipped state.

Some of the main modifications carried out by Ian Metcalfe during 1947/8 were the decentralising of the steering wheel, to accommodate the specially made two-seater body; the fitting of front brakes made up to Lockheed patterns; the additions of a C.A.V. starter motor and an 8-litre Bentley flywheel to provide the teeth; a standard Bentley dynamo; and headlamps which once graced the 1928 Birkin “Le Mans” car (these have since been replaced by smaller ones and, we hear, are due to reappear on its “halfbrother”—the ex-Pacey-Hassan.)

New pistons were made to the original Hassan pattern and the fitting of a 4-mm. compression plate reduced the compression ratio to 6.7-to-1.

A Tecalemit filter was incorporated in the fuel line from the new 52-gallon rear petrol tank, and a spare wheel slung in the rounded tail immediately behind.

At the other end the propeller-driven compressor mounted in front of the radiator, and behind the grille, was retained—a useful legacy from the Track period—thus keeping the three thirsty S.U.s well supplied with the essential fluid.

The above-mentioned grille, after coming in for a fair amount of criticism, has now been remodelled.

Its racing history is as follows:

1935
Class B Brooklands lap Record … 142.6 m.p.h.
All Comers Lap Record (Brooklands) … 142.6 m.p.h.

1936
Gold Star Handicap Retired

1937
Second October Short Handicap … 137.58-m.p.h. 3rd
Second October Long Handicap … 120.25 m.p.h. 1st

1938
First August Long Handicap … 131.74 m.p.h. 2nd
Second August Long Handicap … 131.90 m.p.h. 2nd
Dunlop Jubilee Lap Race … 134.95 m.p.h. 4th
First October Long Handicap … 131.49 m.p.h. Unplaced
Second October Long Handicap … 132.27 m.p.h. Unplaced
Brooklands Trophy Race of the B.R.D.C. … 133.58 m.p.h. 3rd

The third lap in the 1938 Dunlop Jubilee Cup Race is recorded at 143.11 m.p.h., though this was never officially recognised as the Class B lap record, which it still holds at 142.6 m.p.h.

It was authoritatively stated at the time that its non-appearance in 1939 was due to the impossible handicap applied to the car, which, in spite of its phenomenal lap speeds, precluded it being placed. And another thing—each race used up a set of tyres which cost not less than £40!