The Sunbeam “Tiger” — a Progress Report
FOR SOME TIME the famous 4-litre V12 Sunbeam Tiger has been an important exhibit at The Midland Motor Museum at Bridgnorth but after a gearbox failure at a Brooklands Reunion had been repaired and subsequently tested at Silverstone, its proud owner Bob Roberts decided the time had come to have the car completely overhauled, a decision prompted mainly by the multitudinous oil leaks which had been troublesome over most of the car’s life. Soon the refurbished car will be back in the Museum and Bob hopes to drive it at VSCC meetings. The chassis and body of “Tigress” has also been purchased from Tony Jones for restoration and when completed Tony will drive this in VSCC events.
I went recently to see how work had progressed. The project has been entrusted to John Merryfield (who was recommended to Bob by the Bugatti Owners Club to manufacture for him some new 57C brake drums and who, although for many years involved in racing powerboats, was earlier involved with the late Jack Moor’s ON `Wasp” and 500 cc Wasp and latterly with Gordon Chapman’s ERA “E”-type), and John Baker-Courtenay, a Loughborough university trained engineer who normally resides in Blackburn and acts as Technical Adviser to the Museum and with whose cars he has been connected for many years. It will be remembered that, after the War “Tiger” with its great career in the hands of Segrave, Kaye Don and Sir Malcolm Campbell behind it, was raced in VSCC events by J. M. James and was then acquired by Sir Ralph Millais. Whilst in the latter’s ownership it threw a rod causing much damage. The engine was rebuilt, using the “Tigress” crankcase (whilst the
original crankcase was sent for welding) fitted with blocks, timing towers and cambox assemblies from “Tiger”. The lining up of these individual parts obviously proved very difficult and the oil leaks were quite serious.
The object of the current rebuild is not to change the character or power of the car but to reduce the oil leaks to a minimum and to make the engine somewhat quieter in operation soda little more reliable although it seems possible it will develop more like the 300 hp it should produce than the 250-280 of recent years. It should be explained that the Sunbeam Motor Co who built “Tiger” as a land speed record car in 1925 (152 mph at Southport), made the second car “Tigress” in 1926 and sensibly laid down a third engine assembly which was never fully machined, all the
parts of which Bob has. Kaye Don put the Brooklands lap record to 132.58 mph in 1930, in one of them. Over the years the vvhippy chassis of Tiger worked on the engine, solid mounted at four points, splitting repeatedly the main bearing webs at the forward end of the engine and continually working out the gaskets between the mounting plate and supercharger mountings.
At some time in later life the engine had obviously been frozen and whereas a cast iron engine would have cracked, the fabricated steel blocks expanded, twisted and took the aluminium camboxes with them thus distorting the alignment of the roller bearings housings for the camshafts. It was to cure such shortcomings and to generally civilise the power unit and other parts of the car that Bob called in the “two Johns”. Incidentally Bob compares the “Tiger” performance-wise with a 275 Ferrari (with which he has much experience), ie some 155 mph and 0-100 mph in about 141/4 seconds, without a limited slip differential. The engine has been three-point-mounted and the six-cylinder blocks lined up by first building up the cambox mounting faces with a Vs in steel plate and then mounting them on the jigs which John had made to machine up the “Tigress” cylinder blocks, and facing them all off to the original design height to give correct timing gear mesh. Incidentally it was found that during the manufacture of the new cylinder jigs, which had tube done from averaged measurements of some 14 used and misused cylinder blocks, that one of Sunbeam’s original driving jigs did not produce quite the correct valve guide to cylinder centre line relationship, although generally the original Sunbeam machining has been found to be of exceptional accuracy and quality. This engine by the way is a 60
deg VI2, not the 70 deg so often stated in the past, with three blocks of two cylinders each per bank, unlike the two-litre six-cylinder GP Sunbeams (“Cub” etc) which had two blocks of three cylinders each, although Bob thinks the later six cylinders used the two cylinder per block layout.
Developments arc in hand to improve the blower manifolding which led the engine from the front, resulting in poor distribution and poor power output from the rear cylinders, particularly when running on methanol. The new manifold is very similar in construction to the original but has one side inlet from each blower instead of the Siamesed original, which is being kept, together with other modified parts, intact. Apart from that the original I-section con-rods, blocks, blowers etc have been retained with the same blower gearing and 7:1 cr. New camshafts slightly larger in shaft diameter are used, again on roller bearings with revised cam profiles and finer followers designed to give quieter operation with springs considerably lighter in weight than the original triple spring assembly (101/2 oz), whilst retaining the same actual valve motion and timing. Considerable design help here was provided by a well known manufacturer of V12 engines.
A new rubber cushioned hub has been made for the blower drive, which is taken from the crankshaft nose, to m,-Itnise the violence the original had inflicted on the crank and keyway over the years. The old dry sump lubrication system had a scavenge pump only marginally larger than the pressure pump, contributing further to rear main bearing oil leaks. To improve this a larger bevel driven scavenge pump has been grafted on to the original oil pump carrier and a somewhat different sump drain assembly feeds direct to the new pump. A new oil tank has been built in the cockpit in an effort to quickly de-aerate oil
and return it warm to the engine as soon as possible after start-up to minimise engine wear under cold conditions. The warm oil gradually heats up the surrounding oil in the tank bringing this into circulation as the viscosity decreases, a system poached from pre-war aircraft practice.
A camshaft driven air pump has been fitted to pressurise the fuel tank as per original, augmented by the existing hand pump, in place or the cumbersome World War Two fuel pump conversion previously fitted. The magneto platform has also after considerable effort been cured of its incontinence.
The blowers retain their greasers for the end bearings but as the housings for these have been bored out to the correct position in relation to the casings and sleeved back to size it is now possible to obtain the same clearance on both tips of each rotor and reduce the end clearance to a reasonable amount of thous.
For starting John has devised an ingenious service trolley incorporating a tank and SU pump to feed petrol to an auxiliary starting carburettor which for starting and warm-up purposes is fitted to the manifold and used prior to turning over to alcohol mixture. A battery and hydro-electric pump from an early Bentley window lift system is also fitted to the trolley, and used to circulate oil through the engine and particularly the camshafts prior to starting, which is now accomplished, not by pushing or towing but by an electric starter fitted to the side of the crankcase driving via a universal shaft from a three-litre Rover steering column and an A35 Bendix pinion assembly mounted on both the front and rear faces of the flywheel housing to a flywheel mounted ring gear. It was necessary to drill only one 1/4 in diameter hole in the flywheel housing wall to accommodate this assembly. The blow-up Millais experience enabled
one cylinder block to be sectioned up. This shows that even in 1925 Sunbeam had good ideas about water jacketing, and used pipes. with jets to direct water to the hottest areas, whilst the welded and dowelled construction of valve guide housings and port to seat fitting is interesting.
The “Tiger” chassis will not require much attention but larger air scoops will be fitted ‘ to the backplates and repositioned to take air to the shoes instead of merely to the brake cylinders. New drums were being made. It had previously been possible to weld up the broken gearbox casing after the Brooklands incident and to make up a new top gear drum assembly to replace the broken one which was alarmingly thin and not original. Incidentally these Armstrong/ Wilson self-change gearboxes were made especially for the cars when they were rebuilt in 1932 at T&T’s by Reid Railton, for Sir Malcolm Campbell (who took the ultimate Brookiands Class C Mountain Lap Record at 76.31 mph in 1934), and are not ex-LGOC ‘bus gearboxes as is sometimes suggested.
The next task will be to get Tigress back to similar order. To conserve what blower and drive spares exist for Tiger, it has been decided to use a single vertical blower set-up similar to some ERAs, inhaling through two SU carburetters as on Tiger.
It will be remembered that this second chassis lay for a long time at an MG dealers in Staines and was then fitted with an eight-litre Bentley engine before the conversion to the 24-litre Napier Lion engine, now in Peter Morley’s Bentley-Napier.
Tigress will be rebuilt using the third Sunbeam engine parts now fully machined, although the Tiger crankcase, which having been sent for welding some 30-odd years ago and thought to have been lost, has now come into the possession of the present owners. — W.B.