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Aston Martin Third Series, 1934-1935

1½ Litre Third Series.

With the first and second series cars a success and well received, progress continued with the third series of cars. The new car was probably originally envisaged as the ‘Le Mans Mark II’, but was always known as simply the ‘Mark II’. The new design incorporated almost nine years of continuous development. A short 2/4 seater and long chassis variation was produced, with open 4 seater touring, drophead coupe and saloon coachwork built on the longer chassis.

The racing version, the ‘Ulster’, was seen at all major International racing events including at Le Mans, the Mille Miglia, the Targa Abbruzzo, the TT (from which they got their name) and of course Brooklands and Donington Park. The ‘Works’ cars, chassis numbers LM11 through to LM21, became one of the most successful racing teams of the 1930’s, the best result being third at Le Mans in 1935. Twenty one production versions of the works cars were built and all survive but one. Distinguished by a ‘U’ suffix on the chassis numbers (with the exception of the first 4) many are still actively campaigned today.

Bertelli exercised a good deal of influence over the third series cars (particularly the ‘Ulster’) even though more of his time was being taken up running the company. However Claude Hill, who had been with Renwick and Bertelli as far back as 1926 employed as a draughtsman, was rapidly developing into a talented design engineer in his own right. He did a lot of development work on the ‘Mark II’ and the refinement of the original Bertelli designs (the first and second series cars) into the third series ‘Mark II’ owes a lot to his talent.

The Mark II.

The new ‘Mark II’, was almost as different to the ‘Le Mans’ as the latter had been to the ‘International’. The chassis, with much deeper girder construction and of a thicker gauge, was considerably stiffer than the second series chassis. It was particularly heavily braced both in front of and directly behind the engine bay. A cross member with ‘U’ shaped cross section replaced the tubular cross member on the ‘Le Mans’ and an additional similar shaped cross member measuring the full width of the chassis curved directly under the rear of the engine, making this section of the chassis up to the firewall extremely strong. The engine now sat on this rear most cross member on short brackets, and on the top of the side beam at the front on longer ‘H’ section brackets. Both front and back mountings were on ‘Silentbloc’ bushes. The engine was therefore no longer a stiffening element of the chassis but simply sat on it. New higher square aluminium bulkhead plates braced the chassis and not only supported the firewall but doubled as a tool box with neat hinged lids. This in itself was a very stiff structure and helped reduce twist in the chassis, important to keep all four wheels on the ground under heavy cornering. The track and wheel base remained the same, but at the front the shock absorbers were now transverse. With the end of the arm only a foot or so away from the road wheel, the new design considerably increased damping efficiency. A boss on the front axle provided the outboard location for a short link, with small ‘Silentbloc’ bushes top and bottom.

The oil tank, still at the front of the chassis between the front tubular cross member and the front axle, had a larger capacity of 2.9 gallons. The oil pumps were also modified, with female threads in the inlet and outlet bosses, which were much easier and cheaper to machine than the male threaded bosses of the first and second series engines. This sort of detail was carried through the entire engine, with almost no parts common between the ‘Mark II’ and the ‘Le Mans’, except the camshaft (though even this was up-rated from chassis no 500 onwards), the rockers and pins, and the flywheel and bell-housing. All the other ancillaries were redesigned.

The new engine now had an ‘Auto-Klean’ oil strainer instead of the filter meshes to be found in the gallery on the side of the older type block. It also had improved cylinder block breathing via an aluminium casting on the front of the rocker box with convoluted metal hose attached. Coolant was still pumped through the cylinder head and by thermo-syphon through the redesigned cylinder block which had a wider timing chest at the front. This was partly to accommodate improved timing chain damping with a triple bladed ‘Weller’ spring, but also for wider timing gears which also had an increased spiral angle.

Overall, many modifications were made over the previous block and ancillaries, and the tooling up and pattern costs must have been considerable. The new engine was without doubt an improvement over the second series engine and was now capable of 85 bhp at 4750 rpm in ‘Ulster’ specification, which was near 100 bhp at the absolute maximum 5500 rpm; a very respectable performance from a 1½ litre engine in 1934.

The brake system was also improved. The back plates and shoes were identical to the second series cars, but heavier, fabricated steel drums were used. Though essentially still cable operated, the cables were linked to rods half way along their length in neat bronze housings, in order to try to reduce cable stretch. There was also one less brake cross shaft which improved efficiency, with both front and rear brakes now being actuated off the same shaft. The handbrake, now bolted to the chassis frame and mounted on a ‘Silentbloc’ bush, operated directly on all four wheels via a simple cam working on the offside rear brake lever.

Derivatives

1. Mark II 2/4 seater

Specification

Chassis. Third series chassis, heavier gauge steel with deeper side members and all channel section cross members with the exception of the front and rear cross tubes. The chassis frame sides are strengthened by flanges top and bottom where the first cross member comes, with an additional cross member behind the engine. Length: 11’ 6".

Engine
Bore 69.3 mm, stroke 99 mm, 1495 cc.
Compression ratio: 7.5:1 using third series cylinder head.
Power: approximately 70 bhp at 4750 rpm.
Torque: approximately 70 lbft.
Twin 1⅛" and later 1¼" SU side draught. E4 needle in .090" jets.
Magneto ignition by Scintilla type PN4.
Crankshaft now heavier and made out of ’Nitralloy’, is fully counterbalanced. The ‘Duralumin’ connecting rods now have strengthened caps with a heavier web between the connecting rod bolts.
Two SU Type L fuel pumps mounted on the right hand front face of the toolbox.
‘Auto-Klean’ oil filter with larger diameter oil pipes with ½" BSP fitting.
After chassis no. 500, a different cylinder block casting was used and a 3207 camshaft is fitted, amongst other modifications.

Transmission. Aston Martin designed 4 speed crash gearbox with constant mesh mainshaft and layshaft and dog clutch 4th speed with straight cut gears. Aluminium casing, now mounted on four 1.2 BSF studs on the bell-housing in unit with the engine. A ‘Silentbloc’ bush was fitted to the gear lever pivot and blade type springs were attached to the gear lever to hold it in neutral position; this was an attempt by Aston Martin to eliminate a rattle in the gear lever mechanism. A rubber gear lever knob was also fitted. Ratios: 12.698:1, 8.105:1, 5.91:1, 4.66:1. An open propeller shaft was fitted, with two dust proof ‘Mechanic’ type universal joints.

Steering. Marles worm and peg, now with a ball bearing at the top of the steering column tube.

Wheels and tyres. Rudge Whitworth 18" well base wire wheels with sixty spokes and 2¾" rims fitted with 5.25 tyres.

Suspension. Semi elliptic leaf springs front and rear with Hartford friction dampers. Aston Martin designed transverse dampers were fitted at the front with a short link to the front axle with ‘Silentbloc’ bushes top and bottom.

Brakes. 14" diameter steel fabricated drums with cam operated 1" aluminium shoes, mounted on two pivots. Brakes actuated by cable and rod, the two connected in a bronze housing by a brake cable muff, via a single cross shaft in spherical bearings. Handbrake works on all four brakes.

Dimensions.
Wheelbase: 8’ 7".
Track: 4’ 4".
Length: 12’ 8".
Height: (hood raised) 4’ 5".
Weight: 19 cwt.
Fuel tank capacity: 13 gallons
Price: £535 (chassis), £610 (complete car).

Coachwork. The ‘Mark II’ short chassis 2/4 seater was similar to the ‘Le Mans’ model, but with a deeper ‘V’ to the dry case radiator, which had thermostatically controlled louvers. The body was slightly squarer, with flat scuttle and almost square back. The hood frame sat outside the body tub when folded down and the windscreen had wind deflectors which converted to aero screens. An outside ‘slab’ fuel tank was fitted at the rear of the body tub with spare wheel on the first few cars attached to the tank, but later secured by steel rod frame and aluminium shoe. The archetype helmet type wings were retained. Later bodies had a pronounced cutaway on the driver’s door on both ‘2/4 Seater ‘and ‘4-Seater’. Early models had Smiths dual gauges, and matching speedometer and rev. counter. Later cars had Smiths (or British Jaeger) 5" speedometer and rev. counter with separate 2" minor gauges. A Rotax FT91 switch plate was used with switches for magneto, main, and reserve fuel pumps. A ‘Ki-gas’ starter pump was fitted along with a simple starter button. The rear of the car was upholstered with cushions and squab and carpets were fitted throughout. On the short chassis cars simple storage boxes with lids were built into the floorboards directly behind the drivers’ and passengers’ seats. A two part undertray was fitted; the longer part being directly underneath the engine, and a shorter section beneath the flywheel and clutch bell-housings.

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