AJS Stormer. Motocross Pair. 1969.

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33. AJS Stormer Motocross Pair

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 Andy Roberton / AJS Stormer 370cc / 1970

After intensive prototype testing and for the last few months of 1969, with Andy Roberton riding the new model at motocross meetings, the AJS 370cc Y5 is now in production. The established AJS 250cc Y4, updated to include a number of modifications, goes forward as the Mark II version.

Both are powered by the AJS single-cylinder, two-stroke Stormer engines which represent the latest stage of development of a unit, which started life in 1962 as the Villiers Starmaker. There remains a faint family likeness, but the AJS engineers are quick to point out that, since the original Starmaker was announced, over 300 improvements have been made.

The Y5 and Y4 are identical in most respects. However, cylinder bore sizes are 83mm for the 370cc and 68mm for the 250cc, carburettor choke sizes are 34mm and 32mm and the exhuast expansion chambers differ in volume.

Quick identification point is that the two-gallon, glass-fibre fuel tank is coloured yellow on the Y5 and orange on the Y4.

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 Andy Roberton / AJS Stormer 370cc / 1970

The engine is of straightforward, robust design. A crankshaft with full-circle flywheels runs on two roller bearings in the high-grade, light-alloy crankcase, cast with extensive ribbing at the bearing housing to ensure adequate support. Crankshaft sealing is by means of spring-loaded, garter-type seals. The stroke for both engines is 68mm.

New for this year are the massively finned, light-alloy cylinder and head, as used last year on only the factory Grand Prix models and the machines exported to California.

Another change is a more robust connecting rod to cope with the increased power and permit the same bottom half to be used for both capacities. The rod runs on a caged, needle-roller big-end bearing and a plain bearing at the little-end.

A spun-cast, austenitic-iron liner, which has an expansion rate only fractionally less than that of the light-alloy, is fitted in the cylinder. The cylinder head has a squish band and a centrally located spark plug.

Compression ratio is 11 to 1, as compared with 12.3 to 1 used on the 250cc last year. Combined with the larger fin area for the cylinder and head, this ratio reduction gives a lower working temperature and, as a result, peak power is maintained for longer periods.

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 Malcolm Davis / AJS Stormer 250cc / 1970

The gravity die-cast, light-alloy Hepolite piston has two cast-iron rings, and circlips to locate the gudgeon pin. Carburation is by an Amal Concentric breathing through a large-capacity, impregnated-paper filter mounted on the left side beneath the seat.

The exhaust system is a push fit in the port and is retained by frame attachments. A chromium-plated heat shield protects the rider’s legs. Ignition is by energy transfer, with the flywheel generator on the right end of the crankshaft and an external, 6volt Siba coil mounted under the seat.

To withstand exceptionally harsh gear changing, the selector fork pins engaging with the cam drum are high-tensile steel. A duplex primary chain drives the all-metal diaphragm-spring clutch.

The frame has, as its main stress member, a fabricated, tapered steel spine extending rearward from the steering head. A duplex, tubular frame suspended from the main member serves only to retain the power unit. Engine mountings and attachment points for the rear-suspension units are strengthened to make the frame suitable for both engines.

Rear chain setting is made by adjustment at the rear fork pivot. The pivot spindle is located by metal discs, each with an eccentrically positioned hole.

To exclude mud and water from the 5inch diameter brake drums, each shoeplate has a peripheral groove locating a cast-iron ring, which bears on the edge of the steel brake liner.

On the test track, with the ground frozen solid, the conditions could have been better for galloping a pair of motocross machines, such as these competition Ajays, across the icy surface, but there was no choice.

Both bikes, handled perfectly, you could confidently ride them zestfully anywhere and the power could be controlled to a nicety, it speaks volumes for their design.

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 Andy Roberton / AJS Stormer 370cc / USA / 1970

Although their power characteristics are similar, there was no doubting which was the 370cc. Opening the throttle produced a steady increase in purposeful surge right through the full range.

There was no sudden arrival into the power band, which could of spelt more than excitement on the slippery surfaces.

The gearbox was a delight to use. The all-metal clutch took upto the drive smoothly and positively. Once the machine was under way it was not needed, for the cogs could be changed in either direction with just a flick of the pedal.

The 7inch travel of the two-way damped AJS telescopic front forks, and the damping of the adjustable Girling rear suspension units, produced handling which, even under the borderline conditions gave me enough confidence to rush across the frozen ruts and aviate both models off the jumps, it quickly become, pure enjoyment.

This colourful pair, come in at a keen price, and there is little doubt, in my mind, that they are due to become, a hit on our circuits as well.

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