V2.2 November 2020

Alpha Disc Valve Engines

The Alpha Centuri and Hadley Starr engines

Centuri#1 outside NMM in 2003 before fire, Centuri#2 after restoration in 2018

Single cylinder development Alpha Centuri 250cc twin
(#2 restoration log)
Hadley Starr 125cc single
(restoration log)
RD-Alpha Centuri engine
RD-Yamaha crankshaft
Belt drive, Greeves frame
19 November 2020

A Brief History of Alpha

Alpha Bearings of Dudley in the West Midlands, started as many firms do, making hard to source items for the engineering industry in 1946, encouraged and assisted by Vale Onslow. From this start the company expanded to include components to service the motorcycle trade specialising in the supply of both main bearings, big-ends, connecting rods, and cranks.
Alpha came to the forefront as two-stroke performance equipment manufacturers, during the late fifties and early sixties, which coincided the beginning of the Villiers 197 9E becoming popular as a competition engine. As the power outputs of the 9E increased, so did the failure rate of its standard components. The advent of the high performance alloy cylinders - and 250 cc conversions - put so much pressure on the standard crank, that it cried enough far too regularly. Villiers were adamant that there was nothing wrong with their design, and refused to believe that anyone was actually racing one of their commuter engines.

Frank Cutler, then managing director of Alpha, decided to make good the basic inadequacies, a job that should have been tackled by the makers. First item to come out of the factory was the Mk 1 full circle crank and streamlined con rod, but replacement rods for the standard Villiers crank had been available for some time. Villiers used a 0.8 inch crank pin with 26 crowded rollers in the standard road engine, but reduced this to 9 caged rollers in the sport engine. Alpha reduced this even further, using 8 rollers in a better cage for their replacement rod. The Mk 1 full circle crank increased the crank pin diameter to 0.925 inch and hence increased the rigidity of the whole crank assembly. The benefits of the new crank were two fold, its increased rigidity improved reliability, and the full circle flywheels increased the primary compression ratio aiding cylinder filling.

Next to appear was the Mk 2 full circle crank which had gone metric. The crank pin was now stepped, 22 mm flywheel fitments and a 25 mm bearing inner track. The outer bearing track became 32 mm at 16 mm wide instead of the earlier 0.5 inch wide type. This type of crankshaft was adopted by all the works teams of the day, as it meant that even more power could be extracted from their motors, without the reliability fears of the earlier days.
Power outputs continued to creep higher, and this then began to put the pressure on the flimsy Villiers crankcases, the principle problem areas becoming the main bearing housings. To combat this new problem, Alpha then turned their attention to the crankcase castings, coming up with a heavily finned pair that were beefed up in the appropriate places. Instead of the standard two ball and one roller main bearings, Alpha opted for the use of one ball and two roller bearings. The rollers (a single row on the driving side next to the ball and a double row on the timing side) ran in hardened sleeves in the cases, but directly onto the crank drive shafts. As the crank was factory balanced the heavy brass magneto was not required, nor was there any room for it, as the contact breaker ran straight onto a cam ground on the shaft end. The cases were intended solely for racing use, and the use of a total loss battery coil ignition system was deemed to be the most reliable method of generating sparks.
Not content with making replacement parts, Frank Cutler turned his attention to a complete motor. The first engine was based on the Alpha bottom end, and in an attempt to provide a better induction cycle, used a rotary valve. The inlet port was positioned on the right hand crankcase at the front, with a machined cutaway on the ultra close fitting crankshaft, acting as the valve. This engine was soon replaced by a Mk 2 version, which placed the induction tract at the rear of the cases. The crankshaft again acted as the valve, but this time the cutaways were machined on the circumference of both flywheels, being fed by a bifurcated inlet tract. A third transfer port was added to the rear of the cylinder, which connected to the underside of the piston, in the redundant area vacated due to the repositioning of the inlet port. The single failed to come up to expectations, in that it would not rev to the expected rpm, even though both Royal Enfield and Scorpion showed an interest in it for their production racer projects. Was it due to the failure to produce the required power, or the demise of the two racer projects that persuaded Frank Cutler to drop the single in favour of a twin.
The twin (called the Centuri) used a bore and stroke of 54 X 54 mm, with a pressed up crank that used a pair of inboard discs to collect the mixture from a centrally mounted single carburetor. As the disc controlled inlet periods of less than 180 degrees, this system was deemed as perfectly satisfactory for the state of tune prevailing some thirty years ago. With modern high performance engines having an inlet period of truly wild proportions, this system would not be adequate, and a change to twin carburetors would have been required. Conventional ball races supported the centre of the full circle crank, with caged rollers being used on the outer ends, again running in steel sleeves but directly on the crank drive shafts. A new set of cylinders were sand cast (the development cylinders coming from a Velocette Viceroy scooter) and fitted with spun cast iron liners. The new liners featured not only the main transfer ports but an extra pair of auxiliary ports, placed between the main transfers and the exhaust port. This method of gaining an increase in gas transfer was a full five years ahead of Japanese two-stroke wizards Yamaha, who adopted the same idea for the TD2 racers of 1969.
The rest of the engine was quite conventional, except for the star burst cylinder head finning, which replaced the Bantam cylinder heads used for the initial development. Using battery coil ignition and an Albion HG 5 speed gearbox, the motor revved well over 10,000 rpm and produced an excellent power curve. The choice of the somewhat suspect Albion HG 5 gearbox was purely political, as Alpha was part of the E & HP Smith empire who owned Albion. The gearbox was soon changed when Albion came up with their new barrel-cam 5 speed box.

To test and develop the new motor it was installed into a DMW roadster chassis (the old Mk 1 Hornet that originally housed the disc valve single engine) and given 1000 miles of everyday running to see if it would break, but nothing went wrong, and the engine test ended satisfactorily. Road testing was in the hands of Mike Cutler (Franks son) who described the test as the most fun he ever had. This transpired to mean that although the engine power delivery characteristics were as predicted, the handling and road holding of the DMW chassis was not, which produced a fast but evil handling monster.

The first engine was reinstalled in a Royal Enfield GP5 frame (grudgingly supplied by RE on the orders of E & HP Smith) and called the Centuri, and further development carried out. The frame was supplied on the understanding that it was to remain standard, which prevented Alpha from placing the power unit in the optimum position. The result of this ruling was that the engine was placed too far back, to enable the twin expansion chambers to clear the duplex front down tubes. Handling suffered, with Mike Cutler going as far as to say that the handling was not even up to the DMW frame standards. The Centuri power output was gradually increased to some 48 bhp, but in this form it became fragile, and the output was lowered to 44 bhp in order to make it more reliable. The increases came from the use of a new set of seven port cylinders, a bridged exhaust, four transfers and an extra transfer through the piston. The production engines that would have been on sale to the general public, had the project carried on, only gave 36 bhp, which was enough to be competitive against its nearest rival, the 32 bhp Greeves Silverstone. When asked what the difference was between the production and the works engines, Mike Cutler answered "Fred Hadley and his files".

Testing was carried out on the dyno that RE used to develop their single cylinder GP5, Frank Cutler having purchased it, on the demise of RE. The dyno was installed locally at Graham Star Engineering, (a kart specialist) because of the noise restrictions that prevailed at the Alpha factory, and it was from here that most of the development work was carried on. Later Alpha produced their own frame and used Ceriani forks, for the production run. The run was very small with only a handful of machines being produced. Figures indicate that only eight Centuri engines were built, with enough spares to complete a further 25 units. The only paying customers for the Centuri were DMW, and John Kirkby a leading Cadwell Park specialist, who installed his engine in an ex Tom Phillips/Vic Camp Ducati Mach 1 frame. The Alpha Ducati gaining a couple of wins in its first race despite being over geared and not fitted with a rev counter. Promise was shown by the Kirkby machine, as was with the factory DMW framed machine of works riders Don Wolfindale and Dave Browning, but alas nothing was to come of this forward thinking project.

Alpha finally abandoned the Centuri altogether and went back to making crankshafts and connecting rods for the run of the mill motorcycle market, after receiving an edict from E & HP Smith in 1968, which put a stop to, what Smiths called, costly race development. All the work put into the Centuri (much of it being after hours at a reduced cost and well below budget) was lost, for the ruling stated that "No further engines or spares were to be sold". Even the engine sold to privateer John Kirkby was re purchased, thus Great Britain was deprived of yet another project that could have blossomed, or at least kept abreast with the rapidly improving Japanese hardware. The sad fact is that when the Centuri project was wound up, Frank Cutler had already completed the drawings for a water cooled, gear driven primary, six speed, disc valve twin that would have beaten the TZ Yamaha into production by several years. Frank Cutler even had ideas of a four cylinder 500, but that is another story.

Another project that did blossom from the ashes of the Centuri when it was terminated on the orders of the Smith group, was the Hadley Starr 125. Built by Fred Hadley (an unpaid helper on the Alpha Centuri) the 125 was half a Centuri engine with a side mounted 28 mm DellOrto carburetor, which had began to show some promise. Frank Hadley decided to build the Starr from the vast stock of redundant Centuri parts which then lay unloved and unwanted at the Alpha factory. Using a seven port cylinder with even wilder porting, the single gave an amazing 28 bhp at 12,000 rpm, but at the expense of an 1800 rpm power band, which was not helped by the use of the Albion gearbox. The engine was installed in a frame made by Graham Starr Engineering and ridden by Don Wolfindale, but it proved to be fast but fragile and let down by the gearbox which was full of false neutrals. The Smiths "no racing" ruling effectively put an end to the Centuri and its innovative junior offspring, and any hopes of a racing success.

Alpha Centuri and Hadley Starr outside cafe at Mallory Park
Owner Steve Groves 1982 Alan Cathcart test