1 September 2020

Alpha Disc Valve Engines

The Alpha development single cylinder engines


Until 1998 this single cylinder engine drawing (above) was all that I had ever seen of such an engine. The drawing is for the Mk2 single cylinder but before that came the Mk1. It was created by welding on an inlet stub to the right hand crankcase and the right-hand crank flywheel was sculptured to function as a simple disc-valve. Below is a period shot of the Mk1, accompanied by 2003 photos of how the bits have survived.

Alpha1 Alpha1 Alpha1 See how the flywheel is sculptured to act as a disc valve
Alpha2c Alpha2f The bottom half of the engine No. 2 was recovered. I am told that some week previously that the barrel had been with it, and I guess that someone thought it was a Merlin barrel and bought it. Such barrels are quite distinctive of course, as they are 2-stroke barrels without an inlet port. The bottom half of the engine was left to soak in a tub of diesel for six months, and then dismantled. As you would expect, the shafts and the big-end pin were too corroded for use.
The dismantled assembly was taken to Alpha Bearings for review and confirmation that it was indeed a long lost object of desire and in need of tender loving care. Alpha Bearings of Dudley have manufactured some new shafts, and fitted a new big end and con rod assembly. The timing side shaft has been made to accommodate an electronic ignition rotor, and the con rod assembly is a Maico scrambles component complete with silver cage.

What variations were there?

Alpha2i Alpha2j I can see two variants. My casting (on the left) is all alloy, whereas the crankcase casting on the right has a steel insert to face the inlet port.
Whether by design or to rectify wear I cannot say.
Alpha2k Period picture of Bob Curry on a MK2 engine mounted in a Royal Enfield frame (borrowed from Motorcycle 1964). This bike was normally ridden by Peter Cutler. It was the second MK2 engine as Mike Cutler already has a MK2 engine mounted in a Greeves chassis.

When did it all happen?

The project must have started by 1962 since the second MK2 250 single pictured above was made in late 1964, and the first MK2 engine was reported by Motorcycle in late 1963. The engines revved to 10,000 RPM and performance was considered brisk. Motorcycle reports that Mike Cutler's "Number One" prototype has already clocked 90 mph in its Greeves Scottish chassis. I am hoping that my rebuild will show more than that. The MK1 engine with its lop-sided crank was prone to burning a hole in the piston after several minutes of full throttle running. The MK2 engine solved this problem but over 8000 rpm it was reported by Bob Curry that the gas tended to bounce off the flywheel rims and build up back pressure.

The design of the Centuri twin was laid down in 1962, and the prototype was constructed using Velocette Viceroy scooter barrels and BSA Bantam heads, and was mounted in a DMW frame. Frank Cutler, the designer, was constrained to use the Albion HG5 gearbox since they were part of the EHP SMith Group. It was mounted in a DMW Hornet chassis, and the handling was "interesting" according to Mike Cutler who road it for the next few years. The engine was a 180 degree twin using the then almost standard 54mm x 54mm to give a 250cc twin. Carburation was by 29mm DellOrto SS1 through inboard discs so that the single carb fed both cylinders. The porting arrangements were very similar to the Yamaha TD2 racers that appeared almost five years later, smaller additional transfer ports to direct mixture over the piston crown, for scavenging and cooling. The race version , tuned by Fred Hadley, appeared in 1964 without much success. Royal Enfield who were part of the same EH Smith group, complained that Alpha were using a DMW frame when they have a perfectly good GP5 frame available, and the union was not a good one. The engine was too far back in the frame, so a purpose built frame was made. Don Wolfindale had scored three straight wins from three starts on the DMW frame. A second frame, identical to the first, was made, Don Wolfindale and Dave Browning had some good success with the Alpha frames. Dave Browning became the 1968 ACU Clubmans Champion riding several rounds on his Alpha Centuri. The Alpha Centuri featured in the rebuild is that 1968 championship machine. A Centuri MK2 engine was loaned to Fred Launchbury for the 1967 TT and he was clocked at 122 mph, just 2 mph down on the works Kawasaki. With the demise of the Royal Enfield race shop, and the acquisition of their dynamometer now installed at Graham Starr Engineering, further development was possible. John Kirkby had many successes using the only privately purchased Alpha Centuri engine in a Ducati frame. The earlier Centuri engines had four transfer ports (42 bhp at 10,000 rpm), and the later MK2 engine had five ports (works motors courtesy of Fred Hadley had 48 bhp at 12,500 rpm, power band 7000 to 10000, production motors had 36 bhp) but primary chains were quickly destroyed and needed to be changed after every race.

A preliminary batch of 25 engines were scheduled for the 1968 season, and a portion of the works canteen was cordoned off to create a "production line". Parts were made and seven engines were assembled, three of these went to DMW and four stayed with Alpha. The engines were intended for sale to customers to fit into their own machines, at a cost of 200 sterling. The project was cancelled at the end of 1968. EHP Smith even reclaimed the engine sold to John Kirkby. The next year Yamaha came through with an engine that was not quite as good and cleaned up commercially. Fred Hadley was not prepared to stop quite yet, and used some of the Centuri parts to make a 125cc single in association with the kart specialists Graham Starr Engineering of Wolverhampton. Using the five transfer port barrel, the Hadley Starr produced 28 bhp at 12,000 rpm with a power band less than 2000 rpm wide. You need better than a 5-speed Albion gearbox to get the best out of a small power band. Frank Cutler had planned to solve the power and transmission difficulties, he had designed a water-cooled geared-primary six-speed disc-valve twin. It was on the drawing board, ready to go.

Copyright: John Wood 2003, period photos of Centuri: Mike Cutler

What will my rebuild look like?

Alpha2g Alpha2h Well, something like this. Just a mockup of course.
Was to be mounted in an early Greeves chassis, but has been sold on to someone with the correct DMW chassis for a restoration rebuild.