Technical Details of a One-off Two-stroke "Multi" - Designed by Two Coventry Enthusiasts.
The year is 1959 and we are speaking about crankshaft speeds in the region of 14,000 r.p.m., or more, that are likely to be forthcoming from a 49 c.c. in-line, four-cylinder racing two-stroke home built in Coventry. This Unit, based on "square" 0.990in. cylinder dimensions, has recently been completed by a couple of Midlands enthusiasts who hope it will aid their racing prospects in the 50 c.c. class. Those responsible are Maurice Raby, his 50 c.c. Mini motor-powered "Fitz-Ray Special" was seen at Blandford as long ago as 1955 and F. Eric Fitz-Hugh, ideas man, practical engineer and the "Fitz" half of the partner-ship.
Eric, a staunch supporter of the two-stroke had often sought to justify his pet theory that a two-stroke "multi” is best built compactly using the approach of keying together separate crankshaft assemblies. In this design, where the crankcase is a machined unit from an aluminium plate and not cast, his arrangement is that each crank is carried by a 15/16" o.d. ball journal bearing pressed into each crankcase wall. The separate walls or plates are manufactured from 1/2" alloy plate. The whole is then sandwiched together and secured by lateral through-bolts. (Picture: Comparing the size of the engine against the Fitz-Hugh Tiger Cub wheel.
The inner, No. 2 cylinder main shaft is longer than the others to carry, in the centre of the assembly between the two pairs of flywheels, the main driving gear. This arrangement is similar to the primary transmission of the Scott twin. This long section mates with the short shaft of No. 3 section of the crankshaft. Key dispositions around the shafts give a 90° firing interval. The four power strokes per crankshaft revolution have a 1-4-2-3 firing order.
Similar 15/16" bearings are mounted in the outer most crankcase end walls so that there is crankshaft support at six points. There are seals at each bearing to divide off the separate compartments.
The flywheel and shafts are of orthodox design. Flywheels are pierced for balance, the holes being blanked off by caps to minimize crankcase volume. Pressed-in crankpins carry roller big-end bearings.
The four connecting rods are machined from mild steel and cyanide-hardened at the ends. On each a case-hardened, mild-steel gudgeon pin is mounted a turned, light-alloy piston, perceptibly relieved on the skirt, at the gudgeon pin bosses and having a slightly domed crown. Each piston carries two rings turned from Meehanite tube.
A light alloy, one piece cylinder block was cast by Eric, who also made his own wooden patterns for the casting work and tackled the final machining of the casts and the shrinking-in of the four iron liners. After some experiment, the final porting arrangement gives two opposed transfer passages so creating a double-loop scavenge effect. One induction and two exhaust ports are used per cylinder.
This cylinder block is capped by two cast, light-alloy heads secured by 12 Allen bolts extending down through the block and anchoring into the crankcases. Spark plugs of 10 mm. are used.
Four tiny home-made mixing chambers, each with a single jet, are fed from two round style, remote float chambers. Each carburettor has a 7/16in. dia. throttle slide with unified control by a link rod and cable to the twist-grip. The matchbox gives some idea of scale.
The 30 tooth gear mounted on the crankshaft also operates an externally located contact-breaker system. Ignition is by a separate coil to each spark plug and is a battery powered, total loss system. The gear is part of a 3: 1 reduction train to the clutch. A Ferodo three-friction plate assembly incorporates four pressure springs.
Two of the crankcase plates act as walls and extend backwards providing the support for the primary gear train and also housing the gearbox supporting the live-speed gears. This provided two close "top" ratios, two close second speeds and a first (bottom) gear.
The engine was tried with a 10.5:1 compression ratio in the head and 8:1 in the crankcase, the unit did fired up and ran under its own power. A reported looking at the engine made a statement that " until development is completed and there has been an opportunity of a fairly prolonged track test, the ultimate potential of this neat and ingenious little unit will not be known". Unfortunately the engine was never put into a frame by the Fitz-Raby team.
What did happen to that wonderful little engine?
The Motorcycle press said in 1963 "Four cylinders of only 12 c.c. each, you may say model aircraft stuff but Duncan Mitchell, of Stevenage, has brewed up his own racing four engine, but it is a 48 c.c."
Duncan had taken over the Fitz-Hugh project and using that engine as a model and with other parts, designed and manufactured by him, had created the engine that now resides in the Sammy Miller Museum. But, we are going ahead of ourselves. Let's go back to Duncan Mitchell and his then company "MOTO DECLA".
I asked Ian Ager, a well known 50cc racer in the 50s and 60s, about Duncan Mitchell and "MOTO DECLA" including his involvement with the "50", and through emails and 'phone conversation, we penned these words giving a background story.
"I first met Duncan in 1961, even then he was talking about producing a 50cc racer using his own designed cycle parts and using a Demm engine. With his spare time being taken up with getting "MOTO DECLA" off the ground, holding down a full time job with ERA and the duties of secretary for the Racing 50 club, the racer project was put on the back burner".
"With Duncan's history in getting 50cc racing started in England in the late 1950's special thanks should go out to him. Without his actions we would probably not have any racing 50cc motorcycles to enthuse about in the UK and the present Racing Fifty Enthusiast Club would possibly not have been formed today".
"It was at one of the 'New 50 ERA' club monthly race meetings that Duncan approached Brian Cockell, after seeing the frames that Brian and I had made. We had built three of these frames with me using one for building my bike and the two others being used by Ray “Jasper” Smith and Brian Cockell. Duncan asked if we would build a batch of frames for MOTO DECLA Ltd". The picture shows the frame with the water-cooled engine fitted. (Editor: In 1962 there were different chains of thought being developed in the Racing 50 Club that caused a split in the membership. Duncan Mitchel formed a new club with the followers of his way of thinking on how the club should be run. This new club was named 'The New 50 ERA Club').
"Arrangements were made for Brian and myself to visit his "factory" and collect a frame, which Duncan had designed and fabricated, so we could submit a quotation for the supply of 10 frames. Back in Brian's workshop, on checking out the frame supplied, it was found that the swinging arm spindle was neither square with the headstock nor with the axis of the frame. The swinging arm assembly had been fabricated in such a way as to compensate for the frame errors and bring the wheels into line at ground level. The arc welding on the frame was of poor quality with no penetration into the frame tubing".
"Brian and I put pen to paper and sketched the new frame based on those we were using for our own mounts. These pictures are of the finished frame using a Bultaco fairing from a 125cc model".
"MOTO DECLA Ltd accepted Brian's quotation for the frames including the three jigs used in their fabrication. Five frames were built, these were of a 'sif-bronze' welded construction. Working capital must have been tight at Duncan's factory as the five frames were finished awaiting an engine to be supplied, so that engine plates could be made and welded into position, but he could not supply the engine. Brian pressed for part payment of their order to cover the materials and labour used but to no avail".
"Months passed before a second hand DEMM engine was supplied and instructions to use it to fit engine plates into 2 of the frames and use an ITOM engine to fit plates into the other 3 frames. The frames were delivered to MOTODECLA but their cheque could only cover the price of two DEMM engine/frames. So the other three ITOM frames were brought back to the workshop. What happened to these frames in the end escapes me but they kicked around the workshop for quite a period of time".
"The DEMM engine was totally out classed by the ITOM engine in 1962 so their choice of using it was a mystery. The only DEMM engine, which could compete with the ITOM, was the "Pope Special" built by John and Tony Pope in 1961. This was very quick for its day but the gearbox main shaft kept breaking due to the increase in power that the engine was then producing. This engine was finally retired in 1962 and replaced with a home built, water-cooled ITOM unit, which was ridden by little Alan Dawson with great success".
As to how many Moto Decla's were ever sold, I do recall seeing one at the New 50 ERA clubs monthly race meeting and this had the Demm engine fitted. It was complete with lights and number plates etc, this may have been the one and only!"
Editor: MOTO DECLA unfortunately was unable to resolve the financial problems of the company and closed for business around 1963/64. In 1965 Duncan launched a new company called 'The Heldun Engineering Company' which took its name from the founders, Helen and Duncan Mitchell and it commenced production in Dudleston, Shropshire before later moving to premises in Birmingham. Duncan still had the Fitz-Ray 4 cylinder motor to experiment with but this was not for a production racer.
1963 and Mitchell's First Four Pot Fifty
A number of people tried to copy the Fitz-Ray 4 that was created in 1960 but with little success as with out drawings or a prototype guesswork had to be used. Duncan Mitchell, having the F-R engine was ahead of the pack and re-designed some of he parts and built most of the engine, a two-stroke, in a year. Then he made his own diminutive five-speed gearbox. The Bore and stroke are 25.3 x 25mm, compression ratio was 10:1 and maximum power 5 bhp. at 7,000 r.p.m.
Being of 0.5 inch journal diameter, the crankshaft is built up from Four separate full-disc flywheel assemblies pressed together and supported in eight ball bearing races. Stainless steel connecting rods run on uncaged 3/16" x 3/16" rollers. Pistons are in Y alloy and, like the con rods, are machined from solid.
A one-piece aluminium casting forms the cylinder block and the steel liners are pressed in. Two castings are used for the cylinder heads, in which the 10mm sparking plugs are axially placed.
Four Runbaken ignition coils, each with its own contact breaker, receive current from an Exide 6-volt battery. Carburation is by four Amal 10mm-bore carburettors fed from two of the latest flat float chambers.
An exposed spur gear drives the tiny clutch, which has friction plates taken from an old girder fork damper! The five speed gearbox is bolted to the rear of the crankcase. Later a six-speed version is envisaged for production. Tyre size is 2.50 x 17in but the 4 1/2"diameter continental hubs will soon be replaced by full width 6in hubs designed by Mitchell. Shapely glass fibre tank and seat base complete an extremely interesting design. The crankcase is laminated from ½” thick light alloy plates clamped by four long studs.
Already costing over £500, the model needs further development before it is fit for racing. Mitchell intends to use it, water cooled, as a mobile test bed for his firm's Moto-Decla production racers. Already, a 50c.c. twin is taking shape alongside a double overhead camshaft single!
Duncan Mitchell continued the development of the 4 cylinder with some changes to the frame and forks and improvements to the engine and running gear. However the little screamer never saw the race track in anger due to financial problems.
In later years the bike came into the hands of the Sammy Miller Museum and was restored and is now on display. It is well worthwhile paying the museum a visit as the display of bikes is remarkable.
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