Of 250cc Gold Stars
During 1953 a Mr Thorpe persuaded Bert Perrigo that he should allow us to build a 250 Gold Star on a one off basis and a promise not to worry us for special tuning or special parts. I was quite keen to do this having always been interested in the 250 class. We had one of the experimental small valve scrambler heads left over and an experimental short 6" steel connecting rod, also a special pair of flywheels with crankpin holes specially machined to give 63 mm stroke. The crankpin was special for these short stroke flywheels in that the roller bearing in the centre was standard but the portion that fitted in the flywheels was smaller than usual, being 1.00" diameter and 0.00l taper giving a press fit, the nuts just keeping things together. With this very short stroke there was not room for the standard crankpin nuts. We made special long round nuts with a protruding hexagonal head these were cut off close to the pin after tightening. To remove them they were split with a chisel.
A standard 71mm bore piston and cylinder was used, suitably shortened. Alloy tubular pushrods were used, with lightened tappets, eccentric rocker spindles were used. After experimenting with several cams 22bhp was attained at 7000 – 7600RPM. The engine did not give as much power as the MC4 and it is quite likely that with further running the power increased but it proved very reliable and Mr Thorpe ran it for some years. Later on we built another Gold Star 250, this time using a one-piece crankshaft with outside flywheel and a Sunbeam S7 connecting rod with plain big end. This engine had a 1953 350 Gold Star head and cylinder barrel, the cylinder had to be shortened considerably only seven fins remaining. After quite a small amount of running 26.2bhp was obtained, then the inlet valve broke which ruined the head. When the engine was rebuilt a late type big fin cylinder head was used, a different cylinder head holding down arrangement was used, short steel tubes screwed into the crankcase, bolts fitted from the Back to top screwing into tubes, only four were used and we had no head blowing.
This was the final arrangement of our 250 Gold Star, it proved very successful, the best power recorded being 29.6 at 8250. It would run up to 9000 RPM quite reliably. The inlet port was a pressed in piece of alloy machined after fitting which gave a very straight port. Several engines were built to this design, one was a 350cc, using an 85 mm 500 Gold Star piston on a 61.5 stroke, this was later converted to a 250 by fitting a 72 mm Gold Star piston and was used in the Geoff Monty Special (GMS) for several years. Obviously these 250's were what could best be called ‘codge-ups' using 500 style crankcases for lack of a proper one, the cylinder heads being from 350s had unsuitable valve angles and sizes, the compression ratios were restricted, because of wide angle valves. In spite of these disadvantages proved in the GMS that this was the way to go. The lessons learnt from the ill-fated MC1 were not forgotten. The 250 Gold Star engine in a 500 Gold Star frame was timed at MIRA to do 108mph. In the GMS with some fairing it reached 115mph.