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No pain, no gain...

As I described in my previous article, there were (and are) many enthusiasts out there creating their own race engines. Historically, the racing series with the most open regulatory books have been those based on these DIY race engines. One such series with lots of birth certificates for engines originated in the late 1980s, and was the Sound of Singles Series, later called Supermono. This motorcycle series mandated single-cylinder four-stroke engines without forced induction. Later the additional boundary of an 800 cc maximum displacement was added.

Just before the series announced the 800 cc restriction in swept volume, two brothers from the Netherlands decided that, on the basis that "bigger is better", a big race engine would have a significant advantage. A larger displacement would provide a significant gain in torque and therefore increased acceleration out of the corners, and this again would lead to a higher speed on the straights - in itself all very promising for getting as many wins during the season as possible.

Since all Supermono engines from that era were roughly 650 cc in displacement, the idea was to make an engine of around 900 cc. And since an existing Rotax crankshaft (stroke 81 mm) was chosen, the bore size was set at an enormous 120mm! In comparison to usual bore sizes of about 102-105mm this was a significant increase, which would later become a fragile area of the engine.

Most of the parts were made from scratch. The crankcases were horizontally split machined (from castings) parts with wall thicknesses up to 35 mm. The dimensions were chosen not only to withstand the combustion forces but also to enable the crankcase structure to be used as structural part in the motorcycle chassis.


Fig. 1 - Side view of the crankcase, horizontally split and wall thickness up to 35 mm

Later, when it became obvious that the chosen cranktrain components could not quite handle the brutal forces of the 120 mm piston going up and down 9000 times per minute, the choice was made to create these components from scratch as well. Where initially the con rod and crankshaft were borrowed from Rotax, now these components were made in-house, and in the end the con rod was made from aluminium. The only production parts used were gearbox shafts and gears (from a Suzuki GSX-R 750), coolant and oil pump, and some smaller bits and pieces elsewhere.

The cylinder liner, which started life thinking it would be fitted in a truck engine, was machined down to (press) fit into a made-from-solid aluminium cylinder, acting as a wet liner with a top stop. This way the wall thickness of the cast-iron liner could be kept to a minimum, without the risk of deforming under the clamping load of the cylinder head onto the liner (typically leading to higher oil consumption due to piston ring flattering).


Fig. 2 - Side view of the engine, with forward-pointing intake

Another very interesting piece of craftsmanship on this engine was the cylinder head. This component was oriented with the intake pointing to the front of the motorcycle, in order to create a 'logical' airflow through the engine. During its fabrication, coolant channels were machined into the structure that were sealed later on to create a cooling cavity. Sealing was done by a pressed-in tube at the location of the spark plug and welding on the outsides.


Fig. 3 - Bottom view of the cylinder head

Inside this purpose-made cylinder head, initially Opel passenger car valves were operated by camshafts from a Honda Bol D'Or. To be exact, it was a portion of this four-cylinder camshaft, as it needed to fit inside the cylinder head of this single-cylinder engine.

The engine has raced in a number of events and has been praised for its torque and ease of driving. On the other hand, the engine remained fragile, and although excellent lap times could be recorded, the engine had difficulties with finishing races.

In the battle to make this huge single-cylinder engine work as it should, lots of lessons have been learned. These were put into a second engine, which unfortunately never raced. However, after this engine, a number of others have been developed by the same people, among others a diesel race engine.

A number of years after this engine was built, the next generation has stepped in, and at the moment a young man who's in his final year of studying automotive engineering is building his own single-cylinder race engine. Again, a couple of parts came from production models but most of the structural components were made from scratch. It's an interesting study topic, and it might be a good idea to focus on this second-generation single-cylinder race engine in the next article.

Written by Dieter van der Put

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