Half a twin
Andy Gourlay’s Ducati singles
Englishman Andy Gourlay, a resident of New Zealand since 1986, has enjoyed a long love affair with quick singles, having raced Aermacchis and a Manx Norton in the Manx Grand Prix from 1976 to 1980. He recently re-acquired his Aermacchi Metisse that has been in storage for over 30 years and which is currently undergoing a complete rebuild as a retirement project. Andy is also a staunch Ducati fan, with a TT2 600 in the stable, which also includes a pair (soon to be a trio) of totally unique creations – 470cc and 540cc single cylinder racers that started life as 2-valve twins. Andy acknowledges Phil Irving’s famous book ‘Tuning for Speed’ was good for reference with the project. By removing the rear cylinder and head, which is pretty much what Ducati did to create their Supermono, Andy has produced his own versions of the Bologna bullets, and highly effective they are too. The 470 (#34) started life as a 900SS, with a 94mm bore (one of a 1000cc) barrel fitted, and the standard 68mm stroke. The frame is a modified Ducati ST2, with Yamaha wheels and Brembo discs. Rear suspension is by Ohlins shocks taken from a Honda CRF450 enduro bike. The 540 (#159) has a 98mm bore barrel from a Multistrada, with a 71.5mm stroke. This one uses a modified Superlite frame. Both the frames are the work of John Anderson. Spoked wheels are used, and the rear spool hub is a Jawa speedway item mated to a Brembo disc. Specialist machining on the bikes has been done by Ian Sharp, with crank machining by Tony Osmon, and the alloy fuel tank on the 540 was made in the UK by Dave Chadburn. Both run Boyer ignition, and are identical in weight at 126kg, despite the fact that the 470 has a steel fuel tank.
“The secret is getting the balance factor right”, says Andy. “Moto Guzzi used 30% for their flat singles, and Aermacchi used 27%. There’s very little wrong with either of those, so I went for 30% as well. The riders I’ve had on the bikes say there’s a bit of vibration around 5,000 rpm, but it smooths out after that. All singles vibrate at some point. I only considered primary balance for my motors, hence no balance rod as on the Supermono. This limits the maximum revs to approximately 9,300, the same as for twins. The only reason the factory used balance rods was to increase maximum rpm, without vibration limits, and hence increase maximum power.” The 470 was originally tested and raced by the talented Finn Harmon after its completion in 2015. Having built and refined two versions of his own ‘Supermonos”, what’s the next challenge? At the time of my visit, that was taking shape on the bench in Andy’s immaculate workshop. Number three in the series is a four-valve, developed from a 916 engine which is now missing its top tower. Beside it on the bench, on Andy’s intricate crank balancing sat the new crankshaft with its 71.5mm stroke and bob weights carefully sculpted to obtain the vital 30% factor. Combined with a bore of 94mm that equates to 496cc. The 6-speed gearbox comes from the 916. Andy hopes to have the new single on the track at the Pukekohe Festival in February 2019, alongside its 2-valve brothers.