Half a twin

Andy Gourlay’s Du­cati sin­gles

Old Bike Australasia - - DUCATI SUPERMONO -

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.

ABOVE Andy with his handy­work. IN­SET TOP In­no­va­tive think­ing. Andy has used the rear cylin­der blank­ing plate as the front shock mount on the 540. Orig­i­nal twin crank. Close-up of the 540 engine. Young charger Finn Har­man gives the 470 a work­out at Man­field Race­way. Mod­i­fied crank for the sin­gle, with a spacer to re­place the se­cond con­rod.

Third in the se­ries; the nearly-com­pleted 517 4-valve, de­rived from a 916.

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