This is my story about the twin.
How it started…
Alcohol. Lots of it. And bar talk.
But then, it started to stick and soon it became an actual goal: If the twins are of such good stability, why not go against the flow and actually ride parallel twins during the annual Ducati Clubraces organized by the Dutch Ducati club? Target: Clubraces 2014!
And now doing it
Should be easy: I got a road registered 500GTL bike, and a few boxes of what was once a 350SD which I acquired in Belgium for a sack of chickenfeed. Mix those two together and a basic racer should be the outcome.
Or so I thought…
The 500GTL had the horrible tapered output shaft which meant a sprocket is near impossible to find.
The 500GTL crank had a seized left conrod so the crank journal was scored, and no undersize bearings readily available.
The 500GTL had a few worn rocker arms from my green 350GTV and the good rockers from the 500GTL were now in the running 350GTV. And I’m not going to sacrifice a running GTV to get a useable 500GTL head!
All were solved with the 350SD basket case from Belgium: 500 Desmo head, 350SD gearbox, NOS 500GTL crank sourced from a friend.
Building the engine was tedious when it came to shimming the crankshaft in the cases: simply measuring the depth of the bearings and the housings wasn’t enough: after putting the shims for required clearance and closing the cases the crank endfloat was always off by a few tenths mm, and I never could figure out the reason for that. With the frequent removal and insertion, the main bearings suffered a lot, they lost their roundness and the flange wasnt flat anymore. But the endfloat was finally achieved with the right shims! Now I had to find other main bearings, and finding those have the same chances as winning the main Lottery prize!
The gearbox had some parts interchanged from the GTL gearbox so that was sorted. Finding a sprocket took long evenings studying the JT Sprockets catalogue comparing the commercially available sprockets with the dimensions I measured from the SD output shaft. Luck struck: I found a contender! Only needed to machine a spacer for the chain alignment.
The 350SD frame made its acquaintance with the angle grinder, and managed to lose the better part of 5 kilograms!
It took just under a year to do the lot and make it into a running bike with electric ignition. It took a new pair of modified piston rings and a complete head re-weld as the head had cracks between the valve seats and sparkplug holes.
The worked head got new valve seats, smaller sparkplugs (10mm) and 600SS/Monster valves modified to suit the PT rockers. The home-made main bearings failed miserably as they seized the crank twice (!) on the dyno due to galling. Luckily the crank was spared so I replaced the Aluminiumbronze bearings back to original bearings and the bike is still running to this day. Turned out I had a good design but chose the wrong material…to be investigated further.
The engine squeezed out phenomenal values of 37hp and 40Nm with the SACHS digital ignition set to curve nr 5 and rev limiter a 7800rpm (to save the internals).