Sunday, June 15, 2008


While Ducati has a lock on the Desmodromic valve system, they weren't the only motorcycles to use double-acting valve management. Many Velocettes have been subjected to springless valve actuation - first in the mid-1950's, when Sid Willis in Australia made an experimental sohc desmo KTT, then Harry Hinton (also of Australia) in the late 1950's, finally in the mid-1960's, when BMG made a commercially available push/pullrod desmo kit for the M-series Velos (Venom and Thruxton - the BMG kit deserves a post).
Interestingly, in both cases, no power advantage was found, and the conversions were short-lived.

Sid Willis was a champion 250cc rider in Australia, using several ex-works Velo engines and parts. In 1953, he joined the Continental Circus (as the European racing season was called in the 1950's) with friend Tony MacAlpine (pictured with his Vincent racer - second photo), taking his 250cc dohc Velos, built around pre-war Velo ex-works cylinder heads. Even with this advanced engine spec, his bikes still used rigid frames - very light, but difficult to handle over the cobblestones of the Mettet circuit in Belgium. To overcome handling issues while retaining the light weight of his racer, he asked frame builder Doug St.Julian Beasley to bring his chassis up to date. Beasley built the now famous all-welded lightweight double cradle frame which housed several 250cc Velos after Willis' (see top photo), which is similar the Norton Featherbed, being an all-welded double cradle, but isn't a merely a copy, being much smaller in scale, lighter, and with slightly different frame geometry.
With this new frame and his dohc engine, Willis came 5th in the 1953 Isle of Man Lightweight TT, the second privateer home behind Arthur Wheeler's Moto Guzzi, and a field of very rapid Italian lightweights including works Guzzis, dohc NSUs, and DKW two strokes.

At some point in the mid-50's, Willis considered the theoretical advantages of desmodromic valve operation for racing (mainly, unlimited revs without fear of valves dropping), and built up his own desmo head. The lower photo shows his thought process made metal; it shows his cobbled-up model, built to see how/if the project would work. Satisfied that it COULD be done, he set about fabricating a new cambox, which he fit atop a KSS mk2 alloy cylinder head, which he had machined flat. Needing a very tough metal for the rocker arms, he cut up steel wrenches and silver soldered them into shape! Bush engineering at its very finest.
The finished engine would rev to 10,000rpm, and started easily enough, but it had no gas sealing when push starting and thus no compression, as a small amount of valve clearance needed to be maintained when cold, so that the valves would have room when the cylinder head warmed up. When the motor was turned over quickly, gas pressure sealed the valves. Willis tried the completed motorcycle at his local racing circuit (Mt. Druitt), but found that it was no faster than his ex-works dohc engine. Plus, the cobbled-up rockers proved troublesome (he used silver soldered wrench handles for the finished product as well), with breakages at the joints, and as he was only interested in quicker lap times, he decided to give up on the project. The bike languished in his shed for a year, with one further attempt to cure its ills. When this failed, Sid melted down the cambox for another project! No photos exist of the completed desmo machine, although Dennis Quinlan, when interviewing Willis for an article, was able to rescue the model from his scrap metal bin!

This post is based on a story written by Dennis Quinlan which can be found here.

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