BMG Motor-Cycles Ltd of 352 High Rd, Ilford, Essex, developed a desmodromic valve kit for Velocette 'M' series engines (Viper, Venom, MSS), and applied for a patent (#939,895) on May 17, 1962. The drawing reproduced here is taken from their patent application form, and shows the parts which they manufactured as a direct bolt-on accessory, with no major machining required.The kit cost £38, or £46.10 if they fitted it to your machine.
The BMG setup is ingenious in its simplicity and clarity of purpose, and is fairly well made, although some have called it a bit agricultural. Regardless, the kit worked as advertised; this was borne out in a road test by Bruce Main-Smith in 1963 (The Motor Cycle), who said 'My summary is that this is the tool for the rev-happy rider'. His road test isn't exactly a ringing endorsement, although he is able to rev the test bike (a 350cc Viper with Butler dolphin fairing - see period photo) to 7500 rpm in the intermediate gears, although he can 'only' pull 6200rpm in top, as the bike is slightly over-geared - this equated to 93.9mph on the Viper, which is going very well. BMG had tested their own Viper and found 6800rpm/102mph was possible.
Having said all that, Main-Smith seems strained to find a reason to spend money on the kit, as the power gains were difficult to gauge (the fairing would have added to the top speed considerably without the desmo kit). The road test was taken in poor, windy weather, and as far as I know, no other road test was
published in the day, or since. Supposedly, a Thruxton equipped with the BMG kit took a speed run at Bonneville, and managed a record top speed, but I haven't found firm documentation on the results. All other indications point to an overall moot gain in power with the kit, although I'd love to hear otherwise, or from someone who actually fitted the kit to their Velocette.
The lower photographs are courtesy Dennis Quinlan, showing a display cutaway engine built by Australian Velo technical guru, Norm Trigg. The BMG kit has been fitted and the installation can be clearly seen, including the cam shaped, paired cam followers, and the positive valve stem connection. You can see, slightly in shadow, the lower rocker arm which lifts the valve off the seat. The pushrod connection at the other end of the rocker arm is fairly substantial, and is where the valve clearance is adjusted.
As there is no valve spring, the cam isn't fighting 100lbs or so of spring pressure to open the valve, which theoretically would give the cams a much easier life, and have the effect of vastly lightening the whole valve train, allowing the engine to rev more freely.
These BMG kits come available now and then, and I've always thought, 'what if?'
