Back to the future
Our Allen gets on with making an old-school Brit bike into a V-twin! This month he's concentrating on the motor and making it look 'factory'.
Ilike to vary the style and era of my motorcycle projects and not stick rigidly with any one make. ‘If it’s got two wheels and an engine, it’s a motorcycle,’ and that’s good enough for me.
My new Velocette project had got me thinking how amazing this old British bike really was. I remember my dad saying to me years ago how Velocette engines were really well made, and I tend to agree with him. In March 1961 a factory prepared 500cc single-cylinder Velocette Venom set the 24-hour world record at an average speed of 100.05mph, a record which remains unbroken today. The crankcases are very narrow compared to other single cylinder bikes, and it also has a high camshaft like a Honda CX500, but driven with fine helical gears that run almost silent and with total reliability. One advantage for me is that the casting wall thicknesses are relatively thick by modern standards, which gives me more metal to play with to make strong welded joints. The main part of the crankcases were nearly complete and they were taking on the shape of a 50-degree V-twin, but now I needed to concentrate on expanding the timing chest and adding a second camshaft to operate the front cylinder inlet and exhaust valves. The rear camshaft is located in its standard position and engages with a second motion gear that’s driven from the crankshaft. I used a strong magnet to hold the front camshaft in the required position while I measured and marked out where to cut the crankcases to incorporate a new piece of timing chest. The new piece would be welded in place and would house the front camshaft and cam followers. I wanted to retain the Velocette look and not end up with a billet-looking crankcase addition, so I had a look at the parts of leftover crankcases from the initial construction and could see that if I cut out the top section of the timing chest complete with camshaft bearing shaft and rocker pivot shaft, I could then file it to fit into the new V-twin crankcases. Both parts needed to fit nicely together while maintaining a precision engagement of the gears. To achieve this I cut a bit bigger than needed, then gradually milled and filed the edges until they fitted perfectly together and the camshaft gear meshed nicely with the second motion gear when rotated. I then clamped the parts together tightly and started to tack weld using my TIG welder, correcting any distortion with opposing welds as I progressed. Half-an-hour later the welding was complete, and after cooling all seemed to be good except the front camshaft gear was a bit tightly meshed with the second
motion gear, probably due to contraction of the welded area. Looking at the second motion gear bearing shaft I could see that it was adjustable within the crankcases to allow for fine adjustment of the gear mesh, so I loosened and re-tightened the three screws and the gears, then meshed nicely together. At this stage I wanted to check the whole camshaft drive train worked, so I assembled a timing side crankshaft web with first motion gear, second motion gear, both camshafts and the four rocking cam followers. I then rotated the crank web and both camshafts turned nicely. The next job was making the new timing cover from 16mm thick 6082T6 aluminium plate. First of all I placed the plate on to the timing chest and marked around the outside with a marker pen and cut out the shape on my band saw, then I filed the perimeter shape to roughly fit the engine. This was a new machined part, but I wanted to recreate the 'Velocette' look by shaping the edges first on my milling machine to rough out, then with rotary burrs, files and emery paper to create the finished look. While on the milling machine, I machined the circular recess that would house the twin ignition points and manual advance mechanism, and then a shallow circular recess to hold a circular portion of the original Velocette timing cover with the Velocette name and 'made in England' cast in lettering. Once this was done I had to drill the holes for the many securing screws. This is a critical job because these holes need to line up with existing blind threaded holes in the crankcases and any errors would show on the finished timing cover. A simple way to mark out the holes was to machine a 45-degree included angle point on to the grub screws, then screw them into the threaded holes on the crankcases until the tip of the point was about 5mm proud of the crankcase surface, then offer up the aluminium plate and align it to fit precisely over the outer edges of the timing chest, then using a soft mallet tap around the edge of the plate. When the plate was removed, the point on the grub screws had marked the centre of each hole. I then used a centre punch to enlarge the marks ready to drill the 5mm holes. It’s always a good idea to use a smaller drill first, then drill the required size hole, as this reduces the chance of the drill wandering off centre. These holes were then counter-bored to allow the head of the screws to sit flush, just like on the original timing cover. Once all the mounting holes were drilled and counterbored, I checked that it fitted the crankcases by fitting and tightening all the screws, and then finish blended the periphery of the timing cover with a fine file until it blended in with the crankcases. The original Velocette timing cover has oil pipes cast in to distribute oil to the crankshaft and camshaft, so I had to replicate this system in my new cover. To do this I first drilled and tapped the two 1/8th BSP threads in the top of the timing cover where external oil feed pipes will eventually supply oil to each cylinder head, and then drilled the crankshaft central hole, and the oil input hole. I then milled a 8mm wide by 8mm deep slot from each drilling to the oil input hole, followed by a 16mm wide slot over the top, 2mm deep. To cover the slots and make an internal oil way, I cut out a piece of 2mm thick aluminium sheet to fit perfectly into 16mm wide slots and screwed it in place with several M3 screws. On final assembly I would use Permatex Ultra grey sealant to seal the joints. Brass pipe fittings were then screwed into the BSP threads. The last job was to tap the thread in the crankshaft feed hole, fit the oil injection fitting and trial fit the new timing cover to the crankcases. The timing cover would need a bit more machining later on when I fit the ignition parts, but for now I gave it a clean-up with wet and dry paper and a polish on my vintage buffing machine. The crankcases were then bolted up and the timing cover fitted. The V-twin engine was now beginning to take shape ready for the next stage.