ALLEN MILLYARD
Allen talks about the inspiration behind his super-six creation – and where the donor engines came from.
Last July I rode my 8.0 litre Viper V10 to Castle Combe circuit to see my friend Dave ride his Yamaha TZ350 and while I was there I saw Guy Martin and had a close look at the 297cc Honda RC174 six that he was parading. I’ve been fascinated with these bikes for many years and have always wanted to make one, not an exact replica but one similar in appearance, and most importantly sound. We had a long chat about motorcycles, mountain bikes and engineering and after this chance meeting a seed had been planted... While riding home that evening my mind was buzzing, all I could think about was making my version of the Honda six and where I might get a pair of suitable donor engines from. The next day I made a call to Pete Amson at DK Motorcycles, asking if they had any 1980s 250cc four-cylinder engines in stock. My first choice would have been the 250cc Honda MC22 engine as fitted to the CBR250RR and Hornet, but due to their popularity for use in replica parade bikes they are becoming hard to find and quite expensive: my aim was to reproduce the sound and throttle response of the original Honda engine and have a passing resemblance in looks especially as the engine is mostly hidden behind the fairing, so the make of donor engine was less important to me this time. Thankfully, Pete had a pair of identical Yamaha FZR250 engines in stock: superb! I set about stripping them both down and could immediately see the potential for making them into a six cylinder engine. The capacity would be 374cc, which is only 12.8cc per cylinder larger than Honda’s 297cc RC174. The cam-chain was central and the transmission drive was between cylinders three and four, making it easy to add a cylinder to each side of the original four cylinder engine. I rarely do drawings, I just look at the parts in front of me on the bench and visualise how they need to be cut and fitted together. The first job was to trim each side of one set of crankcases on my milling machine to accept an extra bit of crankcase each side at the front for the two extra cylinders. These parts were cut off each side of the second set of crankcases with a hacksaw then machined on the milling machine to ensure a nice flat surface ready for welding it all together. The same process followed for the cylinder head and barrels. All these parts were then dropped off at the vapour blasters for cleaning. In the meantime I machined up three steel mandrels on my lathe. Mandrels are steel bars machined to a precise size to match the line of main bearings and the two lines of camshaft bearings and will be clamped in place during the welding process to hold the parts in line and help to minimise distortion. The edges to be welded are chamfered using a file and rotary burrs, this gives the weld better penetration and increases strength, especially when some welds will be filed down to hide them. The freshly cleaned crankcase parts were then assembled with the steel mandrel in position and all the securing bolts tightened. After a final alignment check I pre-heated the assembly with my oxyacetylene torch then using my TIG welder I welded all accessible joints together. The crankcases were left together while they cooled, then I carefully filed protruding welds flat where the barrels seat. The head and barrel parts were then assembled onto the crankcases and bolted down tight with the two camshaft mandrels in place to maintain alignment. All accessible external joints were then welded. After allowing it to cool I removed the head and barrels and split the crankcases to complete the internal welds. Welds that span gasket joints were carefully filed flat then the crankcases, barrels and head were re-assembled with the mandrels in place and heated up and allowed to cool overnight; this helps to relieve stress from the welded parts. The next day I set about blending in all visible welds with files and rotary burrs. The gasket surfaces were cleaned up using wet and dry paper stuck to my milling machine table with double sided tape.
If done carefully this is quite an effective method producing a flat and smooth surface while removing the minimum of material. The head and barrels were put aside while I completed work on the crankcases. I will be making a pressed up roller bearing crankshaft for this engine so I need to line bore the main journals in the crankcases to fit caged needle roller bearings in place of the standard plain metal shell bearings. This task was carried out with a simple jig that I made to make my Kawasaki Z1600 V8, Z2300 V12, and H1 500LC where the main journals needed machining to a larger diameter. Basically two brackets are machined up and bolted to each side of the crankcases with each bracket containing a bearing. The bore of each bearing is centralised to the main bearing journals, then a steel shaft is inserted into the bearings that has a single adjustable cutting tool in the centre. Using an electric drill the shaft was rotated at medium speed, slowly traversing back and forth allowing the cutting tool to cut metal from the journals. The cutting tool is progressively adjusted to cut a larger diameter after each pass until the required size is achieved. With the main bearings bored to size, the work on the crankcases was finished for now. Moving back to the barrels, I started to press the cylinder liners back into the barrels, but on closer inspection I found that I only had five good liners from the two donor engines. I wanted to retain standard bores so I bought a suitable cylinder liner blank casting so that I could machine a new liner on my lathe. The new liner was machined with the bore 0.25mm undersize and pressed into the barrels. The barrels were then set up and finish bored on my old 1950s mini bar cylinder boring machine. All bores were then honed ready for assembly later on.