Cams & crank time!
Problem-solving aplenty for our man Millyard in his latest column where he’s building a super-sexy six-cylinder motor!
Istarted making my Honda RC174 six cylinder replica last July and one thing that has been on my mind since day one is: ‘how am I going to make the crankshaft and camshafts?’
I remember this happening to me once before when I was making my Kawasaki V12 and I ended up completing the whole bike without a crankshaft because I couldn’t figure out how to fit all the parts into the available space until a while later a solution came to mind. My Yamaha FZR250 donor engines have plain metal bearings and a forged steel crankshaft so this ruled out the possibility of modifying the original crankshaft unlike my two-stroke conversions where the crankshaft can be pressed apart then pressed together with ease, adding webs as required. The chances of me finding suitable crankshaft webs with the required 34mm stroke from an existing engine are close to zero so my new six-cylinder engine will need a crankshaft built from scratch! I am reluctant to go down the route of having parts made by a specialist company because I like to make as much as possible at home in my garage. My dad always encouraged me to resolve problems and work out ways of doing things myself with what I could find or acquire and the solution is usually there somewhere, you just have to figure it out. Parking the crankshaft problem for a while I decided to look at the camshafts first because I could see a way of modifying them from four cylinders to six. The first thing I did was cut the camshafts between the lobes using my angle grinder fitted with a thin cutting disc. These parts were then set up on my lathe and accurately faced off and bored out to accept half a Kawasaki H2 gudgeon pin that will be later pressed in with a tight fit into one end of the hollow camshaft section. The camshafts and gudgeon pins are made from hardened steel but can be machined with tungsten carbide tip tools and once you cut through the hard skin it cuts freely. The protruding length of gudgeon pin would be used to join the camshaft pieces back together and allow me to set the orientation of the lobes to suit my six-cylinder engine. Before I could reassemble the sections of camshaft I had to work out the new firing order, so I bent up a piece of welding rod to make a three dimensional model of a six-cylinder 120 degree crankshaft with a 1-4-2-6-3-5 firing sequence. Using this model as a reference guide, I could reset and press the individual cam lobes back together, and to permanently lock the joints I would be able to spot weld through a small hole that I had previously drilled through to the gudgeon pins. I thought it would be a good idea to have a practise run using spare camshaft sections and was pleased when it turned out well with no distortion after the pressing and welding process. I then assembled the two camshafts using my
lathe with an index plate and DTI to set the angle between the lobes, then pressed them together on my hydraulic press and finally spot-welded to permanently lock the pieces together. The finished camshafts were placed into the head onto pre-oiled bearings and the 24 valve clearances set. At this point I wanted to test the camshafts to check that they would rotate freely and the valve timing was correct, so I made a dummy crankshaft from a length of steel bar that fitted nicely into the main bearings and fitted a spare camshaft drive sprocket to the centre to drive the camshafts. I then dry assembled the crankcases, barrels and head with the dummy crankshaft in place, fitted the cam-chain and carefully turned the dummy crankshaft watching closely as the camshafts rotated. After two full rotations of the dummy crankshaft all valves seemed to work fine, and the camshaft timing marks realigned perfectly. I attached my electric drill to the dummy crankshaft and spun it up briefly. I was pleased to see a blur of spinning camshafts and looking up the bores from underneath I could see all the valves functioning perfectly. Back to the crankshaft, the first thing I had to do was to measure the space available in the crankcases and draw a basic layout drawing to establish the critical dimensions. I wanted to retain the starter motor, but decided to remove the alternator to reduce the crankshaft inertia and engine width. I found a photo of the original RC174 crankshaft online, which was a great help because I was able to scale the dimensions to give me a rough idea of the web proportions. I have a box of connecting rods from all sorts of engines and managed to find one of a similar size to the original FZR connecting rod, but designed to run on needle roller bearings and that was also half the weight. I could now add up the width of the bearings, thrust washers and connecting rods to see what was left for the 12 webs. I was quite relieved when there was just enough room in the modified crankcases. The 12 billets of high tensile steel that I would be machining into the new crankshaft webs were then ordered along with seven caged needle roller bearings and six new connecting rod kits. Machining the crankshaft components was going to be a complicated process to ensure that each web has identical dimensions between bores and shafts so I made a machining jig that would fit in my lathe chuck for the finish machining of bores and shafts and to allow me to replicate the parts. The camshaft drive sprocket and primary drive gear were cut from one of the FZR crankshafts using my angle grinder, faced off and bored to the required size in the lathe, these will later be pressed, pinned and welded onto the new crankshaft components. To simplify the balancing process and test the crankshaft design, I started by making a pair of webs from mild steel then assembled the connecting rod and main bearings. This single cylinder portion of crankshaft was assembled into the crankcases to test rotational clearance and then the barrel and one piston was added to check the piston deck height. The test crankshaft was also used to check and adjust the balance factor to 55% and finalise the web dimensions before making the final versions.