Allen’s back with THREE pages…
Iquite often wake up at night with my mind buzzing in the middle of working out how to make a part, resolve a problem, or in the case of my RC374 engine, thinking ‘what have I got myself into this time?’ The engine I’m attempting to make is based on a very fragile 1960s six-cylinder race engine with a short and limited life that was originally made with space age exotic materials and precision. These engines would barely run below 5000rpm but would top 17,000rpm at full chat and I’m trying to make a similar engine from a couple of old Yamaha engines in my shed with quite basic tools and equipment. The hardest and most complicated part of my engine was making the new pressed-up roller bearing crankshaft from scratch. I explained the initial stages last month; the next stage was to start making actual parts. I had already made a rough drawing based on the original Honda design, noting down a few critical dimensions required for each part. I usually make parts by feel from an image in my mind but this crankshaft was so complicated I found it virtually impossible to visualise all the parts in detail while I was machining. To simplify the manual machining process I made a fixture that would ensure the distance between the crankpin centre line and main journal was identical on all parts; this is critical on a pressed-up crankshaft for it to run true once assembled. I set this dimension to exactly 17mm (half the stroke of my engine) on my fixture using a toolmakers button, slip gauges and a dial test indicator (DTI). This is an old pre CNC method of setting up parts where accurate location of centres is required.
The steel for the crankshaft webs was delivered in 12 pre-cut billets; these were set up on my lathe in a four jaw chuck and machined into six webs with integral big-end shafts and six webs with integral main shafts. About 80% of the material ended up as swarf! I then set up my machining fixture in the four jaw chuck with a DTI checking that the datum face and diameter were running true. The webs were then set up one at a time on the fixture, locating on the finish machined main journal to machine the big-end bore. By using a reducing sleeve in the fixture bore the other six webs were set up locating on the big-end shaft to machine the main shaft bore. The machining of the bores to match the diameter of corresponding shafts was critical to ensure the correct interference fit when the crankshaft was assembled. My earlier trials with the single test crankshaft had helped me establish the dimensions of the counter balance so these sizes were replicated on five pairs of webs during machining, but one web was underweight due to additional machining for the gear drive. This was resolved by drilling two 12mm holes through the web that is next to the geared web and pressing in two slugs of Tungsten. Tungsten metal is one of the heaviest elements, weighing in at almost 2.5 times heavier than steel and it was just enough to bring the web back into balance. The last machining job on the webs was to drill oil feed holes in order to feed oil to the big-ends. The oil flow to the main bearings could be routed to the big-ends by drilling a small hole through the main journal across the web into the big-end journal. This was carried out on my drilling machine with a simple angle fixture to secure the webs in the correct position so that the hole would exit in the correct place. All sharp edges were then removed and smoothed to a radius with a fine file and emery cloth followed by a polish on my buffing wheel. The webs were then cleaned with carb cleaner and an airline to remove all traces of buffing soap, swarf and metal filings. With all the parts machined I was ready to assemble the crankshaft. I had been thinking it through for several days prior, to work out in my mind the best assembly sequence, and decided to start from the centre and work outwards. The centre section was a straight shaft, the camshaft drive sprocket was pressed on to it and located in the centre, then two caged needle roller bearings complete with hardened inner rings were pressed onto the shaft, followed by the first two webs, one each side using my hydraulic press.
The first two webs were easy to assemble because they were in-line for pistons three and four to rise and fall together, this allowed me to lay them flat on a block of steel and tap the webs into perfect alignment with a copper mallet. These two webs were then drilled and pinned to the central shaft. At each stage of assembly I checked that the growing crankshaft fitted into the crankcases and would rotate freely, fettling the crankcases to remove any high spots as required. The next stage was to assemble the webs for cylinders two and five. These had to be aligned at 120º relative to the central webs and in-line with each other. I mounted the part-completed crankshaft in my lathe to set up the 120º angle with a degree disc, then pressing the web on a little bit with the tail stock before carefully removing the crankshaft for pressing together using my hydraulic press. The processes of setting up, pressing, checking alignment and pinning were repeated until the whole crankshaft was assembled. After final checks for run-out the crankshaft was placed into the bottom crankcase and the upper crankcase tightened down. Six bare pistons were attached to the connecting rods then the base gasket and barrels were lowered down and tightened. I carefully turned the crankshaft slowly with a spanner and was pleased to see all six pistons rising and falling in pairs. I then set up a pair of DTIS both set to zero when a pair of pistons were at top dead centre (TDC). When the crankshaft was rotated each pair of pistons rose and fell in unison reaching TDC at the same time indicated by simultaneous zero readings on both DTI. I then mounted a degree disc on the end of the crankshaft and rotated the crankshaft to check that the three pairs of pistons were spaced 120º. I was pleased to see perfect alignment so with the crankshaft complete I was ready to carry out the first complete wet assembly of the engine to check oil flow, leaks and compression. I will tell you how that went next time!
Primary drive webs.
Finished and polished components.
Drilling holes for Tungsten slugs.
Machining a billet of steel into a crankshaft web.
Finished crankshaft in crankcases.
Crankshaft assembly, checking alignment.
Finished crankshaft in my hand.
Drilling hole for location pin.