Crankshaft Dreaming...

The home-build genius tells us how he gets that EUREKA moment when designing and building his amazing specials. Crank it out, Allen…

- Allen Millyard

I quite 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I’m attempting to make is based on a very fragile 1960s six-cylinder race engine with a short and limited life that was originally made with space age exotic materials and precision.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!