STUART LIGHTWEIGHT PETROL ENGINE BUILD

Part six by Jason Ballamy

The crankshaft was worked on over a period of time as and when other parts were made that are affected by it and also to retain the centre holes for as long as possible should any slight adjustments have been needed. Stuarts originally supplied this as a disc that was friction welded to a shaft but I cut mine from solid.

Starting with a length of 2" dia EN8 steel a suitable length was parted off, deeply grooved and then finished with a hacksaw. A Femi 782XL bandsaw is due.

The two ends were faced off with the bar held in the 4-jaw. To drill the centre holes in each end on round stock I find that clamping it to the vertical slide ensures the two holes are in exactly the same vertical plane at each end. The bar is clocked central and a hole drilled then raised up by the required amount and the centre hole for the crankpin drilled. Then rotate the vertical slide 180degrees on the cross slide and repeat the hole drilling in the other end.
I then removed a bit of waste material from each end to reduce the in-balance as much as possible as the pin was turned starting with a parting tool.

Then cleaning up the inside edge of the webs with a HSS tool that had a small radius on the end to reduce any stress points in the corners

A spacer was then turned up to stop things deforming while the shaft was turned, this was retained with a zip tie and a spot or two of hot melt glue to stop it rattling about.
A short taper was turned on one end to take the ignition cam and a longer one on the other end for the flywheel. Mating parts were bored at the same taper setting to make sure they seated well.
An M8 thread was cut for the flywheel retaining nut.
And a slot cut for a woodruff key
An unused bit of my lathe cabinet was put to use for the two counterbalance weights which were machined and filed to shape.
Some matching holes were added to the crank webs with the aid of a long centre drill and extra long 3mm drill.
The weights were then screwed on with a drop of Loctite 648 on the screws and finally skimmed down to match the dia of the webs.
The final job to keep true to the ‘lightweight’ theme was to drill out the middle of the pin and down into the shafts to get the weight down a bit further.

See Part one here  part two  part three  part foupart five  part six  part seven  part eighpart nine