Stuart No 1 - Crankshaft

The critical objective with any crankshaft is to ensure that the crank pin and the main axle are parallel in all planes. If this is not the case there are all sorts of awkward twisting actions applied to the connecting rod causing tight spots and binding. The other dimension to get right is the linear distance between the crank pin and the axle as this defines the stroke of the engine.


First the main axle was cut to length and so was the crank pin. Both were faced square in the lathe and the axle was set to run true in the 4 jaw so that it could be centre drilled accurately in both ends.









The crank webs were machined as a pair to ensure that they were identical.

The two blanks were screwed together and the 2 holes were bored using the 4 jaw chuck on the lathe to get them exactly parallel.

When this was complete the web were cut to length using a hack saw, removing the securing screws in the process.



The rods and webs were assembled and cross-drilled in the drill press to allow pins to be used to hold the assembly together. Both crank webs were clamped together in the drill vice to ensure that they were pinned in line with each other.










Next crank assembly was silver soldered together.

It was left to cool slowly in the hearth so that it didn't harden or distort.









The crank was mounted on the lathe between centres to turn the webs to their final thickness.

The radii on one end of the webs was also turned in this setting. A drilled block of bright mild steel was used to drive the crank round. I didn't have any right handed tools so I turned the assembly over to machine the second web.

Also in this setting the reduce diameter section of the axle was turned to size.











The next process was to remove the section of the axle between the webs and then machine the inside faces of the webs. To do this 2 blocks of bright mild steel were drilled to hold the crank axles and another hole 1 inch from the first hole was drilled to act as a centre to turn the workpiece. The radii on the other end of the crank webs was also cut in this setting.

Very light cuts were the order of the day to avoid the workpiece moving.









The finished crank.

The crank pin was also skimmed in the above set-up to ensure it ran true.





Stuart 1 Index