Originally Posted by
Jim.Thompson
The extension will have a mating flange with a spigot and be fixed with four axial studs.
that is how the hubs are held on the larger zenoah engines but with only two screws. aftermarket adapters to replace the small flywheel are available. no idea if the taper is the same but if it is that would save a lot of work.
how i make single piece hubs: bore a pilot hole in the hub stock that is slightly smaller than the diameter of the small end of the taper. add a counterbore large enough to accept the crank nut and socket. this should have a flat bottom for the nut/washer to seat on. for my engines this is usually 19mm. make the bore about 1mm deeper than needed so you will have something to remove when facing the front of the hub later on. back end of the hub where the taper goes should also have some extra stock left. concentricity between this bore and the stock isn't important. if doing this on a mill then now is the time to add the prop screw holes
in the same setup so they are concentric to the counterbore.
now chuck up a piece of round stock that is larger than your counterbore. i use 20mm ground stock in a collet but a regular 3-jaw chuck is fine as runout isn't important. turn this stock to a shrink fit with the counterbore in your hub stock. make it long enough so that you can still get in to face the front of the hub but no more than that. put a shallow couterbore in the end of your new mandrel that is slightly larger than the small diameter of the crank's taper, this is to allow whatever tool you use to cut the taper to pass through without hitting the mandrel.
mount your hub stock on the mandrel and cut the OD, both faces and the taper. go slow with the taper so you can check it against the crank. to broach the keyway grind up a very sharp tool the same thickness as the key and mount it centered with the hub. use the compound set at the same taper angle to plunge cut and wind out the cross slide a bit with each plunge until you get the depth needed. nice if you can lock the spindle but not really necessary as each cut should follow the last.
important part is to never remove the mandrel from the chuck until you are done.
6061-t6 or 2024-t4 are both fine.