For best visibility while aligning the cutter in my helical gear machining setup, I machined the gears with an opposite handedness compared with those in the full-size engine. The downside of doing this was a reversed direction of the rotor inside the distributor and a backwards order for the plug wires coming out of the cap. A small upside is that the axial thrust created by the driven gear will tend to pull the tiny distributor into the engine rather than push it out.
Fixturing an operation to bore the distributor's mounting hole in the block was a lot easier and much less risky than I had been imagining. After squaring the block on an angle plate, I realized the bore could be accurately located using a driving gear on the end of a rod in the camshaft bore and a driven gear on the end of a dummy shaft in the spindle. The final result was a smoothly meshed gear set with near zero backlash.
The distributor body was machined from 6061 and the distributor shaft from 303 stainless. I replaced the bronze bushings for the distributor shaft with sealed ball bearings to avoid having to manually oil them later. The shaft's long skinny end was turned without using the tailstock similarly to the valves (in a previous post) using short overlapping segments turned one at a time.
A rather sketchy fixture was used to drill a 3/64" hole simultaneously though the assembled gear and distributor shaft for a pin to lock the two together with zero thrust clearance. The pin will be Loctite'd at final assembly but will remain loose for fit testing partial assemblies.
The distributor uses a shuttered type Hall trigger. A machined steel cylinder with six openings rotates between a magnet and a Hall device to provide timed trigger pulses to a CDI. An alternative would be to use six magnets, but this can become tricky inside a tiny distributor where the individual fields of the magnets can interact and reduce the sizes of the flux pulses seen by the Hall device.
A shuttered trigger disk has its own limitations. Even at the ON location in front of an open window, much of the magnet's flux will be shunted away from the Hall device by the low permeability return path offered by the disk. The disk's permeability can be reduced and the flux pulses seen by the device increased by judiciously machining material from the disk. However, if too much is removed, the disk can become saturated and allow flux to pass through even a closed window. Some trial and error is usually required to obtain reliable operation that's also dependent upon the particular Hall device, magnet, and the spacing between them.
The trigger disk was machined from 12L14. It's best to use a soft steel alloy for the disk since hard alloys tends tend to acquire and retain magnetism. I modified the disk design to avoid the angled grub screw suggested in the plans to lock it to the distributor shaft. Instead, the trigger disk attaches to a flange on the distributor shaft with three 0-80 SHCS's. The bolt holes are slotted to allow the disk to be rotated on the shaft for proper timing before being tightened down.
A 1/8" x .100" neodymium magnet was epoxied into a machined aluminum holder that attaches to the floor of the distributor body with stainless steel screws. Hall devices tend to go obsolete very quickly making it difficult to recommend a specific part number. I plan to use up some mystery parts from my junk box on which I've run my preliminary tests. They appear to be marked '738S06L', but I have no idea what the actual part number is or who the manufacturer was. - Terry
