The trigger components for the distributor are located on the distributor driveshaft below the rotor. The distributor driveshafts are driven by the crankshaft through Oldham couplers on either side of the wheel case. Since the triggers are magnetic, the driveshafts were machined from aluminum to avoid influencing the magnetic fields at the Hall sensors mounted near their ends.
The trigger disks in the distributors of the three engines that I've previously built have all used multiple magnets mounted on nonferrous disks rotating in front of single Hall sensors. The sensors were located inside the housings of the distributors but were protected from ionized discharges by spark shields covering them. All three distributors worked reliably, and the only ignition-related issue I ever encountered was with the Howell V-4 distributor whose neodymium magnets slowly weakened over time and reduced the dwell for its transistorized ignition. The magnets' loss of strength was most likely a result of the extremely close proximity of the opposing fields of the distributor's irregularly placed magnet pairs.
The Merlin's magneto housing has room only for a tiny trigger disk, and so multiple magnets weren't an option. Instead, the Quarter Scale uses a single stationary magnet with a ferrous disk rotating between it and the sensor. A pair of holes drilled through the disk exposes the magnet to the sensor as it rotates. As a bonus, the sensor ends up mounted outside the rotor housing and safely away from the thunderstorm at the tip of the rotor.
The trigger disk is driven at 1-1/2 times the speed of the crankshaft, but a gear on the driveshaft drives the rotor at half the speed of the crankshaft. The phasing of the rotor with respect to the crankshaft is easily adjusted thanks to a pair of spacers that grip the drive gear after the assembly is tightened together by a bolt on the end of the driveshaft. Previously drilled index holes in the rotor and rotor housing align when the rotor points to tower electrode number one, and the rotor can be temporarily pinned in this position during alignment. The trigger disk is simultaneously gripped by the same spacers, and this allows the timing of plug number one's firing to also be adjusted.
The ideal material for this type of trigger disk has a high magnetic permeability so it will easily conduct and shunt the flux of the source magnet away from the sensor when the trigger is OFF. It also has a low remanence which means that over time it will acquire little magnetism of its own from the nearby source magnet since this would change the threshold of the sensor. Silicon or 'magnetic' steel is best used in these applications and is found in the laminated cores of transformers and motors. Magnetic steel is typically laminated for ac applications in order to reduce power losses, but laminating provides little benefit in a dc application. I considered salvaging the steel from an old transformer, but its laminations were too thin, and I didn't want to deal with the extra complexity involved with stacking them. A good second choice would probably have been wrought iron, but it can be gummy to machine, and I didn't have any on hand. My third choice was hot-rolled steel. Quality hot-rolled mild steel typically has half the remanence of cold-rolled steel, and even less remanence than most hardenable alloys.
The trigger disks in the Merlin's distributors require a pair of diametrically placed aperture holes to handle the two rows of tower electrodes. In order to reduce timing jitter, the holes must be matched and carefully placed. I made my disks by turning a rod to final o.d., plunge-milling the apertures from one end using an end mill, parting off a pair of disks, and then surface grinding their faces. I verified there were no visible asymmetries by stacking the aligned and anti-aligned pair back-to-back under magnification on a snug fitting rod.
There's probably a good reason why the apertures may appear to be excessively wide to someone who has built a more conventional distributor. Although no ignition details were provided in the documentation, the disk was likely designed to provide dwell for a transistorized ignition. With the disk rotating at 1-1/2 times the speed of the crankshaft instead of at the more familiar half speed of the rotor shaft, the apertures had to be widened accordingly. Since I plan to use CDI modules, the wide apertures probably weren't necessary.
The distributor drawing included a timing adjuster which is a two part Delrin assembly that supports the magnet and sensor while the trigger disk spins between them. Neither a particular magnet nor sensor were specified, but the timing adjuster drawing showed a 1/8" diameter by 1/8" deep flat bottom hole for a magnet. I trial tested an 1/8" long neodymium magnet with an Optek OH090U sensor that I had on hand and found that the sensor would fire through the disk apertures using the .125" magnet/sensor separation shown in the drawing. Details about how a sensor was to be consistently supported in the mount provided was unclear in the drawing. Since I had several Optek sensors on hand, I designed a mount around them with a tight fitting pocket for the sensor and a strain relief for a cable.
I've standardized on Futaba J male servo connector cables for the sensors in all my engines. These are readily available from RC hobby shops, and I've found the connectors to be reliable and easy to work with. Using a standard connector allows the use of common test fixtures which in the past have helped speed testing and troubleshooting. A minor problem is that the order of the three sensor leads doesn't match the order of the color-coded wires in the Futaba 3-wire flat cable. When soldering a sensor to the cable, one of the leads has to cross over the other two, and so I included a milled trough in the sensor mount to allow this to be cleanly done. After soldering the cable to the sensor in its mount, a few drops of silicone windshield sealer stabilizes and insulates the connections. A cover bears down on the wired assembly inside the sensor mount and the pair are held together with the two mounting screws and a short length of shrink tubing on the adjuster arm.
After machining the first part of the timing adjuster assembly according to the drawing, I discovered a significant interference between it and the magneto housing. I modified its fit for use in testing but changed the design before machining the final parts. The test part ended up being put to good use, though.
With all the parts for the timing adjusters finally completed, I assembled the first set to check its functionality but without high voltage. The sensor fired through the aperture holes but would not turn off. The sensor's hysteresis combined with the huge flux level flowing through the wide apertures and around the edge of the disk prevented the sensor from turning OFF. The OH090U sensor is the most sensitive part in an Optek Hall sensor line which also includes the OH180U and OH360U. (The number in the part number is actually the typical flux density, in Gauss, required to turn the sensor ON.)
A shorter 1/10" long magnet gave about the same result, and an even shorter 1/16" long magnet appeared to work but with little margin. I decided to machine a new set of thicker sensor mounts so I could experimentally reduce their thicknesses and find an optimum magnet-sensor separation. After some testing I found that .175" was near optimum using the 1/10" long magnet and the OH090U sensor, and so the whole batch of new mounts were modified accordingly.
During final engine assembly when the rotor and timing disk are initially phased to the crankshaft and locked into position with the spacers, the timing can be adjusted over a limited range by rotating the arm of the sensor mount. The adjuster rotates the magnet/sensor pair with respect to the timing disk apertures causing the plug firing angle to change. After adjustment, it's locked in place with its own screw. The adjustment range is ultimately limited by the widths of the rotor and tower electrodes. It's important for a portion of the rotor electrode to remain overlapped with the corresponding tower electrode over the entire usable range. For the electrode and rotor diameters used, the available timing range works out to a maximum of +/-9 distributor degrees or +/-18 crankshaft degrees around the electrodes' center. Since timing changes are made with respect to the trigger disk which rotates 1.5 times faster than the crankshaft, one degree of crankshaft timing change will require 1.5 degrees of timing adjuster rotation. The total adjustment range of the arm is 38 degrees. - Terry