A bright red magneto prominently mounted on its front end seems to be a signature feature of the Offy 270. The quarter scale's version is really just a distributor meaning its high voltage will be generated elsewhere. Ron sneaked his voltage into the enclosure through the magneto's 'Points' terminal in order to maintain the illusion of a magneto. This innocuous-looking terminal that would have been at several hundred volts potential in the full-scale engine will ride at several thousand volts potential in the model.
The high voltage's side entry complicates the design of the rotor. A slip ring rather than the simple button contact found in conventional distributors was used to complete the path to the rotor. In order to avoid metal-to-metal wear and potential metal dust that could create leakage paths inside the distributor, the actual coupling to the rotor was accomplished through an air gap. Since this gap is in series with the model's tiny plug gaps, it robs energy from the ignition system.
My first inclination was to retain the slip ring but eliminate the air gap with a spring-loaded carbon brush assembly. After prototyping it, though, I had second thoughts about the inevitable carbon dust I'd noticed around commutators in some of the dc motors I'd disassembled.
Eventually, I decided upon a conventional rotor with its axial contact button. Similar to Ron's distributor, the high voltage is brought into the enclosure through an end-mounted terminal on the cover. However, I modified the cover to accommodate a wire running through a drilled passage between this terminal and the pressed-in button contact. One of the photos shows the voltage path through the modeled cross-section of the modified cover.
Although not obvious from the resulting hue in my photos, the cover was machined from red Delrin. The tower electrodes were turned from phosphor bronze and hard pressed into the top of the cover. The rotor contacts on their ends were exposed after boring the opening for the rotor. The side mounted screw terminal was made up with a threaded insert screwed into the cover. The interconnecting wire is sandwiched under the pressed-in button at one end and soldered to the insert at the other.
Finally, the four high voltage towers were threaded for plastic wire retaining caps similar to those in the original design. Excess material left on the cover's mounting flange will be removed later after the enclosure's details have been finally worked out. Those details depend upon the design of the rotor which in turn was waiting upon a proven cover. With it now in hand, work can continue on the magneto which for the most part will follow Ron's original design. - Terry
The high voltage's side entry complicates the design of the rotor. A slip ring rather than the simple button contact found in conventional distributors was used to complete the path to the rotor. In order to avoid metal-to-metal wear and potential metal dust that could create leakage paths inside the distributor, the actual coupling to the rotor was accomplished through an air gap. Since this gap is in series with the model's tiny plug gaps, it robs energy from the ignition system.
My first inclination was to retain the slip ring but eliminate the air gap with a spring-loaded carbon brush assembly. After prototyping it, though, I had second thoughts about the inevitable carbon dust I'd noticed around commutators in some of the dc motors I'd disassembled.
Eventually, I decided upon a conventional rotor with its axial contact button. Similar to Ron's distributor, the high voltage is brought into the enclosure through an end-mounted terminal on the cover. However, I modified the cover to accommodate a wire running through a drilled passage between this terminal and the pressed-in button contact. One of the photos shows the voltage path through the modeled cross-section of the modified cover.
Although not obvious from the resulting hue in my photos, the cover was machined from red Delrin. The tower electrodes were turned from phosphor bronze and hard pressed into the top of the cover. The rotor contacts on their ends were exposed after boring the opening for the rotor. The side mounted screw terminal was made up with a threaded insert screwed into the cover. The interconnecting wire is sandwiched under the pressed-in button at one end and soldered to the insert at the other.
Finally, the four high voltage towers were threaded for plastic wire retaining caps similar to those in the original design. Excess material left on the cover's mounting flange will be removed later after the enclosure's details have been finally worked out. Those details depend upon the design of the rotor which in turn was waiting upon a proven cover. With it now in hand, work can continue on the magneto which for the most part will follow Ron's original design. - Terry
