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Machining a New Ring Gear Holder For Electric Starting

In our restoration-machine shop we are fortunate to work on some very special early cars which we choose not to modify in spite of modern day needs. Most all of the pre-1912 machines are started by hand crank and because of modern day safety issues, we will occasionally convert a car to electric start when needed. There are many ways to do this but we choose to engineer a bolt-on conversion which can be installed without modifying any of the original parts. This way it is totally reversible and does not affect the car’s integrity at all, as would machining and installing a ring gear on the original flywheel. We have not done a technical feature in a while so we thought you might enjoy seeing what is involved to do this.

This is designed to sit inside a Mercer Raceabout flywheel which is dished and to attach onto the front flange of a universal joint which is between the flywheel and the transmission (this u-joint flange always runs true to the flywheel, only the back half of the joint is flexible). The only change needed is six bolts 1/4″ longer to attach it and the u-joint to the flywheel. Follow along below where we describe how it is machined.


Left to right a high-strength aluminum alloy plate is first bored in the center on a milling machine and the corners are cut off to make machining quicker. It then gets set-up and turned round and to the proper thickness on the lathe. It takes extra time but it is relieved in the center to keep it as light as possible. The last photo shows an off the shelf ring gear on for a trial fit.


The plate is then set up in a vertical milling machine on an indexing head which turns and locates precisely at the right angle to keep the holes all evenly spaced. While in the indexing fixture the holes at the center are bored and reamed. More unneeded metal is also removed at the center and twelve lightening holes are bored in the middle section.

The last operation is to install the ring gear (a light press fit) and bore and ream four holes through both for pressed in dowel pins. The other twenty holes are drilled in the plate for button-headed Allen screws and the holes drilled in the gear are tapped for threads. The bolts are installed and tightened and the assembly is complete. The last photo shows it on the back of the u-joint flange.

We have installed several of these along with a hidden starter which also bolts on without any modifications to the car and the results have been excellent. Once the center of the Allen heads are filled (to resemble rivet heads) the assembly is painted to match the original flywheel. When installed, it is all but hidden and only visible if one is under the car. Several thousand miles in all types of driving, including the race track have shown that it works very well and only adds about sixty pounds of weight to the car including the battery and its box.

Cost wise it is comparable to finding a large diameter ring gear today, as they are getting hard to find, or having one machined. In addition to that is the cost of machining and installing one on the original the flywheel.

6 responses to “Machining a New Ring Gear Holder For Electric Starting

  1. was a machinist at boeing for 25 years, retied. your work is truly amazing. i am a car guy and seeing you keep the old ones going makes me happy i all ways thought working on old automotive stuff would be fun.

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