Reply To: Johnson HS39 carbon rods and springs

[Michael Purpura]

For what it is worth the rods are 0.250″ conductive graphite. There is a 1/8″ shoulder on one end turned to 0.200″. The top is radiused to allow it to slide between the bakelite and the coper conductor plate inside the flywheel. The springs are 0.4375″ free length, 0.145″ OD, 0.115″ ID. Coils total 7 – 5 active and 2 inactive on the top gripping the shoulder of the rod. The wire thickness is 0.015″. I still do not know the original length, but I am thinking that there can’t be more than a 1/4″ of allowable travel in that spring. One of my rods is 11/16″ and the other is 7/16″. I will try to determine the original length by measuring the free height inside the flywheel when it is mounted. Then I will add length to compress the spring + 1/8″ for the shoulder. This should give me the overall length of the graphite rods. I plan to turn them on the lathe and radius the top that contacts the underside of the flywheel. Since it is graphite it is self-lubricating but over 50 years or so some wear must occur.

I’ve had several suggestions to stretch the springs. I don’t think this is the right approach. It will change the rate on the spring and you could end up with less force against the graphite rod. I think the pressure has to be consistent to make good contact and conduct the pulse from the magneto. There is probably some limit to the wear on those graphite rods, beyond which the electrical connection is not good enough to conduct the pulse to the coil.

I will let you know how the parts turn out and, more importantly, if the engine runs smoothly after I get them replaced. If anyone has any information on the original length of these rods let me know. McMaster Carr has 1/4″ conductive graphite in 6″ and 12″ lengths.