2 thoughts on “Summary of Distributor Cap to Rotor Gap Testing”
I have a 70 Plymouth GTX with a 440 6pack and the distributor is electronic but the rotor button and cap are arcing on top of the button & the top of the contacts instead of the side trying to figure out why this is happening
Same problem happened to me going cross country…..distributor center contact failures first which later ended up with coil & electronic ignition component failures. The friend traveling with me in his Cuda had the same problem.
Think: HIGHER THAN NORMAL COIL WIRE VOLTAGES INDICATE POSSIBLE LARGER SPARK ARCS IN THE CAP
You can measure the coil wire voltage easily to validate. Voltages below 10,000 volts will give expected life on the cap center contact. Voltages reaching near 20,000 volts will destroy cap center contact quickly & then other ignition components.
The coil wire voltage is determined by the sum of resistances in the cap, coil wire, spark plug wires, and plug.
Simple fixes are to go blindly around changing things like using 8mm coil & spark plug wires and/or shortening those wires thru better routing. Also look for an old distributor cap & rotor (made in the 60’s)
Problem is likely the improper machining of the distributor cap & with some efforts you can verify. Dealers in the 70’s were having the same problem & Chrysler wrote up a 1976 bulletin saying that even though the rotor electrode lengths had recently shrunk….the resulting larger gaps between the rotor & caps were not an issue……didn’t convince many. Essentially it’s possible Chrysler shrunk the rotor so that it wouldn’t bang into some of the 8 poorly machined Cap contacts.
Today some of the 8 rotor gaps can climb near 1/8″ when they use to be less that 1/32″……that introduces a lot of resistance & raises coil wire voltage. Essentially there is more firing going on in your cap than at the plug!
So if the center contact is breaking down chances are your coil wire voltage is high and the other components could fail soon.
There is a lot more to this; for example some of Plymouth 60’s Caps had wide contacts….why then and not later? Thing is, the advance is always changing so at idle the rotor points to one place on the cap contact when firing but at wide open throttle it points to a much different point. So Chrysler used caps with wide contacts to handle both rotor locations. It was later dropped to lower costs……but unfortunately increase resistances. I buy those old vintage caps to get those coil line voltages down and extend the life of the ignition components.
Also in my case I get a longer rotor & machine every cap; essentially fitting the rotor & every distributor contact to get gaps below 1/32″. End result?: coil wire voltages drop to as low as 6000 Volts & spark plugs act as they should.
An analogy could be to think what happens when a beaver builds a dam in a brook……what happens to the flow downstream & to the height of the water upstream? That’s what those high distributor to rotor gaps increases do…..increase resistance to flow and build higher coil wire voltages.
So if you want to improve the cap center electrode life…..get those coil wire voltages down.
I have a 70 Plymouth GTX with a 440 6pack and the distributor is electronic but the rotor button and cap are arcing on top of the button & the top of the contacts instead of the side trying to figure out why this is happening
Same problem happened to me going cross country…..distributor center contact failures first which later ended up with coil & electronic ignition component failures. The friend traveling with me in his Cuda had the same problem.
Think: HIGHER THAN NORMAL COIL WIRE VOLTAGES INDICATE POSSIBLE LARGER SPARK ARCS IN THE CAP
You can measure the coil wire voltage easily to validate. Voltages below 10,000 volts will give expected life on the cap center contact. Voltages reaching near 20,000 volts will destroy cap center contact quickly & then other ignition components.
The coil wire voltage is determined by the sum of resistances in the cap, coil wire, spark plug wires, and plug.
Simple fixes are to go blindly around changing things like using 8mm coil & spark plug wires and/or shortening those wires thru better routing. Also look for an old distributor cap & rotor (made in the 60’s)
Problem is likely the improper machining of the distributor cap & with some efforts you can verify. Dealers in the 70’s were having the same problem & Chrysler wrote up a 1976 bulletin saying that even though the rotor electrode lengths had recently shrunk….the resulting larger gaps between the rotor & caps were not an issue……didn’t convince many. Essentially it’s possible Chrysler shrunk the rotor so that it wouldn’t bang into some of the 8 poorly machined Cap contacts.
Today some of the 8 rotor gaps can climb near 1/8″ when they use to be less that 1/32″……that introduces a lot of resistance & raises coil wire voltage. Essentially there is more firing going on in your cap than at the plug!
So if the center contact is breaking down chances are your coil wire voltage is high and the other components could fail soon.
There is a lot more to this; for example some of Plymouth 60’s Caps had wide contacts….why then and not later? Thing is, the advance is always changing so at idle the rotor points to one place on the cap contact when firing but at wide open throttle it points to a much different point. So Chrysler used caps with wide contacts to handle both rotor locations. It was later dropped to lower costs……but unfortunately increase resistances. I buy those old vintage caps to get those coil line voltages down and extend the life of the ignition components.
Also in my case I get a longer rotor & machine every cap; essentially fitting the rotor & every distributor contact to get gaps below 1/32″. End result?: coil wire voltages drop to as low as 6000 Volts & spark plugs act as they should.
An analogy could be to think what happens when a beaver builds a dam in a brook……what happens to the flow downstream & to the height of the water upstream? That’s what those high distributor to rotor gaps increases do…..increase resistance to flow and build higher coil wire voltages.
So if you want to improve the cap center electrode life…..get those coil wire voltages down.