Broken rotor on crab distributor

[Charlie ny]

I received a bunch of emails on my reply to 34 so bear with me.........
The following is for any front mounted distributor I build......
After disassembly and cleaning I machine the diameter on the rotor shaft that spins in the big bronze bushing. I turn this diameter to a full cleanup to eliminate taper and out of round, needless to say this diameter is already worn under size. The old bronze bushing is removed from the housing and a fresh bushing is installed, the fresh bushing is .060 undersize on the inside diameter. Now dealing with the 'crab' dist I need to know which cap will be used. The crab cap mounts over the register diameter on the housing and the rabbit ear cap mounts on the inside register diameter. I grab the crab
dist housing in a very accurate 6 jaw chuck on the appropriate register diam and bore the undersize bushing for an .0008 to .001 running fit on the machined rotor shaft. Because I trust my machine ( so far so good ) I am certain that the rotor shaft will be spinning in
truth with the cap register diameter on the housing.
Next
The breaker plate with the outboard support bushing is chucked up in the

6 jaw chuck and I indicate the bushing for runout. Runout here is usually .010 to .020.
In many cases I can true up this bushing using a small brass drift and a small hammer
thunking it in to .002 or less. Sometimes of course more serious lathe work is required


My thinking is it is no use to install a new standard size bushing and expect
to accomplish anything when the rotor shaft is worn .007 or .008.

I have yet to see a used housing where the cap register diameter was even
close in concentricity to the bushing diameter.
I took Bubba's advice a longtime ago and use only NAPA Echlin ignition goods.
Charlie ny

[BUBBAS IGNITION]

Originally Posted by Charlie ny

I received a bunch of emails on my reply to 34 so bear with me.........
The following is for any front mounted distributor I build......
After disassembly and cleaning I machine the diameter on the rotor shaft that spins in the big bronze bushing. I turn this diameter to a full cleanup to eliminate taper and out of round, needless to say this diameter is already worn under size. The old bronze bushing is removed from the housing and a fresh bushing is installed, the fresh bushing is .060 undersize on the inside diameter. Now dealing with the 'crab' dist I need to know which cap will be used. The crab cap mounts over the register diameter on the housing and the rabbit ear cap mounts on the inside register diameter. I grab the crab
dist housing in a very accurate 6 jaw chuck on the appropriate register diam and bore the undersize bushing for an .0008 to .001 running fit on the machined rotor shaft. Because I trust my machine ( so far so good ) I am certain that the rotor shaft will be spinning in
truth with the cap register diameter on the housing.
Next
The breaker plate with the outboard support bushing is chucked up in the

6 jaw chuck and I indicate the bushing for runout. Runout here is usually .010 to .020.
In many cases I can true up this bushing using a small brass drift and a small hammer
thunking it in to .002 or less. Sometimes of course more serious lathe work is required


My thinking is it is no use to install a new standard size bushing and expect
to accomplish anything when the rotor shaft is worn .007 or .008.

I have yet to see a used housing where the cap register diameter was even
close in concentricity to the bushing diameter.
I took Bubba's advice a longtime ago and use only NAPA Echlin ignition goods.
Charlie ny

Charlie , While i dont disagree with any of the above , if i did all that the rebuild would cost 5-600 dollars . We shipped a few hundred crab distributors last year, i check the bushings front and rear and do see a few defective ones , a defective one goes in the defective pile to build with new some day and a good used one is selected. Unit is built and spun up on a machine to adjust and place some initial wear on the contact points and lubed. I install a new NAPA cap and rotor and check for hitting , if it strikes i just belt sand the rotor a little bit.
I have tested these using a lab scope checking for rotor cap secondary voltage. A good coil will give you approx 20,000 volts , a good idling flathead will use approx 5,000 volts, opening up the rotor gap may use a couple hundred more volts, not noticed when you have 15000 reserve volts...even hard to see the increase on the scope..
Actually in the old days we used to open the coil wire gap to start a flooded engine and the snake oil folks sold a gap for every wire at the state fairs etc to make the engine run better ......

[drolston]

Originally Posted by BUBBAS IGNITION

Charlie , While i dont disagree with any of the above , if i did all that the rebuild would cost 5-600 dollars . We shipped a few hundred crab distributors last year, i check the bushings front and rear and do see a few defective ones , a defective one goes in the defective pile to build with new some day and a good used one is selected. Unit is built and spun up on a machine to adjust and place some initial wear on the contact points and lubed. I install a new NAPA cap and rotor and check for hitting , if it strikes i just belt sand the rotor a little bit.
I have tested these using a lab scope checking for rotor cap secondary voltage. A good coil will give you approx 20,000 volts , a good idling flathead will use approx 5,000 volts, opening up the rotor gap may use a couple hundred more volts, not noticed when you have 15000 reserve volts...even hard to see the increase on the scope..
Actually in the old days we used to open the coil wire gap to start a flooded engine and the snake oil folks sold a gap for every wire at the state fairs etc to make the engine run better ......

A bit off topic, but I loved watching the "Spark Booster" gadget salesmen at the fair. They would take a rough idling car and pop the booster in or out so quick that the engine would not die. The idle magically smoothed out with the gadget in place. Tuning the demo car to do that must have been fun.