Broken rotor on crab distributor

[34pickup]

I was reading another thread about some people experiencing a broken rotor when converting over to a crab style distributor setup. Well, I ordered the crab conversion kit and had that happen too on my 1948. It came with the cap,matching rotor and clips. I put the distributor together and spun it around and it seemed to turn freely enough, no binding, felt like only the points were offering any resistance. I put it on the engine, it fired once and died. After a while I decided to pull the distributor back off and yep, the rotor was broken off.
So, now I need to buy another new rotor and hope the same thing doesn't happen again,they aren't cheap. I read where I think it was Bubba that files the rotor down, but how do you know how much is correct?
Doesn't anyone make a proper fitting rotor that works without breaking or having to be modified to work?

[drolston]

For starters, a rotor that turns freely in the cap should never break unless it was cracked by misdirected force in putting it on. Or unless the distributor shaft bushings are completely worn out, allowing wobble.

You can buy a rotor on Ebay for $7.95 and up. Cap for $27 and up.

Good luck!

[Charlie ny]

34,
The rotor is not the problem. The problem is the bushing the rotor shaft spins in
is not on the same center line as the diameter that the cap locates on....AND/OR
the support bushing on the breaker plate is off center. Both these situations occur
quite often. Email me at [email protected] , with a bit of common sense machine

work we can get everything spinning nice and true.
Charlie ny

[FlatheadTed]

I set up a distributor for a friend for a Vintage stock car night ,I went down for a look and hear he was parked up after only one race ,We found out later he fitted a new cap and rotor and there was the Rotor in two pieces , So in his case it was the added parts that were at fault ,these repoped bits are not that accurate .In the Ford books they recommend to set the air gap on the rotor divers Helmet so the crab would be the same ,Just don't ask me how you do it a set of callipers maybe ,turn the cap terminals in a laith or go to Charlie. this would be a good reason why people change to electronic ,Air gap to wide = weak spark ,Ted

[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

[Mart]

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Thanks for sharing the inside info Charlie. I might look more closely at my units now.

Mart.

[FlatheadTed]

I also find a lot of the caps have the terminals not centred ,I prefer to chuck them up in a four jaw and true them up as well as filing the Rotors

[Mart]

^^ What do you use as a reference surface to true them up, Ted? The mating diameter to the distributor body?

Mart.

[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 ......

[Charlie ny]

Jim,
I guess I'm fortunate , I'm a one man operation and do all machining in house,
nothing is farmed out. In my relatively little world it seems to pay off in the long run
to operate the way I do. When possible I'll do 15 or 20 of any components that I

build....fuel pumps, carbs, distributors yah da yah da..... to capitalize on the economy

scale.

I thank Jim Bubba Linder most everyday for his assistance when I'm 'up against it'.
Charlie ny

[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.

[FlatheadTed]

Its not something I do every day ,you run the risk of cracking the cap ,your reference is the locating ridge were the clips go .I do quiet a few distributors most I hand work with a dermal like Bubba does ,If The bushes are worm they show a fluctuating needle on the dwell meter ,I have been doing a few electronic refits lately, that solves it .Ted


,

Originally Posted by Mart

^^ What do you use as a reference surface to true them up, Ted? The mating diameter to the distributor body?

Mart.

[bobscogin]

Originally Posted by FlatheadTed

I have been doing a few electronic refits lately, that solves it .Ted
,

Think I'm missing something here. After an electronic conversion, you've still got a rotor spinning around in a cap with the potential for collision.True?

Bob

[barnfind08]

A lot of people forget to have a gasket between cap and body. Sometimes it is just that simple.

[johncusano]

Stick with the rabbit ear one with nice bail clip other crab stuff is cheap,cap flew off twice on me.

[FlatheadTed]

I am referring to the bush ,with electronic is not as critical ,with points a few thou play shows up on the dwell meter, but you yes you still need to clearance the rotor ,

Originally Posted by bobscogin

Think I'm missing something here. After an electronic conversion, you've still got a rotor spinning around in a cap with the potential for collision.True?

Bob

[34pickup]

I believe Charlie is right about the bushings. I didn't think to check them out very good. With the distributor all assembled, it seemed to be tight enough, but when I took it back apart and checked each single bushing on the shaft,they seemed to be too loose. I don't understand.....its only 70 years old....☺

So I ordered a set of bushings. I am determined to get this thing running without having to resort to sending it off to be machined. I think they were designed to be fixed in someones back yard.
I noticed there are oil grooves behind the bushing in the housing. What kind of lubricant is advised to go there?

[BUBBAS IGNITION]

Originally Posted by 34pickup

I believe Charlie is right about the bushings. I didn't think to check them out very good. With the distributor all assembled, it seemed to be tight enough, but when I took it back apart and checked each single bushing on the shaft,they seemed to be too loose. I don't understand.....its only 70 years old....☺

So I ordered a set of bushings. I am determined to get this thing running without having to resort to sending it off to be machined. I think they were designed to be fixed in someones back yard.
I noticed there are oil grooves behind the bushing in the housing. What kind of lubricant is advised to go there?

Keep us informed as you go with the bushing replacement. Engine oil is what get to the back side of the bushing on the engine side and the bushing is oilite and retains enough to get the job done. The outter bushing has a oil weep hole and this oil is supplied by a oil "worm" or wick inside the shaft....i

[cmbrucew]

I believe we are fortunate to have four of the best talking about distributors. Many years of experience shared here.
Bruce

[FlatheadTed]

If I get a bunch of distributors to do I roller out in the sun and go for it, nothing like checking things at high RPM . Ted