> I can feel what I can only describe as a 'teeny' bit of play in the
> dizzy shaft. I have no way to reliably measure the runout in the
Your dial calipers have a depth measurement leg on them. Open the calipers
sufficiently, then take a measurement with the tail end of the calipers
hooked over the edge of the dizzy body, and the depth leg against the shaft,
while holding the shaft away from the calipers with the other hand. Record
the value. Now repeat the measurement while holding the shaft towards the
calipers. The difference is the total runout of the shaft. Don't recall
the spec offhand, but it should be only .002" or so. Anything above that is
likely bushing or shaft wear.
But 'teeny' doesn't sound like enough to cause the sort of problem you're
> I can see some chipping of the plastic surrounding the terminals on
> their trailing edge. (and yes, I'm sure it's the TRAILING edge of
> the terminals) I don't see any evidence of collision on the leading
> edges of the terminals. And I do see wear on the terminals right
> where the outer edge of the rotor's blade would be brushing the
> terminal. There WAS physical contact between the rotor and the
> terminals, but I'd call it very light contact.
If it was whacking the terminals hard enough to crack the cap, then it was
also hard enough to crack the rotor. Because of the leverage, the rotor
edge gets more force than the cap does.
> Now that brings up the question: Is the rotor supposed to physically
> connect with each terminal as it swings past, or is there supposed to
> be a slight gap between the parts, small enough to encourage a spark
> to jump, but large enough to prevent actual contact?
There should be a small gap, something like .005" to .010".
> Second question is, would someone please explain why a crack in the
> rotor body necessarily renders the rotor non-functional? I don't
> really doubt that it does, mind you
> I just want to know WHY.
These are quite high voltages we are dealing with here, and the crack acts
as a conductor to lead the spark to ground. Once the spark has jumped a few
times, it will leave a trail of carbon where the heat of the spark has
decomposed the plastic of the rotor. Carbon is a conductor, although not a
very good one. It takes only a small amount of leakage, resistance far
higher than you can measure with an ordinary ohmmeter, to cause the coil to
never develop enough voltage to create a spark. BTW this is the same reason
why fouled plugs won't fire.
> Third question is: What is the CORRECT way to test the rotor's
The easiest thing is to just replace it with a known good one. If you can
tell the difference, the old one was bad.
You could, I suppose, built a high voltage supply (like 20 kilovolts) and
try to measure the leakage current though the rotor at 20kV, while mounted
on a shaft ...
> Is there a way to test a rotor with an
> ohmmeter to absolutely diagnose it as bogus?
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