[ROVERNET - UK] Dist cap and other HT failure

Fletcher gofanu at usachoice.net
Mon Mar 12 17:33:13 GMT 2007

I posted this to another board recently, in response to questions about 
rotor failure. Note that many failures are the result of bad moves on 
the part of those working on the car. The use of a ballast required type 
coil on a non ballasted system is the overkill version of a coil that 
will put out excessive high voltage sparks, but the developed voltage is 
still a function of whatever takes the most voltage to fire - either the 
plug gap or a disconnected wire. A special note on excessive plug gaps: 
Some platinum plugs, Bosch in my experience, have very skinny center 
electrodes which burn down inside the insulator, giving gaps much 
greater than they appear. I lost quite a few caps on my Dodge truck 
before I figured that one out.

Rotor failures are due to two main reasons:
Faulty molding - all the OE Lucas ones (maybe 3)I ever had bad, had 
bubbles in the plastic when broken in half. There may also be issues 
with repro material since it's the same sort of thing as the rubber bits 
they now make that go away on the shelf in a couple of months. No excuse 
for that with modern materials science.
Overloading of an old tech design - The fundamental limitation is the 
thickness of plastic between the center rotor contact and the dizzy 
shaft. OK for the Max old time 40KV of a Sports coil, which was rarely 
if ever actually developed with .025 plug gaps. Modern systems can put 
out in excess of 60KV, and the use of wide gaps will actually result in 
that kind of voltage. So will broken conductors in plug wires, or 
amateur testing by removing plug wires to see "how far the spark will 
jump", or cranking the engine with IGN ON and plug leads disconnected. 
Solid copper wires rarely had broken conductors, and most coils have 
internal voltage limiting "sparkover gaps"; if the coil is designed for 
50 or 60KV, the internal gaps don't work until well over the design 
voltage. These voltages WILL puncture the plastic at the weakest point 
of the rotor, cap or coil, and will damage plug/coil leads. If you find 
a rotor that will stand this abuse, then the spark will simply track 
across the cap to the nearest ground, either externally or at a 
neighboring plug contact; that's why modern distributors are larger in 
diameter, as in the change from 25D to 45D Lucas dizzies.
For good results, use standard or moderate high performance coils, don't 
use excessively wide (over .035 on 25D dizzy by my preference) plug 
gaps, never disconnect a plug or coil lead with IGN active unless it is 
held no more than 1/4" from ground, and ALWAYS carry spare rotors 
previously tested as good.

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