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 HEI Burned Carbon Button 
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Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Too much shim = no shaft endplay = pulls rotor down from carbon button.

Burned Carbon Button HEI.jpg
Burned Carbon Button HEI.jpg [ 43.77 KiB | Viewed 460 times ]
Fri Apr 03, 2015 7:23 pm

Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
What style rotor, aftermarket? Is there such a thing as an aftermarket rotor with a solid button contact instead of the stocker's springy type? With stock parts there's a ton of travel, I don't think it would be possible for stock parts to move enough to lose contact like that.

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Fri Apr 03, 2015 7:43 pm

Joined: Thu Mar 28, 2013 6:09 am
Posts: 729
Ground head to head to intake to frame to battery ground and it will go away...Ask me how I know it will!!

Fri Apr 03, 2015 8:21 pm
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Sorry guys, I didn't put the explanation for that in the post with the picture. It was a Chevy HEI that someone had shimmed all the end play out of and the almost 1/8" if extra shim between the gear and shaft lowered the shaft, and so the rotor, enough to make clearance between the rotor spring blade and the button. The 8 little burn marks are where the spark was jumping when it fired and the arc was eroding the carbon button. This was just with the regular HEI coil and module.

Burning up the carbon button in a Chevy distributor, HEI or points type, is a common occurrence when the end play is shimmed out of the shaft unless an 'adjustment' is made to the rotor spring blade, bending it up a little to be sure it makes contact with the button.

Another common problem you probably know about is when a standard OE 5000Ω button is used with a CD ignition. The button gets hot and melts the cap. The solution is the low resistance button you can buy from MSD or comes with the economy priced replacement caps. You get the low resistance button for less than ½ the MSD price and a new cap with it. Measure the button with an Ohmmeter from the spring to the tip of the carbon button. The OE 5000 Ohm resistance button is used with the high energy induction type system because it reduces plug electrode erosion.

Sat Apr 04, 2015 3:24 am

Joined: Sat Mar 29, 2014 10:27 am
Posts: 45
Whats your recommendation for endplay on the stock HEI distributors?

Sat Apr 04, 2015 9:54 am
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
I have always left Chevy Delco distributors with what ever end play they have unless there is some reason, such as a hothouse rebuilt housing has been tumbled so many times it needs another thin shim.

In a Chevy the end play is meaningless because the load of driving the oil pump thrusts the gear up. If the distributor end play in a Chevy engine is causing the timing to be erratic, the problem is much greater than eliminating the end play will fix.

The so-called "anti-cavitation" grooves in the oil pump are a good thing. Some people go crazy because they make the idle oil pressure low, sometimes 10 or 15 psi at below 1000 RPM. That doesn't bother me, I drove a 1941 216 Chevy to high school that never had over 15 psi for the 500,000 miles of its lifetime.

MSD distributors at first came with so little end play that, because the aluminum housing expands more than the steel shaft, when the engine got hot it would expand more than the end play and the advance mechanism would lock up and drag until it cooled. MSD made them like that for almost 20 years. People graduate from engineering schools to do things like this. What are they thinking?

Sat Apr 04, 2015 3:34 pm
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