I was working on my engine the other day when I pulled the distributor cap and noticed what looked like grease around the inside of the cap. But what really caught my eye was the strange buildup of crud on one end of each spark plug wire terminal on the cap. What causes this and how do I prevent it?

W.D.

This is the photo of the cap that W.D sent us with his question. Notice the dark iron oxide area on the inside of the cap. Our guess is that the inside of his rotor was absolutely filled with this stuff. Also notice the whitish aluminum oxide deposits on the spark plug wire terminals. That’s not good.

This is the photo of the cap that W.D sent us with his question. Notice the dark iron oxide area on the inside of the cap. Our guess is that the inside of his rotor was absolutely filled with this stuff. Also notice the whitish aluminum oxide deposits on the spark plug wire terminals. That’s not good. (Image/Jeff Smith)

Jeff Smith: At first glance, I thought the dark area on the cap was grease. But looking more closely, I’ve seen a dark brown material coating like this before, appearing like a very fine powder. Most often, I see this on the inside of HEI distributors, but it can be common on any distributor. This material is iron oxide and is the result of misfires inside the cap from errant sparks inside the cap looking for a convenient ground path.

Electricity will always follow the path of least resistance. If the rotor gap is too large, often the resistance from this large gap will be greater than the resistance created to jump down to the steel rotor screws and into the mechanical advance weights in the distributor. When this happens regularly, the electrical energy oxidizes the metal in the mechanical advance, turning some of the iron into iron oxide particles that collect on the inside of the cap when thrown off from the rotor.

So we need to discover why this is occurring.

Our first clue is to look closely at the spark plug terminals on the inside of the cap. Notice that they are not brass. These terminals are aluminum. This is an indication of an inexpensive distributor cap. Aluminum does not conduct electricity nearly as efficiently as brass or copper. This means there will be more resistance in the high tension circuit to the spark plugs. You may not realize it, but aluminum does corrode. It just doesn’t rust like iron components. But this corrosion shows up as aluminum oxide. I believe that’s what we see as buildup on the edge of each terminal. This corrosion creates resistance, which reduces the voltage that ends up reaching the spark plug. Then if your plug wires also have high resistance, this really can hurt spark energy. If you lose enough spark energy, part-throttle power will suffer and the engine will seem sluggish and unresponsive to throttle.

But what’s also interesting is the fact that this corrosion is located on the leading edge of the terminal. This is a clue that the rotor is in the wrong position when the pickup aligns with the spinning pole piece and triggers the coil to fire. This is most commonly referred to as rotor phasing. MSD has a good video on what happens when the rotor is not phased properly.

In this case, the rotor is retarded in relation to the pickup. So when the pickup triggers the coil to fire, the rotor has not yet advanced enough to line up with the terminal on the cap. The deposits on your cap indicate that the rotor consistently fires short of the spark plug terminal. This requires the spark to jump the large gap between the rotor and the cap. If the rotor is retarded enough, it might actually bleed voltage off to the previous cylinder. You won’t necessarily feel this as a dedicated misfire because that voltage ends up in a cylinder that is on the exhaust stroke. So the plug might fire, but it will be doing so into essentially a dead cylinder. The real issue is that some voltage is being siphoned off to the previous cylinder in the firing order.

The fix for this malady is to first ensure the mechanical advance mechanism is fully functional. Remove the rotor and lightly lube the mechanical advance mechanism to ensure it works properly. Next, make sure that the MSD cap is properly registered with the distributor cap pin in the slot in the distributor. It’s possible that the cap was improperly registered on the distributor. Also make sure that the cap you are using is a true MSD distributor cap with brass terminals and not aluminum.

Now mount the rotor on the distributor and bump the engine around until the balancer displays roughly 15 degrees before top dead center (BTDC). This doesn’t have to be exact. Now place the cap on the distributor and carefully place a Sharpie mark on the distributor body that vertically lines up with the Number One spark plug wire terminal on the cap. Twist the rotor to maximum advance and make another, smaller mark on the distributor body. Now remove the distributor cap and evaluate how close the rotor terminal is to line up with either the large or small marks on the distributor. If the rotor is positioned between the two marks, then the rotor phasing is acceptable. The rotor should be fairly close to the mark on the distributor. If the rotor is pointed away from both marks, then you will probably have to buy an adjustable rotor from MSD (PN 8421) to correct the problem.

This is an adjustable rotor fitted to an MSD distributor. Note that we’ve positioned the rotor just before the black line on the distributor body. When the mechanical advance moves the rotor (roughly 21 degrees or 15 initial + 21 mechanical = 36 degrees BTDC), it will swing from before the terminal to after, which will be as close as we can get throughout the entire advance curve.

This is an adjustable rotor fitted to an MSD distributor. Note that we’ve positioned the rotor just before the black line on the distributor body. When the mechanical advance moves the rotor (roughly 21 degrees or 15 initial + 21 mechanical = 36 degrees BTDC), it will swing from before the terminal to after, which will be as close as we can get throughout the entire advance curve. (Image/Jeff Smith)

This adjustable rotor allows the tuner to move the tip relative to its stock location to line up the rotor with the terminal on the cap. We chose 15 degrees because that’s a typical initial timing setting. As the mechanical advance operates, it moves the rotor ahead to advance the timing. We set up one of our test small blocks and discovered that our distributor advanced the timing and moved the rotor away from the static location, creating more distance between the rotor and the cap terminal. An adjustable rotor allowed us to move the rotor so that it remained close to the spark plug terminal.

MSD’s You Tube video illustrates this dynamically with a timing light and a hole drilled into the top of the distributor cap. You can also do this if that’s easier. Our above description is the way to set up a brand new engine and distributor so that it’s all ready to go. Hope this helps.

Keywords
Ask Away, distributor caps, Jeff Smith
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.