For a long time, WWII Jeep restorers have called these Rain Shields. Why is a mystery because they never get rained on. The WWII Jeep parts books, Willys, Ford and SNL-G-503 all call them “Cap – Spark Plug Insulator”.
Our customers invariably search for Rain Shields so we also refer to them as a Rain Shields but they are a Cap that goes over the spark plug insulator and not down on the cylinder head to protect from rain.
If you have experienced these shorting out the problem it is not a material problem, its an installation problem.
Restorers invariably think the Cap must go down to and touch the cylinder head and this is totally wrong.
I am sure you have a copy of TM-9-803 but just in case you don’t here is a copy of one page which shows the rain shield goes as high as possible on the spark plug resistor before the lead is pushed onto the spark plug proper, there should be a considerable gap between the bottom lip of the shield and the cylinder head.
The high voltage in the actual spark plug circuit is likely to be around 20,000 / 30,000 volts and this can skirt across the surface of many non-conductive materials. There are more technical descriptions as to high voltages and its ability to travel over surfaces and jump gaps depending on sharp edges etc but all this is usually more than a Jeep restorer wants to know.
Maybe some can remember children getting car sick and Dad fitting a rubber strap to the car which touched the ground and static electricity was dissipated over the rubber strap and solved the problem, the rubber was not conductive.
So, if you experience this situation make sure the cap is fitted all the way up the copper part of the spark plug resistor and touches the Bakelite part of the insulator.
One final note, we have seen a few early NOS caps with small holes in the top that appear to fit the spark plug and not the insulator. We suspect these are first production and are very rare as we have only seen a couple in the past 40 years. Most NOS Caps we have seen are the larger top holes.
If you have any questions please email firstname.lastname@example.org and we will do what we can to assist.