www.jec.org.uk

I recently decided to replace the spark plug caps on my E-Type 4.2 SII FHC and opted for the WCX 600 cap. I spoke to a reputable ignition parts supplier and before ordering I enquired about the current quality of WCX 600 caps as it is known that there have been issues in the past. I was reassured that no problems have been reported recently and on the strength of this I placed my order. On receipt, I considered it wise to check the resistance of each cap and, disappointingly, one was found to be open-circuit. The supplier promptly organized a replacement and as they did not want the defective cap returned I decided to undertake some investigations.
The current incarnation of the WCX 600 consists of a one-piece phenolic resin moulding having a main body which fits over the spark plug insulator and a side-arm into which the HT lead passes. The internals consist of a brass insert which locates onto the spark plug terminal, a resistor, a coil spring, a screw with flat head and a serrated steel washer. The brass insert has splines machined into the upper land, the purpose of which is to secure it firmly into the resin body when pressed home. The insert has a spigot which extends from the top surface and fits into a round hole in the upper part of the resin body. A spring clip fits into a annular groove machined in the insert and this grips the spark plug terminal. A hole extends along the side arm of the body and breaks into the hole in the top of the main cap occupied by the spigot. The resistor, which appears to be of the surface-mount metal film type, is pushed down the hole in the side arm and one end cap of the cartridge makes contact with the spigot. The spring presses the resistor against the spigot and is held in compression by contact with the screw head; the screw is locked in place in the side arm by the serrated washer which passes over the threads and is pressed into a shoulder. There appears to be evidence of adhesive which is used to bond the washer in place.
Once I had cut the internals out of the resin body, the resistor was found to have a value of 9.9k ohms and therefore OK. Furthermore, there was no evidence of corrosion along the electrical path which could explain the open circuit. On closer examination I noticed that the resistor was binding in the hole and the spring force was insufficient to press it into contact with the spigot of the brass terminal. Hence the defect was not failure of the resistor or corrosion but insufficient clearance within the hole in the side arm for the assembly to function as intended. Thought you might be interested.

Fascinating. I assumed all plug leads and gaps were like my XJS V12's, which AFAICT is simply a metal cylinder with a waist and a longitudinal split that clips over the plug terminal, and what I believe is a length of resistive cable.

Your post lead me to https://www.ngkntk.com/products/ignition-leads-caps/, which helped a bit, but appears somewhat specific to one manufacturer's product line. Despite their statement, I can't help feeling that for cars of this vintage, the different technologies will all work about the same, since the ignition systems were designed in the days of contact breaker points and mechanical distributors, and almost any spark will do. It's not like the engines rev much above 6,000 rpm, or have compression rations above 10:1 (OK, the V12 is special in that respect, and in some of its incarnations the ignition system delivers 12 sparks where the XK engine has only 6, so you could argue that's 12,000 effective rpm!)

Paul
Although there are many views on this topic, general wisdom would suggest that for cars with conventional distributor and CB points ignition, copper-cored (i.e. non-resistive) HT leads, resistor plug caps (5k ohms or 10 k ohms) and non-resistor spark plugs is a good compromise. It must be said that the distributor having an advance curve resembling the correct one and the points in good order and correctly adjusted is of paramount importance. For cars fitted with electronic ignition, either original fitment or retro-fitted, it is generally the case that more sophisticated HT leads are preferred (e.g various designs of carbon cores) with non-resistor plug caps and resistor spark plugs; commonly, the resistance of a resistor spark plug is 5k ohms. This combination helps to quench electromagnetic interference developing in the HT path which would otherwise adversely affect the correct functioning of electronic ignition modules. Screw-in plug caps such as the Champion WCX 600 cannot be successfully used with non-copper cored HT leads. A combination that should not be employed is resistor plug caps with resistor spark plugs. Having no built-in resistance in the case of conventional ignition is OK but you wont be popular with anyone in the vicinity operating equipment sensitive to stray RF interference.
Regards
Clive