Odd (Dist.) Rotor Wear

I guess my tech bud got an itch the other day and decided to pull and check the condition of Superblue’s dist. cap and rotor. I have told him, “if it ain’t broke, don’t ‘fix’ it”, many times, but to no avail. I also had replaced both (along with plug wires and air filter) about two or so years ago (less than 7K miles). I remember the cap was a mess and possibly the original one, as when I went to gently pull the wires out of it often times the metal connectors remained behind, “stuck” inside the terminals. :open_mouth: (on the new one I put a dab of electrical connection “goop” inside each of the terminal holes to protect things against corrosion for future removal! :triumph:).

Well, I guess I had to eat those words this time b/c I was stunned when my tech bud showed me my rotor. The edge of the (brass?) contact plate was almost completely (eaten?) away, such that the plate edge was almost flush with the plastic edge of the rotor. :open_mouth: He also told me the contact points on the underside of the cap were badly carboned up (he said he cleaned that up). I amazed Superblue still ran as smoothly as she did.

What the heck would account for the condition of the rotor? I can’t remember which brand I went with, but my first thought was it must have been a “cheap” one, hence its current poor condition. :thinking: However, since those rotor edges aren’t supposed to make actual physical contact with the “contact” points on the cap but just come close enough for the spark to be able to “jump” from the rotor to them (correct?) how could it wear down like that? There was no sign of burning or pitting on it, btw. :confused:

My tech bud noted that, unlike Superblack’s mega-cap, the cap on Superblue’s 6 cyl. is not held on with screws but with a pair of clips. This does allow for a SLIGHT amount of movement to the cap, but only in terms of rotation, not side-side movement.

Could the rotor wear be a sign that the distributor shaft/gears are a bit worn/loose, although Superblue has less than 95K miles on her? Wouldn’t such a condition also adversely affect her running, though? :thinking:

Also, does anyone know for certain whether the face-lift 4.0s are supposed to have a dist. cap gasket? Superblue didn’t have one present when I removed her old cap, and when I bought her replacement cap that one didn’t come with one, either. :confused: Also, I believe the face-lift V-12s weren’t supposed to have them - only the pre-face lifts (correct?). In any event, even if Superblue should have had one I don’t see how a missing one could account for the rotor wear.

The XJ40s didn’t have a gasket so i would think the XJS would be the same.

No gasket on my 94 4.0 when I got it in 2000 and never had one in twenty years of running since. I have also noticed the slightly loose fit of the cap that allows a small amount of rotation (fractions of an inch). Over the years I have had original Jaguar caps and aftermarket caps from SNG B. they have all had that slightly loose fit with no issues.

Is your rotor wear smooth or pitted? Erosion from the spark is the only thing I have ever seen on a rotor edge.