The engine in my AU2 VCT XR6 is factory except for exhaust and induction. It has 269,000 kms on it, has been well-maintained, and runs beautifully - pulls hard, runs smooth, burns no oil, and the oil doesn't turn dark.

This isn't a problem, more just a curiosity.

I have just replaced the Bosch WR9LCX+ spark plugs with Autolite 985s. The Bosch plugs have probably 25,000 kms on them and look good - the insulator is near-white on one side, tan/grey/brown on the other. I have read that you can tell if you have the right heat range by where the colour changes on the side electrode - ideally, the colour should change at the bend. The attached pic shows three of the Bosch plugs with widely varying colour change points: one near the tip, one in the middle, and one at the base. That indicates that the three are operating at very different temperatures in the one engine - or, that the spark plugs have different heat ranges. What can cause this?

Variation in fuel injector flow rates? I don't have a reliable compression tester, but seriously doubt that compression is the problem, as the engine is as strong as ever and uses no oil. The coilpack and ignition leads are near new and tested well in spec for resistance. There are no symptoms of anything remotely wrong with the engine, as it's running sweet.

Production tolerances in the plugs might be the culprit, but that would be surprising for German-made plugs. The insulator shape determines the heat range, so how likely is it that they used the wrong insulator? I did notice that one of the plugs had the threads cut poorly, as it was very tight going in and out, even with anti-seize; an Autolite plug went into the same hole just fine. So maybe Bosch quality isn't what it once was.

Thanks for your help.

 

 

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I have used the FORD plugs for a long time and to be honest, never had a REAL issue with them.
The last time I remember a HUGE problem with the Ford plugs was for the EA/EB.

I sold HEAPS of the Ford ones for EF/EL/AU/BE/FG and don't remember many issues with them.

I would guess they look different because they have a different rotation when installed. I don't think all threads start at the same point on heads nor on the plugs, so they sit pointing in different directions. Racers actually use shims to get them into optimal position so the turbulence of the incoming mixture doesn't blow away the spark. Or something. In any case, they look ok in all respects.