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Some years ago the Bosch caps on my '93 SL500 were arcing so badly that I bought replacement BERU caps that in my opinion are better than the original Bosch caps.

Recently though even the better BERU caps have been tracking and arcing. I had to stop and do the well practiced caps removal and moisture wipe. I was wiping them with a rag and the Allen key and managed to scratch them!!

Today I removed both caps and polished them using a Dremel drill and a brush and pad with 3M 'fastCut'.
When they were burnished and all traces of tracking removed I coated them with a two part epoxy resin.
They now look almost new apart from the scratches that I made with the Allen key.

Before:



After:



Hopefully now good for a few thousand more kilometres ;)
 

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I put silicone grease on the metal parts of the cap and rotor but my issue was mostly too much current going thru them so it helped reduce burning the metal. The issue is the same as yours, mine was just harsher. So I was thinking you could coat the plastic with silicone grease and it should stop or greatly reduce tracking. I used silicone spray, grease is way better but also a much bigger mess to clean. For mine I'd just spray the cap with brake cleaner, then spray with silicone and done. Tracking starts small and builds up but I can't see how it could start at all on silicone grease. You may not have too much power, but your rotor alignment may be off which causes a similar problem. So imo you need to be sure your rotor is lined up with the cyl when it fires. If not then it has to jump further and you have more wear and tracking as a result. In my Chevota, which used a Chevy eng, I had to adjust the timing plate to ensure the cap and rotor were as close as possible. You may have both mechanical and vacuum adv which can add up to be a lot, so it may look aligned when static but might be way off when advanced. I cut a big window in an old cap so I could see the alignment, and I could actuate vac adv while watching it. I could do this with the eng off, then fire it up and watch it with and without a timing light to be sure it was in the ideal spot. Pita but I was determined to get it right, and I certainly didn't want any spark energy jumping to the wrong cyl.
I also used metal plug wires because carbon wires only make things worse. Not sure but I think most or all have metal now? Another option that I had to do to one car was lengthen the rotor contact because when lined up the spark still had quite a gap to jump. If you get too close it'll hit at higher rpms, which I assume is the cap and rotor moving around a bit, but I didn't notice until very high rpms, like over 7, but of course it depends how close you make it! So soldering or gluing an extension on may extend life too, and optionally you can use a wider rotor tip so when it's not aligned it's still reaching out to minimize that gap.
Ideally you want good connections on the wires to the plugs and cap, and no leakage, so use silicone grease on the metal and rubber boots.
To solve a moisture issue I had on one eng I ran a vac line to it to keep air moving through it and suck up the steam, which worked! Redneck Racing ;)
 

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Discussion Starter #3
I put silicone grease on the metal parts of the cap and rotor but my issue was mostly too much current going thru them so it helped reduce burning the metal. The issue is the same as yours, mine was just harsher. So I was thinking you could coat the plastic with silicone grease and it should stop or greatly reduce tracking. I used silicone spray, grease is way better but also a much bigger mess to clean. For mine I'd just spray the cap with brake cleaner, then spray with silicone and done. Tracking starts small and builds up but I can't see how it could start at all on silicone grease. You may not have too much power, but your rotor alignment may be off which causes a similar problem. So imo you need to be sure your rotor is lined up with the cyl when it fires. If not then it has to jump further and you have more wear and tracking as a result. In my Chevota, which used a Chevy eng, I had to adjust the timing plate to ensure the cap and rotor were as close as possible. You may have both mechanical and vacuum adv which can add up to be a lot, so it may look aligned when static but might be way off when advanced. I cut a big window in an old cap so I could see the alignment, and I could actuate vac adv while watching it. I could do this with the eng off, then fire it up and watch it with and without a timing light to be sure it was in the ideal spot. Pita but I was determined to get it right, and I certainly didn't want any spark energy jumping to the wrong cyl.
I also used metal plug wires because carbon wires only make things worse. Not sure but I think most or all have metal now? Another option that I had to do to one car was lengthen the rotor contact because when lined up the spark still had quite a gap to jump. If you get too close it'll hit at higher rpms, which I assume is the cap and rotor moving around a bit, but I didn't notice until very high rpms, like over 7, but of course it depends how close you make it! So soldering or gluing an extension on may extend life too, and optionally you can use a wider rotor tip so when it's not aligned it's still reaching out to minimize that gap.
Ideally you want good connections on the wires to the plugs and cap, and no leakage, so use silicone grease on the metal and rubber boots.
To solve a moisture issue I had on one eng I ran a vac line to it to keep air moving through it and suck up the steam, which worked! Redneck Racing ;)
Thanks for your useful and informative advice.
I too am a great believer in silicone grease and use for many purposes knowing that it is a very good electrical insulator.
Donkey's years ago my uncle who worked for a UK power line company gave me a tub of silicone grease that he used on 30Kv. transformers that I'm still using to this day.
When I kept the car in the UK I did squirt some WD40 into the caps but the next time I had to wipe out moisture the WD had turned to a yellow sludge!
Using the silicone grease in the caps is a much better idea, but silicone sprays should be used with great caution in the engine bay because silicone can destroy the sensitive element in MAF sensor.
 
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