The bitumen or tar filling of the older Bosch coils seems to last well in moderate to hot climates, but not so well in climates that experience really cold temperatures. At least that's been my experience with older Bosch inductive coils for 60's era Volvos. The coils can fail sitting on the shelf inside the shed without being used. In general I don't trust any old coil, especially German ones. Oil seems to preserve early magnet wire better, but today magnet wire is so well made it probably doesn't matter what you fill the coil with providing it allows for some thermal expansion and contraction of the copper wire and there are no voids. Fred
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Originally Posted by Jonny H
^ What he said.
There is also another 'imposter' made by Beru. There are black and silver versions but they are longer than the Bosch CDI coil.
Having spent too much time researching into the Bosch Black coil, the reason they are so reliable is mostly down to the black 'tar' insulation that they are filled with. When heated, this turns into a thick oil that is pretty much self-healing in terms of filling any voids / air gaps between the windings.
Have spoken to a number of transformer winding companies and essentially you can't get that black tar anymore - most use a resin or a transformer oil to accomplish the same thing but this requires the bobbin to be wound using different insulation materials - e.g. the design has to be different for a resin filled coil.
I have cut/machined open many failed Bosch silver coils and they fail because they are filled with epoxy. Often 'bubbles' or voids are present in the epoxy which points to lack of or incorrect vacuuming of the epoxy when setting. This is a difficult process in terms of repeatability. Temperature, vacuum and time are all critical.
The next part is speculation. Imagine you were the Brazilian coil manufacturer and you were mainly making epoxy filled 'low voltage' coils and were asked to remake the CDI coil. I would wager you would just use the old bobbin design and just fill the coils with epoxy instead of 'tar'. I very much doubt you would go back and check the the design is suitable for epoxy fill. They get away with it on the Kettering coils since they are not under as much stress.
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