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Got some really nice used Scalextric Caterhams and apart from having to change one pinion, even though I could see nothing wrong with it using magnifying glasses, they're great - apart from one.

The rears on that offer no traction whatsover, which seems to be down to perished? hardness rather than the original compound, as the others are ok.

I could swap  the rears with the fronts but I'd like to do away with the magnet really, so can anyone recommend replacements please? 

I have seen elsewhere that they are an odd size?

Thanks for any help offered.