Cracking, rust and, in the case of a plastic cap, tearing, are the most common reasons to renew the oil filler cap.
Of course, this doesn't apply to everyone, but you may also want to renew it because the joint is digested or you just want a new cap. If you are tired of the original, you may also consider fitting a chrome oil filler cap, which is available in several versions (depending on the engine).
If there is a separate seal, it is also indicated on the replacement parts tab.
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