oil tank cap

oil tank cap

The cap for the oil tank on a chainsaw I recently bought has an articulated flip section with a strange golden sponge looking circle in it. What is the purpose of this?

(The oil tank supplies oil to the chain and not the engine.)

  • 1
    Might be a filter to let air into the oil tank to break/prevent vacuum.
    – crip659
    Oct 16, 2021 at 18:53
  • Probably so it won't get lost...
    – Solar Mike
    Oct 16, 2021 at 19:17

1 Answer 1


The articulated part is meant to serve as something to grip so that you can actually unscrew the cap. When not in use, it also provides a "sleek", "flat", "clean" design, which seems to be the mantra nowadays. (Why do chainsaws also need to follow fashion trends is beyond me.)

That "golden sponge" is actually a sintered bronze breather to let air into the tank while not letting any oil out. This is necessary to equalize the air pressure between the tank and surroundings as the oil gets consumed.

  • 1
    the flat top keeps it a bit cleaner -- otherwise you get bits of sticky oil and sawdust sticking to it.
    – gbronner
    Oct 16, 2021 at 19:59
  • 2
    @gbronner Does that not happen anyway? Chain oil is sticky stuff.
    – crip659
    Oct 16, 2021 at 20:25
  • 1
    @crip659 The other reason that the cap is so big is so that you can use it without taking your gloves off. But, at least on the chainsaws that I've used, the vertical fin on the top of the bar oil cap tends to accumulate sticky sawdust, and is rather messy.
    – gbronner
    Oct 18, 2021 at 14:14

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