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.)
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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.