I bought replacement filters on eBay , made by Uni. The were about half the price of the ones from Mikes.
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But they are flawed in design, they are open thorough the center and dust can come right through the center of the filter.
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So, I removed the foam to re use,
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I then cut the old paper from the stock filters, and cut the foam to fit the stock filter cages. Oiled the foam and wrapped it around the cages and held it in place with twist wire.
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You can buy sheets of filter foam pretty cheap, I wouldn’t buy the filter oil again, it’s super sticky, I’d just use motor oil. Originally I used a double layer of foam, but I felt that was too restrictive ,so now I’m running a single layer.