38mm vs 40mm fans

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aarcane

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Feb 16, 2016
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Are 38mm and 40mm fans two different names for the same physical fans? Or are 40mm fans actually 2 mm larger in the square dimension? I'd like to buy some 40mm fans to test-fit them in a 38mm fan location, but I don't want to waste the time and money if it's not going to work out. Also, is there a good list of good, silent 38 and 40mm fans of varying dimensions that are readily available? I tried to search, but all I can find are fans rated for CFM with no regard for noise levels.
 
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aarcane

Member
Feb 16, 2016
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So after much research and a lot of nothing, I decided to buy a trio of Noctua NF-A4x20 PWM fans for USD $15 each. They're a little shorter than the 38x38x38 fans, but the 38x38x38 fans are about half plastic reinforcement and half fan. So I don't have significantly less fan.
That said, when I tried to fit them, I found that 38mm vs 40mm is an appreciable size difference, and a device that uses 38mm fans and are a snug fit will likely NOT support 40mm fans.
If you're a brave DIYer like myself, you can use some high grit sandpaper and just sand 1mm off each side of the 40mm noctua fans and have a solid feeling 38x20mm fan. Just use a hobby knife to clean up the dangly edges and a can of air to blow away the dust.
Do Not recommend for enterprise or production deployments obviously, but for homelab or home switch/1u devices, it's a viable workaround.
 
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