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My car went into a ditch and engine went partially under water. I changed oil and filter, tranny fluid, replaced alternator cuz it got wet. Works ok but I hear a "whir" from the right side of engine and my guess is the AC compressor bearing. Dealer says $250 for clutch/pulley/bearing assembly. Can I pull pulley and press out/replace bearing? I see ebay has OEM (NSK) bearing for $30 but before I buy it I wanted someone with knowledge in this area.

 
Rep Power: 517

It isn't real tough to get the pulley off to replace it if you have the tools and know what you are doing. It can be a bear if the compressor has a lot of age and corrosion on the hub though.
I don't even have to discharge the system to replace the clutch set. I do it all with the compressor still attached on most. Occasionally I have to unbolt the compressor for access, but I try like hell to do the work without having to discharge the system.

But I have never tried to change ONLY its bearing.

Pull it off, take it to a press, try it?
The worst you can do is ruin it, and you already needed the new one.