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Your rpm's should be dead on at 900 rpm. Yes, it should be a TRUE 1800 rpm 5hp motor. The compressor should rotate so that the fan blades in the flywheel push air towards the pump. It will run either way, as it is splash lubed, but you should give it all the cooling it can get. If this is a Model 66 (very square design, rectangular cylinder head, bare tube intercooler behind the flywheel), it will at least have generous amounts of aluminum in the heads and crankcase to help dissipate heat. The serial number leads me to believe it it a Model 60 though, which is a totally cast-iron pump with a cast-iron intercooler bolted to the side of the block. Can't speak to the difference in motors. I've often found the compressor duty designation somewhat dubious. Get a good brand name motor like a Baldor, US Motors, GE, Toshiba, Weg or Westinghouse. Use either compressor oil, a single weight non-detergent motor oil, or a good synthetic like Mobil 1. Change it once a year and you'll be fine. The level is more critical. Pop out the sight glass and clean it if it is dark. Yes, you must have a check valve between the pump and the tank. The pressure release valve (unloader) should be a centifugal style, on the back of the pump in a large circular case. There should also be a breather valve on it. Websters were notorious for wearing out their centrifugal unloaders due to wear in the cast-iron plunger, the flyweight fingers and pivot pins, so don't be surprised if it doesn't work. A simple unloader style pressure switch will fix that, just plumb it to the check valve unloader port with some 1/4" copper tubing. You can use a fairly light rated pressure switch if using a motor starter, since it will only deal with the load from the starter coil. These are fairly robust compressors when attended to. If neglected, they will wear their big and small ends oblong fairly quickly, so don't neglect oil changes. This depends on the age of the design, since very old Websters used bronze rods, later ones used aluminum rods, and the last ones used aluminum rods with inserts and roller bearing small ends (the best ones). The Model 66 was one of the last compressors designed by Webster, so it will not have bronze rods, but will at least have aluminum rods with bare bearing surfaces. This type is most susceptible to wear, hence the need for regular (but not obsessive) maintenance. All Websters with the 5 valve heads put out a lot of air.
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