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Old 07-28-2010, 09:09 PM
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Advice Tapping NPT Thread

Hi All,
I recently bought a building that came with a "vintage" American Brake Shoe / Kellogg-American compressor. The drain plug at the bottom of the tank was well-rounded and I sheared off the "square" portion trying to remove it. I managed to get a plug removed on the end of the tank, inserted the hose from a hand pump, and pumped out about 6 gallons of water. (Fairly clean, though!). I think the simplest fix would be to drill through the drain plug (R size bit), tap it with a 1/8" NPT tap, and install a drain petcock. However, I've never actually tapped a hole, much less a tapered thread. I would appreciate any advice, warnings, or other reactions before I proceed. Thanks!

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Old 07-28-2010, 11:38 PM
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use a good tap lube like tapez, or a high sulpher tap lube ( not motor oil ) and a good tap handle not a cresent wrench, or a socket , taper the hole with a reamer to get the tap to start straight , and when taping its a half turn in a 1/4 turn out to clear the chips and a squirt of lube , and every now and then stop and check the dpth of the hole wiht your fitting ( lubricate with heavy oil to make sure it is in far enough. and most of all be patient doing it and try not to misalign or apply side pressure on the tap as it will snap like glass , if you can go up to a 1/4 npt ,the better off you will be as the tap will be stronger and the drain will not clog from any loose rust and it drains quicker too . . ( I use a 1/4 npt on my compressor to drain the tank with a ball valve mounted on a 90 ) But most of all be patient and take your time .
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Old 07-29-2010, 06:23 PM
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Also, with a tapered tap you won't run the tap all the way through as you would on a machine thread. Check the depth of the threads you're cutting periodically so your plug won't sink in too far. There's prolly a specific method to do this, but I always have the plug on hand and use it to check for proper depth.

Russ
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Old 07-29-2010, 07:15 PM
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Both replies offer excellent advice. The only thing I would add (based on personal experience) is to get the best quality tap you can. In the past, I tried to save a few bucks, and bought some "bargain priced taps". I guess the old adage about "getting what you pay for" applied here. Don't settle for the garbage Chinese crap tools that most big box stores have to offer. Check out the McMaster-Carr website. They seem to have the better quality, industrial grade quality tools, and have quick shipping, and practically anything you could possible need. It's well worth a look.
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Old 07-29-2010, 07:20 PM
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Thanks to all of you; this is really helpful information! I am glad I asked BEFORE I got further into this. I will keep you posted on my progress. You guys are great!
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