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  1. #1
    ercBOOSTED's Avatar
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    How to Tap Pump and Do Thru Hull Fitting

    I've been trying to research this topic for a while and can't figure some stuff out. I have a 5/8" thru hull fitting that I want to install. I will be using one of my existing locations that is not being used. I know I should use a unibit to drill but how do I know what size to drill to? Do I just do a little bit at a time until it fits? What do I use to seal it up with?

    I'm also going to be having a larger diameter fitting installed on my pump. I'm going with a 5/8" fitting, but what does NPT mean? I keep finding 5/8" and 1/2" NPT. Does NPT have anything to do with the size of the opening or does it just deal with the threads? What is the actual process of tapping and getting the threads inserted in the pump? My dad knows how to do this but I would like to have some kind of knowledge of this before I do it.


  2. #2
    hitman's Avatar
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    NPT refers to National Pipe Thread. It's an industry standard among pipe fittings. You can read a little more about it here on Wiki: http://en.wikipedia.org/wiki/National_pipe_thread

    Just use a unibit and keep going a little bigger until it fits. I then used black silicone on both sides of the fitting to seal it up real good.

    If you want to drill and tap something like a strainer plate to accept an NPT fitting, you will need a NPT specific tap. They can sometimes be tricky to find locally but can always be ordered in the interwebs.

  3. #3
    Taking money away from the Z06 Fund
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    +1 with what Todd said. Go buy yourself a nice set of Taps, Standard size. It should come with a guide on what the standards are for the taps. Some are NPT, other taps are different standards. Also get yourself a Tap Wrench for tapping the hole for the threads. Buy some tap oil or lubricant as it helps with turning the tap and keeps your taps in good shape.

    The Unibit is money. Just keep drilling until the fitting is snug and you can barely fit it in. I used some 5200 fast cure around the Thru hull to make sure the seal was watertight.

    The most common NPT tap is a 1/8" tap, but make sure you check the standard on the tap for what you need. The Tap wrench and oil are essential for using the tap though. Go about 1 turn (clockwise) for making the threads, then back it out about 1/4-1/2 turn to relieve some of the metal shavings. Repeat, until you have tapped the hole long enough for the threads.

  4. #4
    Bounty Hunter Carlos Thomas's Avatar
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    Start small and work slowly bigger. Easy.

  5. #5
    ercBOOSTED's Avatar
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    Thanks guys. Doesn't seem too hard. How do you remove the old threads that were originally put in there? I think I will be using the same location that was tapped before. Waiting on a little more info from JD1 which I should be getting tonight.

  6. #6
    SHOBiz's Avatar
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    I use these (different size than what is in the picture obviously)


  7. #7
    Addicted Member 5001craig's Avatar
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    FWIW I just used a normal (sharp) drill bit--no unibit for me.

    Prior to drilling, I measured about five times and then studied about five more times (inside and out) then center-punched and drilled a pilot hole with a long bit. Then I drilled it out with the correct size drill bit.

    I just looked at my pics I have of the install and saw it was a little more than a year ago. And none of the pics were of the thru-hull. They were only of the R & D Cooling kit installation.

    Regarding tapping. You will drill the correct sized hole with a drill bit then tap it with a tap. It's easy--just get some tap lube fluid and take your time. I normally turn a tap 1/4 turn then back it off that much or a little less. This will clean the threads on the tap. Take your time and keep the tap perpendicular to what you're tapping. One thing about tapping pipe threads (NPT)--these are not like a bolt that you can keep turning in a nut. Pipe threads seal themselves as they are tapered. Don't tap too deep. You'll see what I'm saying. Practice on some scrap first if you want to get some OJT.

  8. #8
    ercBOOSTED's Avatar
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    Ok I am an idiot when it comes to some things and since I have never done this before I still really have to clue as to what set to buy. I want something good but not ridiculous in price. I keep looking at all these different sets online but some of the numbers that they list just confuse the hell out of me. I'm looking for a set that has the tap for the 5/8" 1/2" NPT. If anyone can help me find a set that will work I'd be very appreciative. I just keep seeing too many things that I don't understand.

  9. #9
    Addicted Member 5001craig's Avatar
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    Just buy a normal tap and die set. Most come with an 1/8" NPT tap. You can buy a 1/4" NPT tap and then buy the appropriate drill bit. There are charts that will tell you what drill bit to use for what tap. Here's one for NPT threads: http://icrank.com/cgi-bin/pageman/pa...rt_npt.htm&t=2 and here's one for normal threads: http://www.jeepfan.com/tech/TapThreadSizeChart.htm.

    I can't see you needing anything larger than 1/8" NPT or maybe 1/4" NPT.

    Like I said--there's nothing like On The Job Training. Buy a tap and die set, look-up what bit to use and thread something (scrap piece of aluminum) to run a bolt or pipe fitting into. That's the best way to learn. Can't go wrong.

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