No announcement yet.

What did I do wrong...NPT threading

  • Filter
  • Time
  • Show
Clear All
new posts

  • What did I do wrong...NPT threading

    So I decided to get after the plug in the water jacket of my 460 block that I buggard up.

    I drilled it out and got a 3/8-18 NPT tap. Also got a nice brass pipe plug to fit in the hole.

    I drilled the hole out to 9/16" According to this chart that is the needed size hole for that tap.

    This is the tap I bought

    It is a through hole, but threading it to my liking and to the depth a bit greater then the depth of the pipe plug will take about the entire cutting surface of the tap.

    So I would run the tap in using a wrench on the end till I could not turn it with my hand, and use a small pipe and go a quarter turn more. Then back off, add more oil and repeat. All went well untill the tap snapped off.

    I had been going completely by hand at first, but when I would feel resistance and take the tap out of the hole, it was barely cutting more thread.

    If it makes any difference I am tapping cast iron, and the tap is tapered.

    I was expecting it to go much easier.

    Picture 1 is drilling out the old plug

    Picture 2 is a different angle

    Picture 3 shows the stuff the drill press is sitting on so it was at a good height to clear and allow good quill travel.

    Picture 4 is the hole ready for tapping and the new plug

    Picture 5 is the broken tap.

    I am thinking about getting this tool to get the tap out.


    Knows a little about many trades.

  • #2
    Last pictures


    Knows a little about many trades.


    • #3
      You can try the tap removal tool, but in all my years as a Tool & Die maker, I've never gotten one tap out with one of those. I'm sure others will have more ideas, but I would cut that tap out with a torch. Then get a 3/8 npt reamer and a new tap and try again. The reamer is about the size of the tap and tapered the same as the tap so it won't get as tight threading the tap in.


      • #4
        A pipe tap that big is usually pretty hard to break but I don't think those Irwin taps are worth much. Someone ordered a set at work and they weren't worth even trying to clean out a buggered thread, much less tapping a new hole from scratch. With a tap that big you might be able to get it out with the tap extractor you have picked out. When there is that much gap between the tap gullets there the extractor can have bigger fingers and you have a better chance of getting it out. I'd say go for it as you don't have much to lose now.
        Jonathan P.


        • #5
          If you don't want to use the torch or a plasma cutter, you could try backing it out with a punch (make one, it'll trash the end of whatever you use).

          I also second the tapered reamer. CI is a pain.


          • #6
            Picture next to last:
            I see chips in the hole, I thought this was a thru hole? Is there a far side to this, like a water jacket? If so, I guess you bottomed the tap out, (snap).

            Taps that are used for shallow holes are different from normal pipe taps. I forget how many versions, but at least 4 different.

            Next, the tap remover you show is for a 3/8" tap for normal bolt threads. It's too small for a 3/8" pipe thread.

            My experience agrees with Toolguy, tap extractors have never worked for me.

            As for what to do, I gotta think on that one, especially if the tap was bottomed out, in addition to being tapered.



            • #7
              If you can get the tap out and you still neet to go deeper but the tap is bottoming out on the jacket, just grind the tap back a bit until you can tap it to the required depth. If you have a bunch to do it is best to have one regular tap and have one ground back so you can tap shallow holes.
              Jonathan P.


              • #8
                What does NPT reamer do?

                The tap is not bottomed out. I did blow compressed air through the water jacket to get the cuttings out from drilling before threading.

                I got the part # for the tap extractor from here..


                Knows a little about many trades.


                • #9
                  Weld a 1/2 bolt onto the back of that tap and after it cools - back it out


                  • #10
                    Thats one piss poor tap if you can drill a dimple in it with an ordinary drill..

                    i wouldn't buy anything ..

                    i would make tool consisting of two or four prongs welded to a handle.

                    the prongs would go down the flutes ..and you could just wind it out .

                    or better still do what joe sugests.

                    all the best.markj


                    • #11
                      If I had a pair of worn out needle nose pliers I would use them to try to turn that tap out, certainly couldn't hurt a thing.
                      Did or didn't use a tap wrench ? Lopsided turning forces using a wrench and a pipe ?


                      • #12
                        Those type tap removers only work if the tap is loose in the hole but you can't reach it. If it is tight enough to snap the tap off your not going to get it out easy no matter how you do it. The best way now is to buy several masonary drills that are bigger than the web of the tap and drill down through the center of the tap. The good thing is it's probably a carbon steel tap not tool steel and may be easier to drill out. When you drill out the web you will have to dig the four pieces out.

                        I too believe you bottomed the tap out in the hole. I don't usually have much trouble tapping cast iron and I don't usually use oil on it either, sometimes a little kerosene.

                        If it is shallow hole you may have to buy three taps and grind the end off two of them giving three different length taps, one to start, second to open it up and the third to get the size you want.

                        A taper thread reamer may work if the hole is deep enough so it don't bottom out. You have to be careful with the reamer to not go to deep or the hole will be oversize and then you will have to tap it to 1/2" pipe.
                        It's only ink and paper


                        • #13
                          The NPT reamer makes the hole tapered to match the tap so the sides of the hole are parallel to the threads on the tap. That way the threads are cutting an even amount as the tap goes in. Normally on a pipe tap, you want to stop going deeper when about 1/4 of the threads on the tap are above the surface of the workpiece. This is an approximation, you may need to try the fit a couple of times by threading in the piece that goes in the hole until it tightens up to the level you want.


                          • #14
                            Ok, a lot got posted while I typed. If the tap is not bottomed out welding a nut to the tap may work to get it out but I would still drill the web out and dig out the pieces.

                            In the mean time buy a good quality Greenfield tap or some such and use that. As I said cast iron is easy to tap. If you want hard to tap try SS or copper.
                            It's only ink and paper


                            • #15
                              What kind of drill bit should I get to try to drill it out? Im not sure how well masonry drill bits will work on steel.

                              The bits I have are HSS and wouldn't make a dent. The divit that is in the middle I ground there with my Dremel tool.

                              No I did not use a tap wrench.

                              Knows a little about many trades.