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.

iCrank NPT Thread Tap Drill Size Chart (Inch)

This is the tap I bought

Irwin Taper Pipe Tap, 3/8-18 NPT # 1904P by Irwin Tools

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.

MSC Item Detail

IMHO, first mistake,,,NEVER use oil when tapping cast iron, it MUST be done dry. Oil causes the chips to stick together and to the tap which jambs it in the threads...as you found out. Second, a carbon steel tap will do exactly what happened to you, they are too hard and therefore brittle. Invest in a "high speed steel" tap. Get a second one and cut the first half of the threads off. Start with the full length one, when it bottoms out...switch to the shorty. OR...find a pipe plug that fits the hole you tap better, size varies by manufacturer.

High speed steel tap...

Like this?

http://www.amazon.com/Greenfield-Tap-Die-8-18npt-Hi-hook/dp/B002KUBFBQ

heat it with a torch till just a little red then shove a candlestick to it ( don't get burned) let cool then you can stick the ends of needle nose pliers in there and unscrew it right out.

Yup, that's a HSS (High Speed Steel) tap.

I got the broken tap out.

I got my torch and carefully blew a hole about dead center through the chunk of tap. Then I took a screwdriver and a hammer and was able to spin the tap out of the hole by whacking against each of the flutes. I also noted that the block in the immidate area of the hole was still cool to the touch soon after using the torch so I do not think I caused any cracking.

I then went back down to Ace Hardware and got another tap, and a 37/64s drill bit. I looked for a proper sized tap handle but they do not sell one big enough. They sell even bigger tap and dies but do not sell big enough handles to use them?

About this time a guy asked if I needed help and I explaned my situation to him and he said he had also commented a few times about not having tap or die handles as big as the larger taps and dies they sell.

So I bought two cheap 6" adjustable wrenches.

Came back home and drilled the hole out with the bit, and ran the tap in the hole. This time only going one turn forward and then a quarter turn back.

Now my question is...do they make a tapered and a straight pipe plug? The pipe plug only threads in about two threads. I would like for it to thread in a bit farther.

The tap doesnt want to go any deeper and I know I am not hitting bottom. I do not want to break another tap.

Picture #1
The broken off piece of tap. You can see where I torched out the very center.

Picture #2
Threaded hole

Picture #3
Brass plug

Picture #4
Broken off piece of tap, flipped over. If you look close that blue spot is the paint on the engine stand. I do not think I could hit center like that again with a torch.

the correct depth to tap is to leave the top 7 threads of the tap out of the hole.

justa note I never use a Tap without Magic Tap, they make it both for Cast and Aluminum... make sure you do not use the one for cast on Aluminum it will eat it up.....



Randy

jones wrote:I guess I should have told you this earlier but they do make straight taps for blind holes.

yep bottom hole taps.....but they do not work on a taper NPT hole.....

Randy

I got a few steel 3/8 npt plugs. Fit much better.

Now for sealing it up, would Permatex thread sealant work?

yes but make sure it is the soft kind not the perminent


Randy

What is the other allen head plug for on the drivers side of the block, in the side but at the back?