What did I do wrong...NPT threading

[OldRedFord]

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.

http://icrank.com/cgi-bin/pageman/pageo ... pt.htm&t=2

This is the tap I bought

http://hand-tools.hardwarestore.com/22- ... 82558.aspx

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.

http://metalworking.mscdirect.com/CGI/N ... NO=3218877

[OldRedFord]

Two more pictures.

[BobbyFord]

Yikes! That's a big tap. Tungsten carbide drill should drill it, down the center then try to shatter the tap. You could try freezing the tap with freon and hitting it with a center punch.

[71PA_Highboy]

the tap remover should do the trick... but if you can put some freon on the broken tap when you are backing out the tap, it might be easier...

Did you cut enough thread before the tap broke? If not, next time don't use an Irwin...

Try Enco www.use-enco.com

Good luck!

[OldRedFord]

I am thinking about spending the money and getting Greenfield taps and dies from now on.

[Doug Comer]

Cheap taps and dies . I too have learned that lesson the hard way. At work we run up to 2" bolts and nuts. I've had excellent luck with Morse brand. Pricey but no more broke taps. In my opinion I've always had a hard time using a wrench on a tap. Seems to pull sideways. If you don't want to spend the money on a tap wrench, I've used to adjustable wrenches (1 on each side) as a T handle. Hope this helps.

Doug

[eggman918]

NEVER use a cheep tap except to clean up a existing thread,"Greenfield" is a good tap as is most of the enco quality cutting tools not the import stuff .
GO-JO hand cleaner is the best lube to tap cast iron,you need to use a larger drill size 37/64" or 19/32" the larger drill is OK in this application as it is a lo pressure fitting.
The drill you used is for use with a tapered reamer.As a last resort you can blow out the tap with a cutting torch just preheat it real good and let the O2 do the rest
Looking at it you will probably get it out with freon and a good punch and #3 hammer,when you get it out check if it was bottoming out if so you can shorten tap to get clearance
A tap handle or two wrenches is the best way to do it ,also never use more leverage than is needed it is a lot harder to break a tap with a short handle .
Stay calm it looks like you have it under control,you have many options
Pipe taps are the worst as they cut on the whole tap not just the end.

[eggman918]

In small pipe threads under 2" they are all tapered ,if it goes in 2 turns easy and then starts to get tight you are right where you want to be.
Use pipe sealant tape and go 1/2 to3/4 turn after it starts to gets snug and you should be OK
By the looks of block and freeze plug you did it as well as could be expected looks fine ,taped hole looks good also

[OldRedFord]

FWIW the freeze plugs are all brand new.



I guess I better start saving for a nice Greenfield tap and die set.

[BobbyFord]

Awesome job getting the headache out

[fordman]

wow you are good with a torch to make a hole that small.

[eggman918]

Outstanding damage control ,I would think you have done this before

[eggman918]

A cutting torch is a precision tool in skilled hands

[Heathcliff68f100]

A cutting torch is a precision tool in skilled hands

Aint that the truth, you can do alot with one when you know what your doing and have the right setup

[averagef250]

Torch is the most useful tool in my shop.

Cast iron can be incredibly hard. Myself, I would tap the whole to a depth that gets the plug all the way in at full torque.