Tapping for an NPT tapered drainplug - Help?
 

Anyone have experience drilling/tapping for an NPT tapered plug? I have a tapered NPT tap, but do I need a tapered drill bit to make the hole? Or just use a regular parallel drill bit on my drill press?

Straight drill. You can find a chart online for the tap drill size. I just did one yesterday. What size?

3/4". Just found it. Drill size is 59/64ths. Fortunately I happen to have a bit that size!! Tapping a factory drainplug into my Corvair differential case. They quit putting in drainplugs after '63.

Start square.
Go slow.
Try the plug in the hole when you have threads all the way through.
You should have about 3 full rotations of the plug by hand before you start to feel some resistance.
In other words, you can't make the threaded hole smaller!

I also learned that you have to use an NPTF "dryseal" tap. If you use a regular NPT tap then it will leak unless you put pipe sealer on it.

When I learned to tap threads in the old days.....1963, I started using Crisco. It absorbs heat, melts and lubes the cutting very well. I have a small can that is about 25 years old and will be around after I am gone probably. Yes, there are a lot of nice, new modern lubes, but.......

12 rotations of the tap usually gets you to size, a Bill says check. After about 8 turns of the tap it is good to start checking.