This is where I come in 
Taps are very hard and brittle, but that is what makes them so easy to alter
Yep , whenever I need a bottoming tap I just grind a broken Standard Tap off
It grinds very fast. See the picture below. The only difference between a Standard Tap and a Bottoming Tap, is the tip. The Bottoming Tap is flat (this is what you want to clean out old threads. Aa Standard Tap with a tapered lead-in, is my likely choice to cut new threads!
Now be careful not all taps are created equal!!!! Pipe threads are not the same as NC and NF taps Pipe threads are actually tapered, so as you tighten down the fitting it is actually wedgeing its self in the the hole. This helps look the joint for a water tight fit with no teflon or sealers. DO NOT TRY TO MAKE A BOTTOMING TAP OUT OF A PIPE TAP- you have no way to know whee the correct size it. Take a look at the Pipe Tap below-
