• 1/8"-27 NPT National Pipe Taper HSS Pipe Tap
• 1/4"-18 NPT National Pipe Taper HSS Pipe Tap
• 3/8"-18 NPT National Pipe Taper HSS Pipe Tap
• 21/64" HSS Drill Bit
• 7/16" HSS Drill Bit
• 37/64" HSS 1/2" Shank Drill Bit
You should replace the 21/64" drill with a letter R drill which is .339 instead of .328.
The other 2 drills are correct.
Are taper taps appropriate for doing this job? They seem to be more commonly available then plug taps which I can understand might be better.
The simple answer here is yes, the taper taps are what you are looking for. A plug tap is a description of the point style available. There are plug, gun, and bottoming taps for most straight threads, but tapered threads, and taps, are different. Unless the hole you are tapping is fairly deep, go with the shorter length tap, otherwise you'll bottom out before getting enough thread to seal properly.
Reamers to create the taper prior to tapping – necessary or not?
Not completely necessary, but recommended. Tapered taps, especially the bigger ones like 3/8-18, require a LOT of torque to cut. This is where the possibility of cracking the metal around the hole, (the boss), can occur. I recommend you try tapping a hole in a scrap piece of aluminum, or an old engine case to get a "feel" for it, before you tap something as precious as an engine block. So, to answer your question, yes, use a tapered reamer to substantially reduce the possibility of cracking the boss that you are tapping.
Hints and tips would be much appreciated.
There are staggered tooth taps available for these thread sizes. Staggered tooth taps work very well in aluminum alloys and require less torque to cut a thread. Use a light cutting oil or even kerosene for aluminum.
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