I've got a Atomic 4 manifold with a stripped-out drain plug. I've found it difficult to tap the cast iron, so I went looking for some information on how best to create new threads in a hard iron casting. Along the way, I found some interesting web pages:
Thread repair how-to using a thread insert
Thread repair how-to by rethreading in place, or using a thread insert
Installing Heli Coil® thread inserts and Primer on Using Heli Coil® Inserts
Tech tips for drilling and threading.
Drill sizes for taps are here, here, here, here, and here. Probably other places, too, but this should be enough.
Understanding tap designations:
- For example, 10-24GH2: 10 is the size, in this case a #10. 24 is the pitch- 24 threads per inch. G means the tap is ground all over; cheap taps are not necessarily ground all over and give substandard results. H2 means the pitch diameter is .001 (2*0.0005) over the basic size.
When buying your taps, it's best to use an industrial supply house, you'll get a better quality tool. Nearly all have websites- Mcmaster-Carr, J&L, Enco, etc
- Pipe taps come in two versions- straight and taper. Most pipe taps are tapered- the taper is obvious. Buy your pipe taps at the same industrial supply you get ordinary taps from; they are designated with NPT- for example, 1/8-27NPT deciphers to 1/8" pipe size, 27 threads per inch, National Pipe Tapered.
Additional note on aluminum: it's best to make use of helical coil inserts (Helicoil) right from the start-the resultant connection is stronger, and is more easily disassembled in the future. Apply zinc chromate paint to the helical coil insert, and install while wet. Helicoil recommends this procedure for use around salt water- also be aware that the inserts are available in free running and locking versions.
For cutting oil, use what's at hand; I've had good results with 2-stroke premix oil; TapMagic is excellent and is available in small containers. While I haven't tried it yet, I suspect canola oil would work well, too.