My thoughts exactly. Except I would add that even a bottoming tap will have 1-2 partially formed or unformed threads at the bottom. If possible, many would prefer to use the taper tap only and not have a second or third operation with the plug and bottoming taps. Add chip clearance and even a small extra factor for safety or to avoid cutting a standard length screw and you can easily get a drilling depth of three diameters or more. Oh, and you do chamfer the holes, don't you. That will easily wipe out a thread, more if you are generous here.
Originally Posted by Carld
In most cases it does no harm to go deeper but if there are too few threads holding you can easily have problems. I would not try to minimize the holding power of the screws in an engine. And drilling is perhaps the easiest shop operation.
Last edited by Paul Alciatore; 12-04-2010 at 08:19 PM.
Make it fit.
You can't win and there is a penalty for trying!