Standard taper pipe taps are designed to produce sufficient thread depth for standard pipefittings. This would include 4 to 5 turns or threads for assembling the parts “hand tight”. These are known as L1 threads. Following these are three additional turns for “wrench” tightening to provide the sealing of the assembly. These are known as L3 threads.
Since all taps have a chamfer for producing the threads, the tap must be fed into the part deep enough to produce the full threads plus allow for the incomplete threads produced by the chamfer.
Most standard pipe taps have a maximum chamfer length that ranges between 3 and 4 threads. Therefore, the tapping depth should be the approx. 5 threads for the “hand tight”, 3 threads for “wrench tight”, and 4 threads for the chamfer, or approx. 12 threads total.
We call this the “12 thread count”. In other words, count approximately 12 threads back from the front of the tap. This is the position (thread size) that should be flush with the face of the part or top of the tapped hole when the tap is fed to full depth.
When using Short Projection taps, (taps designed for holes shallower than those found on standard fittings) the tapping depth should be the projection marked on the tap plus the L1 length. The L1 length can be found in the Machinery Handbook, ASME thread and gaging standards, etc.
There is no standard thread or tapping depth for straight pipe taps. However, when producing straight pipe fittings that will be assembled with tapered external pipe nipples, the user should allow, at a minimum, for the full thread length on the external part.
This length is called the L2 length, which is approximately 8 threads. As with tapered threads, you must also account for the taps chamfer length when setting the taps stroke depth.