The taper pipe thread plug has a step at the L1 distance from its small end. When turned hand tight into the products internal pipe thread ( do not force the gage ), the step is required to be flush with the large end of the product thread within a tolerance of plus or minus one turn.

The taper pipe thread ring gage has a thickness equal to the L1 length.The ring’s large end (marked face) is turned hand tight onto the external pipe thread until it stops ( do not force the gage ). The small end of the ring gage is required to be flush with the small end of the pipe within a tolerance of plus or minus one turn. 

      NPT L1 Basic Type – one step is provided which corresponds to basic size of component.

      NPT L1 Limit Type – Gauge has three steps. These steps corresponds to minimum, basic and maximum size of components.

 

NPT

Gage Code	Thread/perinch	Pitch(mm)	Taper
1/16-27npt	27	0.941	1:16
1/8-27npt	27	0.941
1/4-18npt	18	1.411
3/8-18npt	18	1.411
1/2-14npt	14	1.814
3/4-14npt	14	1.814
1-11.5npt	11.5	2.209
1 1/4-11.5npt	11.5	2.209
1 1/2-11.5npt	11.5	2.209
2-18npt	11.5	2.209
2 1/2-8npt	8.0	3.175
3-8npt	8.0	3.175
……	……	……
npt	8.0	3.175