Pipe threads are manufactured in many forms, but the most common types are straight and tapered threads. Straight pipe threads are gaged with the “GO” & “NO GO” process like regular threads, but tapered pipe threads require an entirely different gaging method: A series of one to three “L” checks. In this video, we’ll cover how to gage the three types of American tapered pipe threads: NPT which stands for National Pipe Taper, and NPTF which stands for National Pipe Taper Fuel and comes in Class 1, and Class 2 classifications.
When assembled tightly, NPT threads leave voids at the root and crest of the threads which need to be filled with a sealing compound at assembly to avoid spiral leakage. NPTF is also known as American Dryseal Pipe Thread and is designed to leave no voids, and therefore requires no sealant.
The L-1 Check
For all three types of tapered thread, the L-1 gage is the first required check. This gage checks the Hand Tight Engagement Length which typically includes the first 4 to 5 threads. For internal threads, the L-1 gage will be a plug gage with a notch ground into the side for a reference surface. For external threads, the L-1 gage will be a ring gage where the flat surface of the gage acts as a reference. The labeled side of the gage faces the end of the thread. The part surface where the pipe thread starts is used as the reference for all measurements.
Each type of tapered thread requires a different use of the L-1 gage.
For NPT threads there is only one test required. Thread the L-1 gage onto or into the tapered thread until the fit is hand tight. When the gage stops, the notch on the plug gage or the flat of the ring gage, should be within plus or minus one turn from the entry face of the thread.
NPTF Class 1 threads require the same check with one major difference, you need to know the distance from the face of the thread to the reference notch in the plug gage or to the surface of the ring gage to complete the next check in the series.
In our example, the L-1 gage is ½ turn below the thread surface which is .036 inches when measured.
NPTF Class 2 threads a third step. You must determine if the thread is at the minimum, basic, or maximum condition. To do this is slightly different for internal vs. external threads.
For external threads if the surface of the L-1 ring gage is within 1/3 of a turn above or below the thread surface then the thread is in the basic condition. If the surface of the L-1 ring gage is more than 1/3 turn above the thread surface, then the thread is in the maximum condition. If the surface of the L-1 gage is more than 1/3 turn lower than the thread face, then the thread is in the minimum condition.
For internal threads, the conditions are reversed. If the notch on the L-1 plug gage is above the thread face more than 1/3 turn then the thread is in the minimum condition. If the notch is more than 1/3 turn below the thread surface, then the thread is in the maximum condition.
In our example, the L-1 gage is ½ turn below the thread surface which puts it in the minimum condition.
The L-2 and L-3 Check
Both classes of NPTF threads require the use of an L-2 gage for external threads or an L-3 gage for internal threads. The L-2 and L-3 gages check the Wrench Tight Engagement Length which generally checks the 5th, 6th, and 7th threads. These gages look similar to their L-1 gage counterparts, but they are designed to verify the taper of the thread.
The L-2 and L-3 gages are used in a similar manner to the L-1 gage. Thread the gage into or onto the thread until the fit is hand tight. To pass this test the surface of the L-3 Gage or the notch of the L-2 gage must be within plus or minus one half turn of the L-1 position. This is the final check needed for NPTF Class 1 threads.
In our example, the L-2 gage lies within the pass region that was determined by the position of the L-1 gage.
The Six Step Gage
NPTF class 2 threads require the third gage in the series: the 6-Step Gage also known as the Crest-and-Root Truncation Gage, or the Crest-Check Gage. This gage is not threaded, but is a tapered plain gage, with six steps ground into it. This gage is simply inserted into the hole, or over the external threads, without turning, as there are no threads.
The six steps are labeled in three pairs, giving a high and low limit for each of the three conditions determined with the L-1 gage: minimum, basic, or maximum. If the thread is in the maximum condition then the face of the thread should fall between the two steps labeled “MX” and “MXT”. If the thread is in the basic condition the face of the thread should fall between the two steps labeled “B” and “BT”. Finally, if the thread is in the minimum condition the face of the thread should fall between the two steps labeled “MN” and “MNT”.
In our example, we determined that the thread was in the minimum condition in our L-1 check. The thread surface lies between the “MN” and “MNT” steps of the 6 Step Gage and has passed the test.
Still have questions about gaging your pipe threads? Contact one of New Vista’s Applications Engineers today for expert assistance and see how much time and money New Vista Systems can save you.