M8 x 1.25 6H Precision JO-PLUGS

Application of the Precision Jo-Plug®

The Precision JO-PLUG® tapped hole location gage solid thread form provides the best method on the market for determining the true position of a threaded hole, as well as perpendicularity and the projected tolerance zone of a tapped hole in conjunction with a CMM. Fundamental to our superior design is the Precision JO-PLUG® solid thread form which has a measurable pitch cylinder and coaxial spindle which extends to establish a projected tolerance zone of the threaded hole.

The solid thread form is significant because ANSI Y14.5 – 2009, “Dimensioning and Tolerancing,” Section 2.9 states that “each tolerance of orientation and datum reference for a screw thread applies to the axis of the thread derived from the pitch cylinder.” The Positional Tolerance Method (Section states that the pitch cylinder is the default condition. Rarely will a major or minor diameter be specified to measure true position.

JO-PLUGS® are manufactured to the minimum pitch diameter of the internal thread, with a tight minus tolerance, giving Precision JO-PLUG® centerline location plugs a measurable pitch diameter.

This is essential in establishing a cylindrical axis which is coaxial to the stem of the JO-PLUG® which is probed using a CMM in the established projected tolerance zone.

The standard Precision JO-PLUG® design is best utilized when a machined surface, perpendicular to the tapped hole, acts as a seating surface. Upward pressure on the thread flanks, which are also held to a tight tolerance, align the Precision JO-PLUG® to the center of the threaded hole.


Independent repeatability and reproducibility tests by a national laboratory have produced very favorable results for the JO-PLUG®. Standard deviation numbers versus other methods are the lowest on the chart. Results are available on request.

In applications lacking a seating surface, such as cast or irregular surfaces, Spence Industries, Inc. suggests considering a JO-PLUG® with a tapered pitch diameter. We have the ability to manufacture tapered threads using GO and NOGO pitch diameter dimensions. This style of gage locks into position prior to seating surface contact. We also manufacture other special gages for applications with counterbores or other unique circumstances with extended projected tolerance zones.