BRITISH & METRIC FLUID CONVEYING PRODUCTS

The calipers in the kit can be used to help you identify both American and Foreign thread forms, including internal and external threads.
The top scale is used to identify all inch threads and the bottom scale is used to identify all metric threads. When using the scale to correctly determine your thread size, use the first, or left, line in the window area. See Figure 1 below for clarification.

To identify external or male threads, use the bottom jaws of the caliper and to identify internal or female threads use the top jaws of the caliper. See Figure 1 above for clarification.

All thread pitch gauges in the kit are used the same way in identifying internal or external threads. Each leaf has a number on it telling you how many threads per inch you have. The only exception to this is on the metric gauge. On the metric gauge, the number on each leaf represents the thread pitch (distance from the crest of one thread to the crest of the next thread.) in millimeters. Figure 2 below shows you the proper use of the thread gauge.

One important point illustrated above is the use of the correct thread pitch gauge on the threads. Incorrect thread identification can cause: equipment failure, leaking connections, thread damage to mating parts, or possible serious human injury.Additional thread identification is explained throughout this catalog.

General Information

In identifying fittings, you should always be sure to identify both sides of the fitting. Except for plugs, caps and unions, all fittings grip a rigid tube or a hose and are screwed into some type of port. Try to divide each fitting into two separate identifying sections – side A (grips tube or hose) and side B (the port threads).

Identifying the Port Side of Fittings (Side B)

There are six common port threads on fittings. They are as follows:

1. British Parallel Pipe Threads
2. British Taper Pipe Threads
3. Metric Threads, Parallel
4. Metric Threads, Taper
5. NPT
6. SAE Straight Threads

Parallel Thread vs. Tapered Threads

To identify any port side thread, first determine if the thread is tapered or parallel by inspection and/or use of a straight edge. A parallel thread will have some means of sealing at the top of the thread, either by O-ring, metal seal, or possibly by a machined ring in the hex itself. A taper thread seals on the threads themselves by wedging into the female port and sealing by thread contact.

There are various American Pipe Thread series in use today. Two of the more popular thread forms used are the Dryseal American Standard Taper Pipe Thread (NPTF) and American Standard Straight Pipe Thread for Mechanical Joints (NPSM). Unlike the British threads, our pipe threads have a 60º thread flank angle. See Figure 6. (Refer to Chart #3 for additional information.)

Two of the more popular thread forms used in the world today are the British Standard Pipe Tapered (BSPT) and the British Standard Parallel Pipe (BSPP). Both of these thread forms have a 55º thread flank angle. See Figure 7. The BSPT or BSPP cannot be interchanged with our own pipe threads due to the difference in thread flank angle.
The British thread forms use sizes similar to our own pipe threads – 1/8, 1/4, 3/8, 1/2, etc. First, determine if the thread is tapered or parallel, then measure it with the thread gauges supplied in your Thread ID Kit under British thread forms. All BSPT and BSPP sizes can be compared to our own NPT for sizing. (A 1/4″ BSPT or BSPP thread, if held end to end with a 1/4″ NPT will have approximately the same thread O.D.) Refer to Chart #2 for additional information.

Neither the British Standard Parallel Pipe or the British Standard Pipe Tapered are true metric threads, although many people find these threads “foreign” and conclude that they must then be "metric".

The Japanese Industrial Standard (J.I.S.) tapered and parallel threads are the same as the British Standard Pipe Tapered and British Standard Parallel Pipe threads. In addition to normal BSPP hose fittings, there are also flare fittings with a 30º or 45º flare in combination with either metric or BSPP threads that meet the J.I.S. standard. These are commonly called Komatsu and Mitsubishi fittings.

Another port thread form found on overseas equipment is the true metric thread. This thread form is probably the easiest to measure and identify.
First, determine if your thread is tapered or parallel by the same methods employed under the British Threads Forms Instructions. If you believe you have a metric parallel thread, you can measure it as follows:
First, measure the outer diameter of the thread in millimeters as shown in the following diagram using your calipers. (D = 24mm). Next, use your metric thread gauges to measure the distance from the crest of one thread to the crest of the next in millimeters. (P)

This thread has 1.5 millimeters between threads. Combine the O.D. of the thread measurement with this second measurement to form a metric thread callout (i.e. M24 x 1.5).

To measure metric taper threads, follow the same instructions as for the metric parallel thread. Since it will be difficult to measure the O.D. of a tapered thread, measure the O.D. at the third thread from the end. Then use your metric thread gauges as before. The thread callout will be M24 x 1.5 tapered. Information on additional thread identification is shown in Chart #1.