Whitworth threads use a 55 degree flank angle, and the threads has rounded roots and rounded crests. These features of Whitworth threads are significantly different from moddern metric and American threads. To properly measure Whitworth threads, specific tool are required because the 55 degree angle of the threads spread the load in a specific way.
If the wrong tools is used to measure Whitworth threads, it is possible to guess the pitch of the threads. Guessing the pitch of the Whitworth threads will result in forcing the parts of the fastener, which can lead to damage to the fastener. Using a thread pitch gauge remove the guesswork of measuring the pitch of the fastener.
How to Measure Whitworth Threads
A thread pitch gauge contains a set of blades marked with the size of Whitworth threads. You can press the blades against the fastener until they seat against the threads. The blade that seats against the fastener indicate the threads per inch of that fastener.
Using the threads per inch value on a thread reference chart will provide the size of the fastener in terms of it’s major diameter, pitch diameter, minor diameter, and the size of the drill bit that will cut the thread. The rounded profile of the Whitworth threads reduce the amount of stress that is placed on the root of the fastener. High level of stress on the root of the fastener can lead to cracks in the fastener after many decade of use.
A sharp 60 degree angle on the fastener creates stress on the root of the fastener that can lead to cracks. The use of a radius on the Whitworth thread reduce the stress on the root of the fastener. It is possible that some of the studs or bolts must be replaced in older workshop equipment.
The original studs and bolts were machined to the Whitworth standard. If modern studs or bolts are used that have a different thread angle, the contact between the studs and bolts will occur at the wrong point. This incorrect contact angle can lead to galling and stripping of the studs or bolts.
British Standard Fine (BSF) threads use the same 55 degree angle on the threads as Whitworth threads. The threads also have the same rounded profile as Whitworth threads. However, BSF threads has a finer pitch than Whitworth threads of the same nominal size.
Because BSF threads have a finer pitch, they have better resistance to vibration. BSF threads are often found on cylinder heads and wheel spindles because they resist vibrations caused by the movement of the car. BSF threads and Whitworth coarse threads are not interchangeable.
For instance, a 26 TPI BSF bolt will not have proper engagement with a 20 TPI BSW bolt. The pitch of the BSF bolt do not match the pitch of the BSW bolt. A thread pitch gauge must be used to determine if a thread is a BSF thread before any tap are used on that fastener or if replacement fasteners are ordered.
Pipe threads use a 55 degree angle in the same fashion as Whitworth threads. However, the use of pipe threads serve a different purpose than fastener threads. Parallel BSPP threads create a seal at the face of the fastener or with an O-ring.
However, tapered BSPT threads create a seal between the flanks of the threads. If the wrong type of pipe thread is used, the joint made with the pipe threads will leak under pressure. No amount of pipe tape will fix the leak created by the wrong type of pipe thread.
A reference material must be used to determine the correct type of pipe thread to use for the application of the fastener. There are steps that can be followed to identify the fasteners that is present in the workshop. First, the outside diameter of the fastener can be measured.
Second, the number of threads on the fastener can be counted with a thread pitch gauge. Third, the angle of the flank of the threads can be measured with a 55 degree template if available. These three steps will provide the best understanding of the fastener and eliminate the guesswork when replacing those fastener.
Often, the fasteners from the older era of the workshop will not have any markings on their heads. The only way to determine the size of the fastener will be through these physical measurement. Having a thread chart can be of great value to the workshop mechanic.
Using a thread chart will prevent the common mistake of treating all imperial sizes as if they is interchangeable. Some metric sizes are close to imperial sizes. However, the threads of the two systems are not the same.
Metric threads have a different angle to their threads than imperial threads. Attempting to use metric and imperial sized threads will only result in the two piece of metal with threads in close contact with each other. The threads may appear to be engaged, but they will only be touching on a small fraction of their total area of contact with each other.
If the threads only have minimal contact with each other, it is likely that the fastener will eventually loosen of its mating component over time. Much patience and the use of correct reference material will make for an efficient workshop. Once the thread pitch gauge and thread chart confirm the pitch and drill size needed for the fastener, the workshop mechanic can proceed with the work on the component.
Overall, the design of the Whitworth threads to last for many decade will continue to last if used with the accuracy and care that the age of the fastener demand.
