On 3/22/2011 Doug Schimke wrote:
"What are the treads on the temperature sensor that screw into the engine"?



This question comes up often enough to merit its own web page. And the answer is:

1/4-19-BSP

Nominal major diameter is 0.518-inch (about 0.510 over a slightly truncated thread). Thread pitch is 19 threads per inch.

All of the threaded fluid port threads in the Austin B-series engine block (and cylinder head) are BSP standard threads. British Standard Pipe threads are straight threads based on the Whitworth thread standard. Whitworth threads have 55-degree included angle (not 60). You likely will not find BSP thread taps and dies at your local hardware store, but they are readily available at any machine tool supply house, and they can be easily found with an internet search.

A pressure tight seal requires a sealing surface either before or after the threads, as the thread itself does not seal. Hex head plugs, hose connectors and drain valves use a copper washer for sealing. The temperature sensor bulb seals between the flanged collar on the bulb and a mating machined shoulder in the bottom of the port, so it does not need any other sealing device.

Always use a flare nut wrench for this device so you won't damage the nut. The nut cannot be replaced without breaking the fluid pipe or disconnecting the soldered end connection. Repairing the pipe and replacing the volatile fluid inside is an expensive repair.