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:
5/8-18-UNF

A pressure tight seal requires a sealing surface either before or after the threads, as the thread itself does not seal. 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.

Senders for Smiths Temperature Gauges
Mechanically operated temperature gauges come fitted with a flexible capillary tube leading to a copper bulb. MGA dash gauges are (originally) Jaeger brand parts, but they were made by Smiths. There are two types of Smith safety gauges, both normally supplied with an adapter which provides a 5/8" UNF male thread for screwing into the block. (Click for larger image).

The adapter was used on Austin Healey Sprite and MG Midget 1968-1974, and Morris Minor 1952-1972. This fitting screws into the cylinder head using a copper washer for sealing. The female thread of the adapter is shorter to allow use of the shorter flare nut (without need for shimming). Adapter part numbers: 11K2846 (BMC), 361-065 (Moss Motors USA). Copper sealing washer AED172 (BMC), 324-805 (Moss Motors USA)