Here's a new part (new to me) that was brought to my attention in mid May 04. Thanks to Phil Bates in Idaho for the notice and the pictures. This is Moss Motors part number 434-155. It has a bypass closing sleeve and is a 160°F thermostat. This is specified for the Austin Healey 100 and 3000, and for the Triumph TR2 through TR4A cars, but it works for the MG also. The difference here is that the sleeve is in the full raised position when cold, nearly touching the top mounting flange. The closure to block fluid flow is between the inner cylindrical body and the top flange.

When the unit approaches the preset operating temperature and begins to open, the sleeve moves downward to block the bypass port. The port opening is 3/8" down the wall, and 3/8" tall (bottom of bypass port 3/4" down the wall). This thermostat has a sleeve about 7/16" tall (and also 1-29/32" diameter). This will overlap the top of the bypass port slightly when at rest (cold). This is no big deal since the port opening is quite large relative to the smaller drill hole within the port.

When hot and fully open the 7/16" tall sleeve needs to be fairly accurately positioned to completely cover the 3/8" tall bypass port. This reportedly works okay as the opening distance between the mounting flange and the top of the sleeve is 5/16" or just a smidge more. This will block all flow above the ring when it is within 5/16" of the top flange. Like the original moving sleeve thermostat, the coolant flows through inside of the moving ring where there is substantial flow space between the sleeve and the inner cylindical body.

Unlike the original moving sleeve thermostat, this one can be purchased today (at least when this article was written). Pity that it is only available in 160°F specification. If anyone knows where this type thermostat could be purchased in 180 to 190°F operating temperature, please holler.