In 2007 Moss Motors offers a close functional reproduction of the original moving sleeve thermostat, Moss part number 434-156 rated for 160dF operation. While there are minor variations of construction detail, the overall design is very similar to original with the bellows pressurized by volatile fluid and the sleeve ring moving upward with original motion and dimensions. This will operate to expose or block the bypass port in the head in original fashion.

At date of issue of this web page I have not personally tried one of these thermostats in a car, and I have not tested it in any way. I have some question and reservations about how it might perform in actual operation in the pressurized cooling system.

Addendum, November 2008
Okay, I gave it a test run. I had this one in my MGA for about 5000 miles, mostly during July and August, plenty of hot weather, and one 1300 mile trip towing the trailer with expressway speed. It did pretty much what I expected, lengthening warm up time a little, opening at 160dF and remaining wide open nearly all the time as engine operating temperature typically runs between 190-210 with hard running. It was difficult to tell it it reduced warm weather running temperature, meaning not more than a few degrees and not definitive, so no great improvement.

Now the bad news, also about what I expected. As soon as ambient temperature dropped below 45dF the engine began having mild running problems due to running too cool. Given a few minutes to warm up a bit, then a 10 mile brisk run on the expressway, the engine would misfire noticeably with some loss of power in the 3500-4000 rpm range (70-75 mph with 3.91 final drive gearing). Pulling the choke out a little did not help, and additional driving made no improvement. I was beginning to worry about some sort of possible ignition problem, coil or condenser, but it was just running to cold.

Before my traditional Thanksgiving morning tour run I changed to a 195dF thermostat, primarily so the heater would work better in cold weather. This made huge change for the better. From a 45dF start in the garage to 30dF outdoor run, coolant temperature came right up to 195 by the time I made it up the expressway ramp one mile from home, and the heater was working well. Coolant temperature wavered a little when the thermostat opened, swinging slowly between 195-200, back to 195, then up again, finally stabilizing at 200dF on the gauge. I will have to test the gauge to see if it might be reading a little high. More important, the engine was running smooth and strong again never missing a beat from the first minute on. Since I don't like changing a thermostat twice a year, I reckon the 160dF thermostat will be looking elsewhere for a new home.