Nick, yesterday I stated that the sleeve on the Smiths X.85025 thermostat (as well as other period thermostats) does not have the original .375 inch sleeve travel that the XK120 Service Manual indicates for the original Smiths X.43655 thermostat. I said that I measured the sleeve travel of my NOS Smiths X.85025 at only .325 inches. This was a hot water measurement, which is difficult to photograph.
An easier way to determine the sleeve travel of any of these thermostats is to measure the distance between the moving bellows and the built in “hard stop.” All of the bellows & sleeve thermostats have some type of hard stop that limits sleeve travel.
The Smiths hard stop is a fixed brass bridge running the width of the thermostat body. AC, Remax, and QH used a similar type of hard stop. In the photos below, I show that the hard stop of the Smiths thermostat limits sleeve travel to .326 inches - roughly what I measured in hot water.
The first photo shows a “feeler gauge” I cut from sheet metal. The feeler gauge is the exact width of the distance between the Smiths movable bellows and the brass bridge, which acts as the hard stop.
The second photo shows the measurement of the feeler gauge, which is a distance of .326 inches. I challenge you to provide photographs showing how you determined your Smiths thermostat sleeve travel at 9.5 mm to 10 mm (at least .375 inches).
The distance the sleeve travels is critical to whether the bypass port fully closes when the main valve of the thermostat fully opens. Last week I posted photos on the E-Type forum showing the Smiths thermostat inside an XK120 thermostat housing. At the time I assumed the Smiths X.85025 has .375 inch sleeve travel just like the original Smiths X.43655. Assuming a .375 inch sleeve travel, I stated that the XK120 bypass port does not fully close. Now that we know the later bellows thermostats have even less sleeve travel, the problem for the XK120 is that much worse.
There may be some hope for the Moss thermostat because of the design of its hard stops. The Moss hard stops are two brass tangs that would lend themselves to shortening. However, as you admit, the sleeve travel of the Moss is only .3 inches. What is unknown and untested is whether shortening the hard stops by .1 - .2 inches for the necessary increase in sleeve travel would damage the thermostat.