Following my recommendation of the TC oil thermostat several years ago Frank in Germany purchased a TC thermostat and installed it on his triple.

He reported in the second half of 2018 on TOL that the TC thermostat failed on the closed position after about 1000 miles of satisfactory service.

By mutual agreement Frank sent the failed unit to me for examination.

On receipt the unit was dismantled and examined. There were no faults immediatelv visible. The thermostat was then tested in the configuration of the wax capsule and metal shaft only.

At room temperature the shaft was mostly within the wax capsule.

The combination of wax capsule and metal shaft was then placed in the test kettle and the water temperature increased.

At approximately 185ºF the metal shaft started coming out of the wax capsule and was fully extended at 212ºF.

As the water cooled the shaft retracted into the wax capsule and was fully retracted when the water had cooled down to approximately 170ºF.

This indicated normal operation.

The thermostat was then reassembled and put through the tests previously carried out on the new TC and the DM thermostats.

At a "cold" water temperature of 73ºF the Frank thermostat gave the same results as the new TC thermostat.

At 212ºF the Frank TC thermostat gave exactly the same results as the new TC thermostat at 212ºF.

As the water was allowed to cool naturally the following was observed:

At 185ºF the Frank TC thermostat gave exactly the same results as the new TC thermostat at that temperature.

At 170ºF the Frank TC thermostat gave a lot of bubbles out of the "to oil tank" connection.

At 160ºF and 150ºF the Frank TC thermostat gave the same results as it did at 170ºF.

The physical examination and the two tests on Franks TC thermostat show it both to be configured and assembled as standard and to perform as expected.

The known failure mode of the wax capsule (which is an extremely rare event) is when the rubber sac of the wax capsule is punctured or its seal to the outer housing fails. When this happens the thermostat is heated and the phase change occurs then the wax takes the easy way out through the leak path rather than exerting force on the metal shaft. In this failure mode the shaft is not forced out of the wax capsule on heating and the unit fails in the "cold" position.

However there is no sign at all of this having happened on Franks TC thermostat.

So, unfortunately, I have no explanation for the failure of Franks TC thermostat based on my examination and testing of his thermostat.

One could hypothesize that the failure was due to - perhaps -

a) The springs inside the thermostat getting hung up internally in some way.
b) Some problem in the bike installation (a flap of rubber hose blocking a connection perhaps).

But these are just guesses and not backed up by any evidence.

My suggestion to Frank is that he reinstalls the TC thermostat and looks out very carefully for any further malfunction.

Alternatively he may of course decide to throw away the TC thermostat and continue with the standard configuration to which he has reverted.