Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing 4/4 1600 Sigma Thermostat Housing

The past 2 services on our 4/4 have flagged a very slight coolant leak from the thermostat housing. I decided that a mild afternoon in January was the ideal time to fix this. Parts ordered and delivered, I set to straight after lunch on Monday. Dismantling was straightforward. The thermostat housing is fitted underneath the alternator on our 2014 car - and just above the engine mounting. The two hoses were OK to remove from underneath with the car on axle stands. I placed a container under the hoses and caught most of the coolant that escaped.

Working from the side of the car, with a 1/4" drive socket set and a long extension I was able to remove the 4 set screws quite easily. The thermostat housing then pulled away and it was easy to spot the problem. There is a small gasket on the smaller of the two pipes and the thermostat housing has a rubber ring which acts as the other gasket. I replaced the thermostat housing and the thermostat. Looking at Ford forums, these plastic thermostat housings are prone to cracking. Best to consider them as consumables.

Re-assembly got me flummoxed for a while. I ran out of time on Monday as it soon became time to get the horses in. The problem was that I could not hold the thermostat housing in place and insert one of the 4 set screws at the same time.

I used rubber grease (red) to coat the two rubber gaskets and to hold the new thermostat in place. The thermostat is on the side of the engine so I ensured that the small pimple (bypass thingy?) was in the same orientation as on the thermostat that I removed.

This afternoon, I finished the job. In order to get the thermostat housing ti stay in place, I cut the head off an M6 set screw and then cut a slot in the end. I was able to get the resulting threaded bar started in one of the fixing holes using some gaffer tape and a length of electric cable. Then the thermostat housing stayed in place long enough to get one of the other set screws started. A dab of grease in the socket held the set screw in place so that I could feed it past the assorted pipes, wires etc. Once all three other set screws were in place, I unscrewed my makeshift dowel and replaced it with the set screw. Torque settings for the M6 set screws (from a Ford forum) are 9Nm / 7 lb ft (I had to buy a new small torque wrench too!).

Once all was back together, I refilled with coolant and ran the engine until the thermostat opened. So far so good. No time for a test drive today - getting the horses in intervened.

Old thermostat housing showing tell-tale seepage & 2 rubber gaskets.


Old thermostat housing showing 2 pipes - this is upside down compared to the orientation on the engine. Not sure where they crack though.


The answer to my inability to get any screws started.