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.

Thanks Richard re draining. I was a bit too hasty and didn’t spot the drain on the rad until I was underneath re-fixing the hoses.
Simon SiFab will cringe given that he thought to include the drain.


Also, I forgot to include the part numbers:

Housing = Ford part number 1707050

Thermostat = 1712228

Got them from the Ford eBay shop. £31.23 for the thermostat and £38.99 for the housing.