capese 21,
You ask the legitimate question " Is the OEM plastic thermostat housing actually that bad?" Yes, it is.
I suggest that the plastic thermostat housing is an excellent example of 'just adequate' design.
Adequate for say, 100,000k km or 8 years in ambient temperatures of -20C to + 30C, but quite inadequate for ambients of +20C to + 45C in tropical or desert locations.
Out here in the Antipodes, we Rover 75 owners are forced by the lack of service to help each other. So I've chatted with several other R75 KV6 owners over time.
So far, there has been a 100% failure rate of thermostat housings reported.
Some owners have even had the replacement OEM part replaced.
A Singaporean and South African member report similary in this forum.
Plastics degrade exponentially with temperature.
So the life of Rover's caprolactam derived component may be just adequate in temperate climes, but inadequate in severe service.
The ambient underhood temperature would exceed the 82C coolant temperature by many degrees. This component works in a very hot environment.
That you have had no problems in 6 years or 100,000km is no real test.
My thermostat housing failed quickly at 130,000km and 6 years.
This seems to be a common Time to Failure out here.
The Time to Failure in Europe may be longer.
But the OEM part is still poorly designed, and made from the wrong material.
Replacing plastic with metal is the only sensible fix.
Mr Kaiser's metal thermostat housing is the best option for replacement, especially if you intend to keep your Rover 75 for many years.
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