While shopping for a new radiator outlet pipe assembly, I came across an “upgrade” for the OEM XJ8 thermostat housing which, like the pipe assembly, is also made of plastic (what was Jag thinking??? 
). Reason I need a new pipe assembly is that, after 180K+ miles, a crack/leak has formed in it, and I’m wondering whether it is worth it for the extra $40 + time for labor to r/r the thermo housing with the upgraded aluminum version at the same time. Has the OEM thermo. housing really proven to be problematic? I r/red the thermostat last year when it stuck closed and didn’t notice any weakness or leaks in the thermo. housing at that time. 
I always replace the water outlet pipe when I replace the thermostat housing. (as well as the short bypass hose)
I like the aluminum units and all my XJ8s have them installed.
The RESIN housings do deteriorate over time. The outlet pipe is NOT available as an aftermarket aluminum part that I am aware of.
bob
I also agree , an aluminum Thermostat housing is not available for the X350 . Also it is not recommended to try and fit one . There is a heat problem with them on the X350 which the earlier models did not suffer from .
The plastic is used for a reason according to my dealer . They replaced mine with the plastic again . Also the small rubber hose under the housing should be replaced at the same time .