So I received the new thermos housing plastic cap (the one with the Allen key slot in the top) and I also ordered what I thought was part of the cap in the parts diagram but it is just a shiny metal cap that seems to cover the plastic cap. That part is not currently on my vehicle, should it be?
If so, then how is it supposed to stay in place as currently if I were to place the metal cap on top of the plastic cap it would just be loose with no apparent way to keep it in place?
Thanks.

 

I replaced the tstat tower with the non-biodegradable aluminium aftermarket version, but I retained the plastic cap as the seal seemed better than the one in the new.
The domed metal finisher just wedged on top of the plastic cap.

There's no obvious requirement for the finisher that I can see: it's just to make things look pretty.

 

You think it's ok just to crimp it a bit so it doesn't fall off? Its got some little indentations in the side like punch indentations but they don't grip the plastic, crazy.

 

TBH I'd probably just put a dab of high-temp RTV in there to hold things in place. That way you should be able to pull it off again without too much drama if you ever need to.

The shiny metal cap ('Thermostat Housing Cover', according to JEPC) didn't grip the new metal top which is a secondary reason why I kept the original plastic. Perhaps your replacement top is a tad smaller in diameter than OEM hence the slack fit?

Hope the extra bling didn't cost you what JEPC is showing