Hi, It's a little bit rude but :

I'm a from france, and I have a donkervoort D8 Cosworth, sorry it's not a Sierra , but the engine in it is a Cosworth YBG, and I have the same thermostat housing.

My problem : The o-ring between this part and the engine block is broken, and
I look at every place possible to find the corresponding o-ring, but I can't find it,
It's very disappointing ><.
Can somebody help me?
If this part isn't anymore manufactured can I just put a flat gasket in place of this damn o-ring?

Thanks a lot in advance for your answers!

Boubak