I've a brand new 4WD Thermostat Housing & Elbow.

I've fitted a brand new Ford Thermostat as well.............now its leaking from the joint between the two.

Both faces are flat and clean !?!

I've a new rubber seal (O-Ring) between the two (bought from Burton Power) - is it worth making a thin gasket or using a sealant ?

Can't really understand why it's strung a leak ?

Anybody had the same issues