When I found chocolate mousse under the oil filler cap of my 91 164S, I just changed the coolant, oil, and filter, and then retorqued the heads to the maximum as shown in the workshop manual.
Turns out the head torques had relaxed to at least 10 ft-lb below the average workshop value, the front head was at 65, the less accessible rear head averaged about 55!! That was at 100k miles, and now at 190k miles, the problem has never returned. The engine runs just fine, with as new oil pressure, ~55 psi at hot 3000 cruise rpm, ~25-30 psi at hot idle. No oil in the coolant overflow tank, no chocolate mousse under the cap.
These engines do seem to require a head retorque every few thousand miles, maybe at 80k miles? I think that sometimes head gaskets are changed even if they really don't need it, just requiring a retorque per the specs.
"the engine has less than 10k miles since I refurbished it in 2012"
You should not be having this problem. Certainly didn't in my two GTV6s or our Milano, let alone the two 164s, except for loose heads in the 91S. Are you sure you are torquing the heads properly with high enough levels? I don't think you have a head gasket quality problem. Something else is going on.
1989 Milano, Shankle Sport
1991 164S, stock
1994 164LS (~Q)
1972 Morgan 27
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6
Last edited by Del; 06-20-2018 at 10:42 PM.