Cylinder Head

Posted On: February 27, 2015

8 cyl heads:

850X and taxis used a cast iron head A90403.

850Y, 851, 852 used A90418 aluminum head which was superseded by A90539. The date or serial number of change and the difference between part numbers is unknown.


  • All 6 cyl used A90431 aluminum head throughout production.
  • All 8 cyl head gaskets are the same A90404.
  • All 6 cyl head gaskets are the same A90432.

Fasten 8 cyl heads using 27 SZ730 cap screws and 27 WZ9048 special washers. Opinion though recent times was that head bolts were cad plated, but old factory and era photos depict head bolts as black. We believe black Parkerized is correct.

There are two PF6 3/8″ npt square head pipe plugs marked A in the cylinder head. The front PF6 is next to the thermostat housing and used as an air bleed when filling the engine with coolant. The rear PF6 plug orifice is used for the supercharger heater line if applicable or in some instances if a cabin heater is installed with this as the hot water input.

Water outlet / thermostat housings A1-233 are the same for all cars. Held in place with three SA508 cap screws, typically marked W, lock washers and gasket A1-119. These are dark Parkerized, like the head bolts.