As the head runs at a fairly high temperature the cooling water must also be at a reasonably high temperature. This further thermal stressing. It is therefore usual to have the cooling water for the head in series with the jacket. The covers are attached to the cylinder block by means of large diameter bolts. The gas loads acting on the head are thus transferred to the cylinder block from which the tie bolts transfer it to the bedplate and then to the hull of the ship.
The original Sulzer engines employed single piece cylinder covers, but thermal stress cracks developed in relatively uncooled section were the conical part of the combustion chamber changed to the flat top.
In order to avoid this problem some allowance was required for thermal expansion, and this was provided by having a two part cover with an inner and outer section.
The inner section was of cast iron due to the complicated shape and the outer section cast steel for strength, a soft iron ring provided the joint between inner and outer sections. When the two parts are bolted together the head may be treated as a single unit.
For recent engines the single piece bore cooled steel cylinder cover has been developed and presents no particular problems.