While changing a head gasket it is recommended to resurface the heads at the headshop. It is an extremely important task. The surface finish affects not only the head gasket’s ability to cold seal fluids and combustion gases but also its long-term durability.
Smoothness has become a major issue with bimetal engines. If the surface finish is too rough, the difference in thermal expansion rates between an aluminum head and cast iron block can cause a delaminating effect in the head gasket causing it to leak and fail.
The only thing you don’t want on the head or block surface is scratches. Each scratch is a potential leak path through which fluids and gases can travel. If the scratch is deep enough, coolant can enter the crankcase or cylinders before the engine starts. Or, combustion gases can force their way through the gasket into the cooling jacket or adjacent cylinder, eventually causing the gasket to burn out and fail. Bad news anyway. Therefore, to avoid cold seal and durability issues the head and block require resurfacing to the manufacturer’s specifications.