From: http://www.dupont-dow.com/Applications/Automotive/head.asp

The cylinder head gasket is arguably the most important seal in the automotive engine. Since the birth of the internal combustion engine, gasket designers have specified many materials to meet this critical sealing challenge.

Traditional copper and brass gave way to metal and asbestos in the 1950s, superseded in turn by composite metal and impregnated fibre or graphite composites by the 1980s. However, those systems were largely overtaken by the development of the Multi-Layer Steel (MLS) gasket in Japan during the early 1990s.

MLS gaskets remain the automotive industry's preferred method of sealing between cylinder head and engine block, as yet unmatched by any other sealing system. Today an estimated 80 percent of new engines are designed with MLS gaskets as standard equipment, and further growth is projected.

In the MLS system, multiple thin layers of cold-rolled spring grade stainless steel are coated with 7-25 microns of elastomeric material. The resilient elastomer is essential to the structure by providing micro-sealing of metal surface imperfections while resisting aggressive combustion gases, oils and coolants at temperatures up to 250�C.

From: http://www.aa1car.com/library/ic697.htm

For years, most aftermarket gasket manufacturers have said surface finishes with a roughness average (RA) of anywhere from 55 to 110 microinches (60 to 125 RMS) are acceptable. The preferred range they have recommended is from 80 to 100 RA. Even so, as long as the surface finish on the head and block end up somewhere between the minimum smoothness and maximum roughness numbers, there shouldn�t be any cold sealing or durability problems with the head gasket.

But like everything else, these numbers have been changing. These recommendations were primarily for older cast iron heads on cast iron blocks. As castings have become lighter and less rigid, the need for smoother, flatter surfaces has become more important. Consequently, some aftermarket gasket manufacturers now recommend a surface finish of 30 to 110 RA for cast iron head and block combinations, with a preferred range of 60 to 100 RA for best results.

For aluminum heads, the numbers are even lower. The typical recommendation today for an aluminum head on an OHC bimetal engine is a surface finish of 30 to 60 RA, with the preferred range being from 50 to 60 RA�unless, of course, it�s one of the Japanese engines already mentioned with the MLS steel head gaskets which requires an even smoother finish (typically 20 to 30 RA).

It�s not difficult for the OEM�s to achieve this type of mirror-like finish when they manufacture a brand new engine on an assembly line. But not every aftermarket machine shop has the proper equipment to reproduce this kind of finish. So some experts say heads that mate to MLS gaskets should not be resurfaced unless absolutely necessary. For this reason, you should use extra care when removing the old head gasket so you don�t scratch or change the surface finish.