What surface finish is required to us an MLS (Cometic) head gasket?

A surface finish of 50 RA (roughness average) or finer, is recommended for a proper gasket seal. Anything rougher may conflict with the gasket design.

Roughness is commonly calculated using one of four methods: Ra (roughness average), RMS (root mean square), Rz (a 10 point average), and Rt or Rmax (maximum between peak and valley). Different methods can be combined to give more detail on roughness.
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The most common measuring method is Ra or roughness average. Metal finishing rolls range from an Ra of 0 to 0.5 micro-inches for a highly polished mirror finish for optical-grade products, up to Ra 50 or 60 micro-inches for satin-finished products, or possibly as much as Ra 300 micro-inches for embossed or coarsely textured products.