.030" MLS. Both water & oil cooled heads must be plugged. No embossing on heads for water or oil cooling ports but for bore & oil drain only. Therefore cylinder heads must be plugged or welded in all cases. MLS (Multi Layer Steel) explained: MLS consists of a top and bottom layer of embossed stainless steel with both side coated with 250°C high temperature polymer rubber layer. The inner ring(s) are also stainless steel. The gasket layers are rivited together; outside the gasket surface; for ease of installation. The outer layers are embossed and both sides coated with a 250°C resitant polymer rubber layer. MLS head gaskets eliminate the use of any dowel pin o-rings.