.040" MLS (multi-Layer-Steel). For water & oil cooled bikes. Special design allows the gasket to fit into the cylinder bore embossement... while even keeping the oil cooling port open on oil cooled heads. With this gasket the 4.250" big bore size is a true bolt-on application for both oil & water cooled M8 models. 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.