MLS (Multi Layer Steel). Front & rear gasket for one engine. 4.00" fits 110" OEM H-D Twin Cooled engines only. Will not fit 110" non-Twin Cooled models. Stock Twin Cam gasket is approx. .040" thick. OEM replacement reference 16500168. 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 dowel pin o-rings; when in use.