api36You get limited on thickness using A36A36M to 0.750 inches20mm or less. For example if thickened inserts are required, on a 20mm tub, the shell plate could be A36 butAPI :Degrees API Gravity. API Gravity Definitions: Definition of API Gravity of water provided by Pedia:. The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or lht a petroleum liquid is compared to