How do you select the thickness of replacement crankcase gasket Briggs model 281707 Craftsman mower?

put the crankcase together with no gasket and move the crankshaft. the amount of movement you get will determine the gasket thickness. if it is a lot of play then use the thick gasket, if there is little to no play use the thinner one and if the movement is in between use the middle thickness gasket. not a super technical answer but it should help you out