1969 Evinrude 40 HP Big Twin, Big Twin Electric Lark Service Manual 4596, Page 50Get this manual
CLEANING, INSPECTION, AND REPAIR CYLINDER BLOCK AND CRANKCASE Check cylinder walls for excessive wear and check cylinder ports for carbon accumulationCylinder walls wear in various degrees, depending on lubrication and conditions under which the motor is operatedMajor portion of wear is in the port area and the area covered by ring travelCheck cylinder for size and wall straightness by using an inside micrometerRefer to Section for specified dimensionsReplace cylinder block, or rebore block for oversize pistons if wear is in excess of .003"Pistons and ring sets are available .025" oversizeNOTE
If your shop is not equipped to rebore cylinder