1978 Johnson 4HP outboards Service Manual, Page 63Get this manual
connecting rod in position, then proceed to install the wrist pinThis can be accomplished more easily if the piston is heated slightly, causing it to expand PISTON RINGS Install the piston rings on each pistonSpread each ring with ring expander just enough to slip it over the head of the piston and down into placeBe sure the rings fit freely in the piston ring groovesThe ring grooves are pinned to secure the position of the rings, primarily to prevent ends of the rings from catching on the edges of the ports in the cylinders, but also to assure staggering of the ring gaps
m;;"I II!lJNOTE
One of the piston bosses is bored for slip fit on the wrist pin and the other for press fitWhen installing the wrist pin, drive from the side marked ilL Use an appropriate fixture to guard against distortion or damage during the operationdReplace retaining rings, making certain they come to rest in the groove provided for this purposeeCheck piston with micrometer to determine whether piston has been distorted during assemblyIf out of round more than 0.0025" (0 .064 mm), replace piston