Questions after tear down

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green72

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So I’m starting the rebuild of my 1972 slant. I believe that this motor has never been touched. Got it out of the car and disassembled today. I pulled the pistons out to clean the carbon off the top (really thick). Also discovered the top ring on three pistons broken. After cleaning I reinstalled them so I could measure how far in the hole they are. The number 4 piston is marked STD. Is it normal for only one piston to be marked? The pistons all measured very close to .172 in the hole, EXCEPT the number 4 piston (the one marked standard) it was about .190 in the hole. Does this seem “normal”?
 
green72 said:
So I’m starting the rebuild of my 1972 slant. I believe that this motor has never been touched. Got it out of the car and disassembled today. I pulled the pistons out to clean the carbon off the top (really thick). Also discovered the top ring on three pistons broken. After cleaning I reinstalled them so I could measure how far in the hole they are. The number 4 piston is marked STD. Is it normal for only one piston to be marked? The pistons all measured very close to .172 in the hole, EXCEPT the number 4 piston (the one marked standard) it was about .190 in the hole. Does this seem “normal”?
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Nope. #4 is a replacement.
 
Compare the compression height of the # 4 piston to one of the others. Compression height is the distance from the center of the pin bore to the top of the piston. One way to do that with out a lot precision measuring equipment is, with the piston pins pressed out and the rods off, have your machine shop turn down a piston pin so it is a slip fit into the piston pin bore. Then insert the pin part way into the #4 piston and part way into one of the other pistons so the one piston pin is centering and holding both pistons. Then set them on a flat plate with the top of the piston down and use feeler gauges to measure the gap if one pistons top is away from the flat plate. I bet that you will find the #4 piston has a smaller compression height.
 
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