You won't find the serial number on the block of the 82 and earlier motors. Later on there was a freeze plug with the serial stamped into it. After 92 the serial was also stamped into the side of the block and the front half of the motor.
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Lots of parts are interchangeable. I run a 1979 mercury V200 powerhead with the updated enrichener, on a 1986 150 XR2 mid section (3 ram trim and tilt) with a 1991 Lower Unit, and a 1992 175 XRI cowling.
The heads are the dead give away that powerhead is a pre - 83 motor.
