M- 0.010' undersize connecting main bearings One or more letters may be present: B- 0.010' oversize cylinder bore The letter is located on the boss directly above the oil filter on six cylinder engines, on the valve cover tag on V8 engines.

MID 1967 AND LATER: A different letter represents each part and the modification. The code is located directly below the serial number or Engine Day Build Code on 172-195.6 six cylinder engines, on the boss directly above the oil filter on 199-258 six cylinder and AMC built four cylinder engines, and on the valve cover tag on V8 engines. In any position, an 'A' indicates standard size, 'B'. The code represented the bore (1st letter), main bearings (2nd letter), and rod bearings (3rd letter). No engine was fitted from the factory with parts that were over. 010 inch over or under would not correct the fit of the part(s), they were replaced. All three letters will be present if any part was factory altered from original specifications.

This code is used to indicate under and/or over size parts. UP TO MID 1967: Some engines will have a second three letter code under or beside the serial number or Day Build Code.

Decoding willys stamped engine numbers