Olds block #'s(big) 1965 - 1976

Big Block Identification A big block measures 14" between the heads at the manifold base.
Facing the front of the engine, at the top surface of the timing chain area, to the left of the oil fill tube, there is a code about 1.5" high followed by the casting number. A letter indicates a big block, a number indicates a small block.
Big blocks have a little hump in that area, whereas the small blocks are flat the whole way across.

ID/ Casting
Code         Year(s)            CID                 Number Notes
A               '65                 425                  381917?? 1st year, big cars.
                                                              386525
B               '65                 400                  389298 442 only, one year only.
D               '66 - '67          425                  389244 Big-car engine. Toro is different internally.
E               '66 - '67          400                  390925 442 engine [VERY rare].
F               '68 - '72          455                  396021 Most common big block. Two types of "F"
                                                               letters: stick type, and one with two vertical
                                                               hangers and a base.
Fa              '72 - '76          455                   396021 Very common big block.
G               '68 - '69          400                   396026 442/Vista/etc. engine.
L                '76                 455                  231788 Motor home, maybe marine also.   GM made  motor homes from '73-'80 maybe in all?

Look for the casting number and letter on the horizontal shelf right behind the water pump, just to the [driver's] right of the oil pressure sensor. The casting numbers are usually about ¾" - 1" tall, and the ID/Code is usually a bit larger, a little over an inch, and covered with schmutz. The most common block will be the standard 455: "396021 F" or "396021 Fa". The "a", as in Fa, is actually an upper case A, but about ¼ as big as the F, and subscript to the F [set lower]. Also, on the 231788L block, the "L" is the same size as most of the rest of the digits, whereas the center "7" is noticeably smaller. 400's were bronze in color, most 425's and early 455's (1968, 1969) were red, while Toronado 425's and later 455's were blue. However, since engine color may have been changed, use this as supporting evidence only.