Blocks 1949 - 1964
The casting number is located at the rear of the bell housing.
The number are about ¾" high, and oriented to the right (passenger) side.

YEAR	SIZE	CASTING	NOTES
'49 - '53	303	555614	Green
'49 - '??	303	555641	Green
'54 - '56	324	568922	Green
		568928
'57 - '58	371
'59 - '60	371		Gold
	394		Green
'60	394	578135-4 1/8
'61 - '63	371		Gold
	394		Lo comp: Green, Hi Comp: Red
'64	394	585786	This specific casting number seen on an "Ultra-High Compression" engine. The number "4 1/8" follows casting number.

Small Block Blocks 1964 - 1990
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.
A small block measures 12" between the heads at the manifold base.

CODE	YEAR	SIZE	CASTING	NOTES
1 1A	'64-'66	330	381917	45° cam bank angle.
2	'68-'76	350	381917
2	'68-'70	350	395558
3	'66-'67	330	394417	39° cam bank angle.
4
5	'73,'74	350	395558
2A	'75-'81	260		Solid main webs for 2A's only? At least for '76.
2B	'??-'81	260	557751	Windowed main webs. Windowed main webs for 2B's only?
3A	'79	350
3B	'77-'80	350	557752
4A	'77	403	557265 553990 554990	Solid main webs possibly for these three casting numbers.
4B	'77-'79	403	557265	Windowed main webs always.
??	'85	307	556607
5A	'81-	307	3161
D3		350	7582	Diesel

Big Block Blocks 1965 - 1976
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.
A big block measures 14" between the heads at the manifold base.

CODE	YEAR	SIZE	CASTING	NOTES
A	'65	425	381917?? 386525	1st year, big cars.
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 - '70	455	396021	Common big block. Stick type F (sans serif).
F	'70 - '72	455	396021	Common big block. Serif type F. Two vertical and a base serif.
Fa	'72 - '76	455	396021	Very common big block. Marine also (L VIN derivative).
G	'68 - '69	400	396026	442/Vista/etc. engine.
L	'76	455	231788	Motorhome, marine and irrigation. GM made motorhomes from '73-8, maybe in all? Casting number appears as "231 [tiny 'L'] 788" on the block.

 
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