Differences Between 1970-71 And 1972 and Later Type Buick Big Block

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Buick Service Bulletin

Differences Between 1970-71 And 1972 and Later Type Block
Model Year	Production Head	Sevice Head	Head Gasket	Intake Gasket	Cylinder Block
1970	(1231786)	1240144 (1240149)	1241749 OR 1247657	1231776	(1231738)
1971	(1237661)	1240144 (1240149)	1241749 OR 1247657	1231776	(1231738) 1ST JOB (1238861) AFTER JOB
	1971 CHANGES:	VALVE SPRING PILOT DIAMETER REDUCED FROM .838 TO .818 AFTER JOB BLOCK FEATURES ENLARGED SUCTION OIL HOLE
1972 1st JOB AFTER JOB	(1238148) (1242445)	12424443 (1242445)	1241749 OR 1247657	1238514	(1238861) 1st JOB (1241735) AFTER JOB
	1972 CHANGES:	A.I.R. HOLES ADDED TO INTAKE GASKET FACE/ EXHAUST HEAT CROSSOVER HOLES REVISED. AFTER JOB: PUSHROD HOLES ENLARGED TO .5625 FOR STEEL ROCKER ARMS. (SERVICE STEEL ARMS DO NOT REQUIRE LARGER PUSHROD HOLES.) BANK FACE OF BLOCK 1241735. REVISED TO HAVE TWO BOSSES AT CENTER INBOARD LOCATION TO SEAL NEW CORED HOLES IN 1973 HEAD (SEE FOOTNOTE 2).
1973-1974	(1241860)	1242001 (1241860)	1241749 OR1247657	1238514	(1241735)
	1973 CHANGES	tWO 3/4" ROUND CORED HOLES IN HEAD AND BLOCK REVISED TO 3/4" X 1-1/8" (SEE FOOTNOTE 2). TWO NEW 3/4" CORED HOLES ADDED TO HEAD.
	1974 CHANGES	HEAD GASKET 1247657 RELEASED WITH OVERHEAD OIL FEED HOLES ADDED TO SERVICE ENGINES PRIOR TO 1970 (SEE FOOTNOTE 3).
1975	(1246322)	(1246322	1241749 OR 1247657	1238514	(1241735)
	1974 CHANGES:	CASTING NEW DUE TO CORED HEAT CROSSOVER HOLES AND ROUND COMBUSTION CHAMBER. COMPRESSION RATIO LOWERED TO 7.9:1 BY INCREASING COMBUSTION CHAMBER TO 4.73 CUBIC INCHES (WAS 4.33 CUBIC INCHES) EXHAUST VALVE SEAT WIDTH INCREASED FROM .060 TO .090.
() DENOTES CASTING NUMBER. (1) 1970-71 HEADS ARE NOT INTERCHANGEABLE WITH 1972-5 HEADS DUE TO REVISED INTAKE MANIFOLD FACE AT THE HEAT CROSSOVER AND A.I.R. HOLES. (2) 1973-75 HEADS REQUIRE '72 AFTER JOB OR LATER BLOCK TO SEAL THE ADDED CORED WATER HOLES (SEE SERVICE BULLETIN 73-1-23). (3) 1242694 SERVICE BASIC ENGINE FOR 1970-75 455 USAGE FEATURES ENLARGED SUCTION OIL HOLE AND DOUBLE SCALLOPS FOR THE COOLANT SEAL (SEE SERVICE BULLETIN 73-1-23)
	CAUTION:	If the Basic Engines have the "double scallop" per illustration and the "oblong" holes, do not use the 1970-1972 Cylinder Head Gasket, 1237849. Use new Cylinder Head Gasket, 1241749.
	If the above Basic Engines have the "single scallop" per illustration and the "round" holes, the specified Cylinder Head Gasket, 1237849, for 1970-72 models may be used, or the 1973 Gasket, 1241749, may be used optionally as available.