Chevrolet Big Block Heads – Casting Numbers and Specifications

Mark IV Chevrolet Big Block Heads

Chevrolet Big Block Heads

Chevrolet Big Block Heads were offered in many different variations within their 51 years of production. GM Production of the Chevy Big Block engine began in 1958 with the Mark I W Series and ended with the Gen VII in 2009. However, in the aftermarket production of heads, blocks or complete engines continue till this day.

Over the years depending on the application Chevrolet used different head configurations for the Big Blocks. GM installed these engines on grocery getters, high-performance street machines, all-out race cars, trucks, boats, and also industrial applications. To accommodate all these applications the heads were designed with different combustion chambers, valve sizes, and port dimensions. They also produced heads in cast iron or in aluminum to achieve different power output and performance.

Chevrolet Big Block Heads Combustion chamber types

All early Chevy Big-Blocks used closed chambered heads. In 1969 Chevy changed to the more efficient open chamber design.

Closed Chamber Head Combustion Chamber Design

Closed Chamber.

The Closed Chamber heads have a smaller combustion chamber. The Chamber is flat and only has a recess cast around the Valves and the spark plug hole. The flat part creates a quench and squish area in between the piston and the head as the piston reaches top dead center. This helps create turbulence pushing the air-fuel mixture to the spark plug.

These heads have a lower combustion chamber volume usually below 100c. Engines with Closed Chamber heads usually use flat top pistons and give a higher compression ratio. However, the compression ratio can be reduced by using a dished piston. They’re not as efficient for emissions purposes as Open Chambers Heads. However, when set up properly Engines with Closed Chamber Heads can provide great low and midrange power.

Open Chamber Head Combustion Chamber Design

Open Chamber.

The Big Block Chevy Open Chamber Heads have a larger combustion chamber compared to the Closed Chamber design. These types of heads have a round pocket cast int to the head just about the size of the cylinder diameter. This design moves the combustion chamber further up into the head and helped increase airflow at higher RPM.

Chevrolet change to the Open Chamber head design in the late 60s to improve combustion for emission purposes and also lower compression ratios. However, a domed piston can be used to increase the compression with this type of head.

Most engine builders prefer using open chamber heads due to the increased flow at higher RPM. Most aftermarket performance heads use this design.

Chevrolet Big Block Heads Intake Port Types.

All Chevy Big Block Heads came in 3 different intake port configurations. The Large Oval Port, the rectangular port, or the Small Oval Port aka Peanut Port. Early engines used either the large oval port or the rectangular ports heads. The oval port heads were used for the standard output engines in cars, trucks, and boats. The rectangular ports heads were used for high-output performance engines.

In the 90s on the redesigned Gen V Big Blocks, a small oval Port(Peanut Port) was introduced for use on Truck applications. These heads produce more torque at lower RPMs.

Big Block Chevrolet Intake Port Types

Chevrolet Big Block Heads With Rectangular Intake Ports

Rectangular Port.

The Chevy Big Block’s Performance Heads have very large Intake runners with rectangular-shaped ports. The intake ports are just about 2 ¾” tall and 1 ¾” wide. Chevy only installed these heads on their high-output performance engines. The heads were used on Corvettes, Chevelles, and other special applications.

These are the heads you want when building an all-out race engine that constantly runs at wide-open throttle. The large runners flow best from 4000 rpm to redline. However, these larger ports don’t generate good air velocity below 4000 rpm and don’t perform as well as the large oval port heads at lower rpm.

Chevrolet Big Block Heads With Oval Intake Ports

Large Oval Ports.

The large oval port type is the most common Chevy Big Block Head. These heads have large oval-shaped ports that are about 2” tall and 1 ¾” wide. These heads work well across the full rpm range. The port is ½ shorter than the rectangular port which helps generate air velocity at lower rpm. With some porting work and larger valves, these are the heads that may be the best choice for a high-performance street engine.

Chevrolet Big Block Heads With Small Oval Intake Ports

Small Oval Ports(Peanut Port) Heads.

The Peanut Port heads were introduced on the Gen V big blocks and were designed for truck applications. The ports are 1.56” tall and 1.44” wide. The smaller runner was designed to increased flow at lower rpm to help increase low rpm power and torque.

These heads are not recommended for high revving performance applications. However, they can work well for low revving high torque applications such as pick-ups, tow vehicles, or rock crawlers.

Choosing a performance head for your Chevrolet Big Block.

If you are building a Big Block and need to purchase heads it’s important to talk with your engine builder to come up with the best possible combination for your build. There are many aftermarket heads to choose from. However, if you want to use OE heads the large oval port types are easier to find due to the sizeable production run of the Chevy big-block. The high-performance rectangular port and factory aluminum heads are harder to find and can get costly to purchase.

If your building a full race engine for the drag strip or oval track which will be always be running at high RPM the rectangular heads may be what you are looking for. However, with some porting work and larger valves, the large oval port can perform just as well as the rectangular port heads. Actually, some bulders now prefer the oval over the rectangular port.

There has been many dyno and flow test done over the years to compare the 2 types of heads. The consensus is that with proper porting the oval port heads can flow just a well at above 4000 RPM as the rectangular without losing the middle and lower-end performance.

Building a road course or street and strip engine then the oval port may be the way to go. Remember the old saying that bigger is not always better. The oval port will provide great power and torque and have the good drivability that is needed for these applications.

Casting Numbers to look For.

Oval Port

Casting No. 336781 Mark IV, oval port, open chamber with 120cc.

Casting No. 353049 Mark IV, oval port, open chamber with 120cc

Casting No. 3964291 Mark IV, LS6. rectangular port, open chamber with 108cc

Casting No. 3931063 Mark IV, Oval port, open chamber with 100cc

Casting No 3964290 Mark IV, Oval port, open chamber with 100cc

Rectangular Port

Casting No 3919840 Mark IV, rectangular port, closed chamber with 107cc

Casting No 6272990 Mark IV, rectangular port, open chamber with 118cc

Casting No 12562934 Gen V, rectangular port, open chamber with 118cc

Casting No 12562932 Gen V, rectangular port, open chamber with 118cc

Aluminum Heads

Casting No 12363391 Mark IV, aluminum, oval port, open chamber 110cc

Casting No 3946074 Mark IV, L88 Zl-1, aluminum, rectangular port, open chamber with 118cc

Casting No 12363401 Mark IV, aluminum, rectangular port, open chamber with 118cc

Casting No 14044861 Mark IV, aluminum, rectangular port, open chamber with 106cc

Chevrolet Big Block Head Casting Numbers

Casting numbers are found under the valve cover.

Casting No.	Year	Engine Size	CC	HP	Port Type	Chamber	Intake Valve	Exhaust Valve	Notes
330864	70-81	366/427/454			Oval	Open	2.06	1.72
330865	70-84	366/427/454			Oval	Open	2.06	1.72
330866	68-78	366/427				Open
330867	68-73	427				Open
336765	73-84	427/454			Oval	Open	2.06	1.72
336768	73-76	427			Oval	Open
336781	73-85	454	113		Oval	Open	2.06	1.72
343771	68-85	366/427/454			Oval	Open	2.06	1.72
343772	76-84	366/427/454			Oval	Open	2.06	1.72
343783	75-78	454			Oval	Open	2.06	1.72	Car or Truck
346236	75-87	454	113		Oval	Open	2.06	1.72
346238	75-87	454	113		Oval	Open	2.06	1.72
352625	70-76	454				Open
353049	70-73	402/454	113		Oval	Open	2.06	1.72
366765	73-84	427/454				Open
473328	68-85	366			Oval	Open	2.06	1.72
3732791	58	348
3758379	59-61	348
3759256	58	348
3767738	59-61	348
3781147	60-61	348							High Performance
3795586	61	409		360
3814690	62-64	409							High Performance
3819333	62-65	348/409
3830817	63-65	409		340
3837731	63	409							High Performance, Z11
3852583	65	409		400
3856206	65-66	396	98	325	Oval	Closed	2.06	1.72
3856208	65	396	109	375 425	Rectangular	Closed	2.19	1.88	Z-16 / L-37
3856213	66-70	366			Oval	Closed	2.06	1.72	Truck
3856260	68	396			Oval	Closed
3872702	66	396/427	98	325 / 360	Oval		2.06	1.72	L-34, L-35, L-36
3873858	66-67	396/427	109	375	Rectangular		2.19	1.88	L-78, L-72
3876875	66-70	427			Oval	Closed	2.06	1.72	Truck
3904390	66-67	396/427	98	325 / 350	Oval		2.06	1.72	L-34, L-35, L-36
3904391	65-67	396/427	107	375	Rectangular		2.19	1.88	L-78, L-71, L-72
3904392	67	427	107	430, 435	Rectangular	Closed	2.19	1.88	Aluminum L-88, L-89
3904393	66-68	366			Oval	Closed	2.06	1.72	Truck
3908952	67	427			Oval	Open
3909802	66-67	396/427	100	325 / 350	Oval		2.06	1.72	L-34, L-35, L-36, L-68
3917215	67-69	396/427	100	325 / 350	Oval		2.06	1.72	L-34, L-35
3917219	66-70	366/427			Oval	Closed	2.06	1.72	Truck
3919840	67-69	396/427	107	375	Rectangular	Closed	2.19	1.88	L-78, L-71, L-72
3919842	68-69	396/427	106	375	Rectangular	Closed	2.19	1.88	L-88, L-89
3931063	69-70	396/427	100	325	Oval	Open	2.06	1.72	L-34, L-35
3933148	69	366/396/427	122		Oval	Open	2.06	1.72	Car or Truck
3933149	69-70	427	122		Oval	Open	2.06	1.72	Truck
3935401	68-76	427			Oval	Open
3946074	69-71	396/402/454	118	375	Rectangular		2.19	1.88	Aluminum L-88, ZL-1
3964280	66-70	427/454		335, 385, 390,400	Oval	Closed	2.06	1.72
3964290	69-70	396/402/427/454	100	325, 330, 350	Oval	Open	2.06	1.72
3964291	69-70	396/402/427/454	108	375	Rectangular	Open	2.19	1.88	L-78, L-71, L-72, LS-6 , 325cc Intake Port
3964292	70	454	109	450	Rectangular	Closed	2.19	1.88	LS-6
3965198	69	396			Oval	Open	2.06	1.72	Truck
3975950	68-70	366/427			Oval	Closed	2.06	1.72	Truck
3986133	70-85	366/427			Oval	Closed	2.06	1.72	Truck
3986136	70-84	366/427			Oval	Closed	2.06	1.72	Truck
3993820	71	402/454	113	300, 330	Oval		2.06	1.72	LS-3
3994025	71	454	118	425	Rectangular	Open	2.19	1.88	LS-6
3994026	71	454	118	425	Rectangular	Closed	2.19	1.88	LS-6
3999241	72	402/454	113	240, 300, 330	Oval		2.06	1.72	LS-3
6258723	71	454	118	425	Rectangular	Open	2.19	1.88	LS-6
6272292	71	402/454			Oval	Open	2.06	1.72	LS-3
6272990	71-94	454	118	425	Rectangular	Open	2.19	1.88	325cc Intake Port
10045427			91						Aluminum
10049875									Aluminum
10051128	90-94	454	72		Rectangular	Semi-Open	2.19	1.88	400cc Intake Port
10052902		427				Open
10101136	91-94	454			Oval	Open	2.06	1.72	Truck
10101140	91-01	366/427				Closed
10114156	91-01	454				Open
10121033	78-88	454			Oval	Open	2.06	1.72	Truck
10141279	96-01	454	100						Vortec 7400 100cc
10487052	77-90	366			Oval	Open
12352783	79-93	454			Oval	Open	2.06	1.72	Truck
12363391	Mark IV		110		Oval	Open			Aluminum Performance
12363401	Mark IV		118		Rectangular	Open			Aluminum, Bowtie, Performance
12558162	01 & UP	496							Vortec 8100 8.1L Truck
12560241	98-01	454	100						Vortec 7400
12562932	91-95	454/502	118		Oval
12562934	00 & UP	502							Marine, GenVI
14011077	89-94	454	118		Rectangular	Open	2.19	1.88	Aluminum, 295cc Intake Port
14025175	83-93	454			Oval	Open	2.06	1.72	Truck
14044861	89-94	454	106	425	Rectangular	Open	2.19	1.88	Aluminum, 380cc Intake Port, bowtie, W port
14081044	86-90	454			Oval	Open	2.06	1.72	Truck
14081045	78-87	454				Open
14081052	85-87	366/427/454				Open
14092359	86-89	427			Oval	Open	2.06	1.72	Truck
14092360	85-89	454			Oval	Open	2.06	1.72	Truck
14096188	87-94	454	118	425	Rectangular	Open	2.19	1.88	325cc Intake Port
14097088	89-94	502	118		Rectangular	Open	2.19	1.88	325cc Intake Port
14101398	86-90	454			Oval	Open	2.06	1.72	Truck