Big block Chevy heads

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by: Cobalt327, Jon, Mortec
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Contents

[edit] Chambers

BBC heads have a few different chambers. But usually what you'll hear is "open" or "closed" chambers. In the image below, the closed chamber heads are on top, open chamber heads below.

There are also "semi-closed" chamber heads like the c/n 215 heads. They are shaped more like an open chamber head, but with less volume. These heads are a decent choice for a production head especially on a smaller displacement engine (up to 454 cid). Casting number 820 heads are similar to the 215 semi-closed head, but with less quench pad area.

Closed chamber top, open chamber bottom
Semi-closed chamber, c/n 215 shown
C/n 820, note how the exhaust valve has less shrouding than the other chambers
Closed chamber,c/n 702 shown


[edit] Exhaust valve guides

Warning Note: The following is an excerpt from an AERA technical bulletin:

Exhaust Valve Guide Caution On 1965-98 GM Big Block Engines

These engines' valve guides were not intended as a replacement item from GM. The guides were located into the head castings after the head was cast and the exhaust guide is "water cooled". The water-cooled guide also has two different size press fit diameters. Adding additional confusion to the matter, they also switched the location of the larger diameter sometime in 1985.

To determine which outside diameter guide you have and which direction to remove an exhaust valve guide, the following procedure is suggested:

Drive the guide no more than 0.250" (6.35 mm) toward the valve spring side of the head and stop. Then, measure the newly exposed area of the guide OD next to the spring pad.

If the measurement is 0.620" it is the first style guide. To remove this style guide, continue driving it toward the valve spring side of the head.

If the measurement is 0.616" , it is the second style guide. To remove this style guide, you must drive it the opposite direction toward the combustion chamber side of the head.

[edit] BBC L29 Vortec heads

ChevyBigBlockL-29vortec chamb.jpg 454 vortec head.jpg

[edit] Mark IV cylinder heads on later Mark V block

There are gaskets made specifically for this swap. Use other gaskets at you're own risk- these are what you want (from a V/R press release, presumably prior to the Gen 6 engine release):

General Motors 7.4L Head Gasket Issue:
General Motors (GM) 7.4L (454 CID) engines use two types of engine blocks: the Mark IV and Mark V. The Mark IV is found on 7.4L engines in model years from 1965 to 1990, and the Mark V is found on 7.4L engines in model years from 1991 and newer.

Often, installers will attempt to adapt a Mark IV cylinder head for a Mark V block. This conversion can be made if attention is paid to the coolant circulation. Mark IV and Mark V have different coolant flows and were originally designed for different head gaskets. If the conversion is not performed correctly, the engine will overheat, causing premature engine wear and damage.

Resolution:
Victor Reinz has designed two Nitroseal® head gaskets to specifically allow for this conversion. The installation requires Victor Reinz part number 4918 be installed on the right cylinder bank to maintain proper coolant circulation, and part number 4923 to be installed on the left cylinder bank for the correct coolant flow.

Application:
Victor Reinz part numbers 4918 (right bank) and 4923 (left bank) are available for GM 7.4L (454 CID) engines.

[edit] Spark plugs

During the BBC production, the spark plugs changed from gasketed to tapered seat. Some heads are almost identical, the only significant s=difference being the type of spark plug used, so be sure to inspect the head closely to be sure the right plug is used.

[edit] Torque sequence for big block aluminum heads

Bbc head bolt tightening sequence.jpg

[edit] Special considerations

Aluminum heads require parallel ground hardened steel washers under the bolt head or nut to prevent galling the aluminum seat. Some or all bolts/studs are often a different length than the bolts used on the stock cast iron head. If aftermarket hardware is used, follow the manufacturer's instructions for lubrication and torque specs. Otherwise, tighten the hardware in at least three steps. New hardware should have the threads burnished before final torque is applied (see manufacturer's instructions).

Use anti seize compound on all threaded aluminum bolt or sender holes, especially the spark plug threads. Only remove fasteners or senders when the head is cold.

[edit] Common BBC casting numbers

Casting No. Part No. Application
Year CID/HP RPO
Chamber
(CC)
Intake
(Inches)
Exhaust
(Inches)
336781 336780 '74 454/270 HP 113 2.06 1.72
353049 353047 '73 454/275 HP 113 2.06 1.72
3999241 3999240 '72 454/270 HP 105 2.06 1.72
3993820   '71 454/365 HP 113 2.06 1.72
3964291 3964287 '69 427/435 HP 118 2.19 1.88
3964290   '69 427/390 & 400 HP; '70 454/390 HP 106 2.06 1.72
3946074   '69 427/430 HP L88 & ZL1; '71 454 LS6 118 2.19 1.88
3931063   '69 427/390 & 400 HP 100 2.06 1.72
3919842   '68 427/430 & 435 HP L88 & L89; '69 427 L89 106 2.19 1.72
3919840   Late '67-'69 427/435 HP L71 106 2.19 1.72
3917215   '68 427/390 & 400 HP 100 2.06 1.72
3909802   Late '67 427/390 & 400 HP 100 2.06 1.72
3904392   '67 427/430 HP L88 & L89 106 2.19 1.72
3904391   '67 427/435 HP L71 106 2.19 1.72
3094390   Early '67 427/390 & 400 HP 98 2.06 1.72
3873858   '66 427/425 HP 108 2.19 1.72
3872702   '66 427/390 HP 98 2.06 1.72
3856208   '65 396/425 HP 108 2.19 1.72

[edit] BBC casting number resources

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