this intake manifold gasket on Gen II motors. This gasket has a heat riser passage. Using an intake manifold gasket with a blocked heat riser passage can help keep the temperatures in the intake manifold cooler; however, blocking the heat riser passage makes cold morning operation more difficult.
The PN 19301685 Intake Gasket is designed for the Chevrolet Performance Fast Burn Aluminum Vortec heads (PN 12558060, PN 19300955, and PN 19300956) only. The gaskets are 0.120-inch thick and have both the early six-bolt pattern and the Vortec four-bolt pattern.
For production Vortec heads, use gasket PN 89017465 with four bolt attachments to the heads. This installation requires attaching bolt PN 12550027 because the bolt has a ball design on the end that sits in the head so it will not crush the intake manifold gasket.
Many aftermarket gaskets are available with larger intake port openings, but unless you’ve port matched your intake and heads for all-out performance, stick with the original sizes.
Chevrolet used a variety of carbs and intakes over the years, which means there are a lot of different gaskets. On many of the intake manifolds made in the 1960s, a heat riser channel was cut across the carb mounting deck, which required a traditional gasket and a metal gasket. Newer-model gaskets do not seal these. In short, be aware of the year of the intake you are using to get the correct carburetor gasket.
If you plan to swap an LT1 (Gen II) engine to accept a carburetor and distributor, use intake gasket PN 12524653.
PN 12528884 is made for Gen II 350 LT4 raised intake port heads. There is no coolant passage in the gasket because these are reverse-flow heads. The manifold bolt holes have also been relocated (used on engine PN 12371172).
Chevrolet Performance offers other unique intake gaskets for use with different performance and racing cylinder heads. PN 10185042 is required with splayed valve heads (PN 24502517). For 18-degree high port heads (PN 10134363 and PN 24502580), use gasket kit PN 10185007.
Aftermarket gasket makers also can supply intake manifold gaskets with larger port openings for racers who are using heads with large port opening dimensions. Be sensible here. Don’t buy the largest intake port gasket you can get, thinking that if you open the port opening to the gasket size, you’ll improve the performance of the engine.
If you are building a street engine, the only thing you need to remember is to make sure that the intake manifold port opening and head port opening match and that there is no ledge formed by the gasket or the head to impede flow. If you are building a race motor and you have substantially improved the flow in the port runners and increased the size of the head port opening, think about increasing the gasket size opening to match the port sizes.
Carb Gaskets
Over four decades of small-block production, a number of different carburetors have been used, and all these carburetors used different-style gaskets between the carb base and the top surface of the intake manifold.
Heat riser passages were built into some intake manifolds to feed warm air to the base of the carb to warm it up when first started on cold mornings. The carb gaskets must match the heat riser passages in these manifolds. You can check the intake manifold casting date to help determine which carburetor and carb gasket are correct for your application. The intake may require the use of a Rochester 2-barrel, Carter WCFB or AFB, Rochester Four Jet, Rochester Quadrajet, or Holley 4-barrel. One of the more common misapplications I have seen is the use of a later-style Quadrajet carb gasket on a 1969 or earlier Quadrajet intake, which has cast-in heat riser passages and a channel that runs across the front side of the carb mounting surface. This channel requires a particular carb gasket to seal it. If a later-style Quadrajet gasket is used on the 1969 and earlier intakes, the result is an air leak, because the later gasket doesn’t seal the intake surface and the heat riser channel completely.
Chevrolet used to offer two types of carb heat shields that fit between the intake manifold and the base of the carb and incorporate a carb gasket into the heat shield. PN 3969835 fit Holley square-flange 4-barrels and PN 3969837 fit Rochester Quadrajet spreadbore carbs on 1970-on intakes. These heat shields can reduce heat absorption by carb fuel bowls and can cure some fuel percolation problems. Aftermarket models are available.
Head Gaskets
Most production engines that have cast-iron heads were built using factory steel shim head gaskets. Engines with aluminum heads always received a composition-style head gasket. Composition head gaskets can be used with iron heads, but never use a steel shim gasket with aluminum heads.
Head gaskets are made in various compressed thicknesses and bore sizes. Altering the thickness of the gasket affects the measured static compression ratio of the engine. It also affects the clearance between the open valves and pistons and between the piston deck and the head deck. You need a minimum of 0.035- to 0.040-inch clearance between the piston deck and the head quench area deck with steel rods. When using aluminum rods, your engine may need more piston-deck-to-head-deck clearance to accommodate aluminum rod stretch.
Bolts, Studs, and Torquing Tips
Unless you’re building a turbocharged or supercharged engine that sees extended periods of severe service in a race car or a boat, there is little reason to use cylinder head studs. Studs are needed to ensure even gasket crush and promote proper sealing in engines with very high cylinder pressures. The brake mean effective pressure (BMEP) in the average street engine rarely approaches the point where gasket integrity is threatened. Blown head gaskets are normally the result of improperly torqued bolts, detonation, or improperly faced surfaces on the head or block.
For street and bracket racing applications, it is usually safe to use stock Chevy head bolts. Factory fasteners have the proper elasticity to provide effective clamping at the recommended torque specs. Aftermarket bolts—such as those available from ARP or Milodon, and others—perform very well.
If you angle-cut the heads to gain compression, be certain to have the machinist spot face all the head bolt seats to make them parallel to the deck surface. Remember also that the bosses along the outside of the head for the short bolts can get very thin after angle milling. It is possible to break or crack the head in this area if care is not exercised when torquing the bolts.
Another good move is to install the head and gasket on the bare block and observe their relationship through the empty cylinders. In this manner, you can determine if any part of the gasket or combustion chamber is overhanging the bore. This problem can be remedied by using the correct head gasket for your bore size and by lightly blending the edges of the combustion chambers to match the bore size. ■
Steel shim or copper gaskets must be coated with a head gasket sealer before installation.
The head gaskets for a 400 small-block have six additional steam holes between the cylinder barrels to help keep these siamesed blocks cool.
Some gaskets do not require hot re-torquing. However, it is not a bad idea to do a hot re-torquing
