![]() Aftermarket studs usually get torqued to 60 ft-lbs, but check with the fastener manufacturer for specific torque recommendations. Use thread sealer on all bolts that go into the water jacket, which means all blocks except Bowtie and aftermarket. Stock head bolts get torqued to 70 ft-lbs in three steps of 40, 55, and 70 ft-lbs. Note that the tops of the ports are almost even with the bottom of the center bolt-hole.īig-block head bolt torque pattern. Stock iron rectangular ports from a “990” casting head. Note distance from the ports to the center bolt-hole above the pair of runners Stock iron oval ports from a “206” casting head. On engines approaching or exceeding the 1,000-hp mark (normally aspirated), the use of race-only spread port or Big Chief–style heads serves to even out the port volume, shape, and airflow in all eight intake ports. This minor idiosyncrasy of the big-block’s design has never been a real problem in terms of limiting the engine’s power potential, until extremely high airflow levels are reached on large-displacement or very-high-RPM engines. The port on the right side of each pair (as you face the intake flange surface) is longer and directs the inlet charge more to the center of the chamber, and is referred to as the “good” port the one of the left, obviously referred to as the “bad” port, dumps the air/fuel mix toward the cylinder wall and usually doesn’t flow as well as the right port. As a result of this arrangement, each siamesed pair of intake runners features non-symmetrical left-hand and right-hand ports. Rat motor heads feature a unique combination of siamesed intake ports mated with symmetrical combustion chambers and equally spaced exhaust ports. ![]() Late-model trucks feature an even smaller oval intake port, frequently referred to as the “peanut” port, on heads that appear to be nearly round at the port entrance. ![]() In the aftermarket, most performance heads are aluminum rectangular port, open-chamber designs reminiscent of the original LS6 casting, although you can also buy aftermarket oval port and cast-iron heads.įactory heads are classified as either high-performance (rectangular port) or standard passenger car (oval ports). This compound arrangement of valve angles gives the big-block head its characteristic “valves pointing everywhere” appearance when the valve covers are removed and is responsible for the early “porcupine head” nickname when the big-block made its debut in the mid 1960s.Īluminum aftermarket cylinder heads offer tremendous airflow potential to maximize power potential.īig-block Chevy factory heads are offered in aluminum and cast iron, with either open or closed combustion chambers, and with oval or rectangular intake ports. The exhaust is tilted 17 degrees relative to the deck surface, and both are inclined 4 degrees laterally. The original head design is often referred to as having a 26-degree valve angle, although this intake valve angle of inclination is only one of four angles needed to numerically describe the big-block’s valve angles. ![]() The big-block Chevy’s canted valve heads are largely responsible for the tremendous power capability of the Rat motor and its continued popularity.
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