Engine Swap - Easy Big Block Swap

Installing the driver-side header was even trickier. After numerous failed attempts we realized that you must remove the driver-side motor mount bolt, slightly lift up on the engine with the hoist, slip the header in from below, lower the engine, and then install the gasket and bolts.

Once the engine is bolted to the motor mounts and the headers are in place, bolt the tranny to the back of the block. Then, from under the car, bolt the big-block's flexplate to the torque converter. Both the bellhousing and the flexplate use the same bolt pattern as the small-block, so that's no big deal. If you're running a manual tranny, you'll need to swap the 502's stock 12-inch flexplate for GM's 14-inch flywheel (PN 14096987).

The 502/502 comes with a specially calibrated HEI distributor. To avoid breakage, we removed the distributor during engine installation and reinstalled it once the engine was in place. Afterward, we installed the heavy-duty 8mm spark plug wires included with the 502 crate engine.

To retain original heater hose routing, we used OPG's big-block water pump nipple kit (PN WP21551) with heater hose nipples that thread into the intake manifold and water pump (arrows).

Big-block-equipped GM A-bodies used a slightly different (actually smaller) radiator fan shroud than small-block cars. To maintain proper fit and cooling, we installed OPG's reproduction big-block fan shroud (PN PZ01017).

Small- and big-block clutch fan assemblies differ in size and strength. Our stock small-block clutch measured 6x21/4 inches, while the new OPG big-block clutch (PN FC57) measured a larger 71/4 x21/2-inches. The stock fan, however, interchanges small- to big-block.

Rather than use universal flex-type radiator hoses, we installed new LS6/L78-type lower (PN 3959188) and upper (PN 3942453) molded radiator hoses. We cut off 1 inch of hose on the thermostat housing side of the upper hose for an improved fit.

The new Holley carb didn't like the Q-jet linkage brackets from our small-block, so we procured the proper Holley carb throttle bracket (PN KM02019) from OPG.

After the 502 was running, we needed to fab some pipework (arrow) to connect the Hedman headers to our existing Borla 3-inch exhaust. After illegally cruising open-headered to Borla and dishing out some serious schmoozing, Borla fabbed up the needed pipework.

We went to a local speed shop to buy a new Holley carb inlet fuel line (PN 34-160) only to find out that it was the wrong unit (arrow). Actually, we needed PN 34-150 to properly fit the dual-metering-block 850-cfm vacuum secondary Holley carb fitted to the 502/502. After scouring an auto parts store, we came up with a PCV valve that had a 90-degree bend (Auto Tune PN PT3771) making for a sano installation of our PCV breather system that tapped into the back of the Holley carb.

We filled the 502 big-block with fresh oil and coolant, rough-timed it, then cranked it over to bring it to life (the engine had been dyno tested and was already broken in). With the distributor vacuum advance disconnected, the initial timing was set to 12 degrees BTDC with a total advance of 36 degrees, then the carb floats and air idle bleeds were adjusted. Because the 502 was taller than the 355, we had to switch to a drop-base-type K&N air filter assembly (arrow) for hood clearance.

With its all-steel bodywork, six-point rollbar, 4L80E overdrive tranny, and a half-tank of fuel, our Chevelle (without driver) weighed in at 3,710 pounds with the 355 small-block. In the same condition, but with the aluminum-headed 502 big-block in place, our Chevelle's weight swelled to a porky 3,840 pounds and the nose dropped down about 1/2-inch.

We wheeled the car to the high desert merriment of L.A. County Raceway for some drag test action. Speaking of a drag, for our earlier cross-country roadtrip we replaced our drag-friendly 3,000-rpm stall converter with a cruise-friendly 1,400-rpm converter that killed us off the starting line. Regardless, with pump gas, full exhaust, and wearing a pair of Mickey Thompson E.T. Street 26x10.5-15 tires, the 502-equipped Chevelle hustled to a 12.41 at 111.41 mph. With a performance converter it would run 11s, dammit!