1972 350 Chevrolet Pickup Engine - Tech - Build Your First Engine

A close look at the piston, rings, and cylinder bore didn't reveal any failures, so we turned our attention to the valveguide clearance. Sure enough, the No. 8 exhaust valve rattled in its guide like a twig in a pipe, showing 0.010 inch of stem clearance. That's five times the proper 0.002-inch clearance we measured on the other 15 valves. Then we noted that the vintage cast-aluminum valve covers were nonvented and had no provision for the PCV (positive ventilation crankcase) valve. It looked like normal crankcase pressurization was forcing oil past the loose guide and making an oil-leak path into the No. 8 cylinder-oil fouled the spark plug every few months. Stop and explore any instance of isolated wear during initial teardown using the principles of root-cause analysis. You may discover the tip of an iceberg or a simple problem. And naturally, always use a PCV valve to vent the crankcase.

With the oil-fouling mystery solved, the teardown continued. The cylinder bores measured 4.030 diameter, 0.030 over the standard 350 bore diameter of 4.00. There was minor wear, with no ring ridge present, and hone marks from the last rebuild were still visible. We may be able to get away with a simple hone job to restore the bore finish. If you find even the slightest ridge, the block should be overbored to correct the wear and allow positive ring seal and piston fit. On most 350s you can safely go 0.060 inch over. Anything more is a gamble unless you sonic-check the block and confirm a wall thickness of at least 0.180 inch on the major thrust side of all cylinders after the block has been bored to its finished size.

Even though we'd be replacing the pistons, we could learn more about the condition of the block by examining the existing piston-to-wall clearance. With its cast-aluminum pistons inverted and inserted (minus rings), we measured 0.0015 inch skirt clearance. This is well within spec and helped us determine that the bores were round and the engine hadn't experienced massive detonation, which can collapse the outwardly tapered skirts and increase the measured clearance. Keep in mind that cast, hypereutectic, and forged pistons all have different clearance settings, from 0.0015 to 0.015 inch. This was still the inspection stage, and our main concern was to see consistent readings from cylinder to cylinder.

Never rush the disassembly process. Before removing each piston and rod assembly, make sure they're correctly marked for position. The pointer highlights the numbers typically punched into the rods and caps at the factory. These critical markings prevent mixing rods and caps and are not always present, so be sure to add them (gently) before disassembly. If you don't have a punch set, get one. A second choice is to clean the rods and apply paint markings. Never cut, grind, or scratch notches into the rods to mark them. These surface blemishes become failure points in operation.

When removing the pistons, rotate the crankshaft away to ensure the rod bolts don't nick the rod journals. Never strike the rod bolts or any part of the rod during removal. Instead, use the long wooden handle of a hammer to tap the bottoms of the pistons until they exit the deck and pop out. Be sure to have hands ready to catch them to prevent broken pistons. The wooden handle can contact the machined crank journal without doing harm. Inspect the rod bearings for wear, discoloration, and size stampings on the backs of the shells. Worn bearings are normal, but any grooves in the crank journals you can catch a fingernail on must be polished or machined out.

Our harmonic damper exhibited signs of having been brutalized during installation. Notice the hammer-blow marks on the inner hub. The energy flow path of such force is the crankshaft thrust flange and the thrust bearing at the No. 5 position in the block.

The crankshaft endplay helps determine if the harsh damper installation has harmed the crank or block. The 0.018-inch reading is outside the preferred 0.005-0.010 range but passable. Many shops will junk crankshafts with higher endplay readings because weld-repairing and machining the flange typically costs more than a replacement crank. Always use a damper installation tool during assembly and a damper hub-pulling tool during disassembly. Hammers are for cavemen.

With the crankshaft removed, we inspected the main bearings and block saddles for obvious signs of damage: deep grooves, scratches, or discoloration. The nasty, dark region shown here on a 454 big-block is clear evidence of a spun main bearing. Badly discolored main caps are always tossed and replaced. While block damage like this can be corrected, severe localized heat cycling tends to breed cracks in the saddles and webs. Anything can be fixed, but it gets costly. So unless it's the original block to Duntov's personal '57 Corvette fuelie, think about a replacement. Fortunately, our 350 was shiny and happy in this department.

With the visual inspection finished, the block needed to pass some physical tests before a dime was invested in machine work. The first is a pressure test. JMS' Freddy Hernandez sealed the coolant passages and pumped compressed air-30-psi, nearly twice the typical pressure of hot coolant-into each bank to check for leaks. He then squirted soapy water on the block and head to identify escaping air bubbles...and potential cracks. The key areas for inspection are the lifter valley, bases of bores, and external surfaces of the block and heads. Note that each head is torqued to the block with a gasket during testing to duplicate the stress patterns of normal operation. Nontorqued (hand-tightened) fasteners may not open and reveal existing cracks, especially in the cylinder heads. Validate parts in their as-run state whenever possible.