We Build a Tri-Power 427 for Today’s Driver

Rebuilding the Heritage

Andy Bolig Dec 1, 2001 0 Comment(s)

Step By Step

The casting numbers (361959) tell us that the block we located for this story is a ’73-’85, 454ci block. On production 454 and 427 Mark IV blocks, some have added water passages in the decks depending on when they were made. Check the head gasket for proper fit when assembling.

The date code is located at the rear of the block opposite the casting numbers. Our date code reveals that this block was cast on April 4, 1975.

An indicator of a block’s use is the block stamping code. The assembly plant, date of installation, and how it was equipped in the vehicle are all stamped on the front surface of the block just below the passenger-side head. In this instance, the engine was assembled at the Tonawanda plant on February 19. It was initially installed in a passenger car in a 230hp configuration using a four-barrel carburetor and a Turbo-Hydramatic transmission.

The presence of the broach marks and the absence of any stamping code means this engine could have been purchased over the counter instead of having been installed in an automobile. This is confirmed by the fact that this engine was originally a known race engine.

Both the 427 and 454 had a standard bore of 4.250. They both can be bored .060-inch oversize safely, but you must make sure the bore isn’t already at the limit. If it needs to be honed to clean up the bore, you’ll be exceeding the safe bore size, and hot spots and cylinder-wall cracking could result. Purchasing machined parts should always be on a “buyer beware” basis. You can never check too many times to make sure the surfaces aren’t damaged beyond repair. Purchasing a complete, running engine from a well-known source (so it’s easily located if there’s a problem) is always the best way to purchase an engine. Some sellers simply disappear with your money, leaving you with a part that would require more to fix than what a guaranteed good part would have cost. When checking the block make sure the threads aren’t stripped, check the bearing surfaces to see if the engine had a spun bearing and, in cooler climates, check the freeze plugs to see if the engine has ever been frozen. Questions can have any number of predetermined answers, especially if the seller has had an entire weekend to find out which ones sound best. Taking along tools you know won’t lie is the best way to protect yourself. Inside-diameter micrometers and a dial caliper are the best ways to check the bores for wear or excessive boring. Check both at the top of the bore and midway down the bore.

Chevy made big-block crankshafts in two different strokes. The 396, 402, and the 427 all use a stroke of 3.76 inches. These are internally balanced crankshafts. The 454 uses a 4-inch stroke and is externally balanced. All the main-bearing journal diameters for the Mark IV big-blocks are 2.7482 to 2.7492 inches, with the rear main-journal diameter at 2.7478 to 2.7488 inches. The rod bearing’s journal diameters measure 2.1988 to 2.1998 inches, making the crankshafts interchangeable. We’ll be using a crankshaft with a stroke of 3.76 inches to make our engine a 427 just like the original. There are two different types of big-block crankshafts—cast or forged. You can tell the difference by the parting line on the crankshaft throws. The cast crankshafts will have a thin parting line where the two halves were joined to make the complete crankshaft, while the forged crankshaft will have a wider parting line. The 427 Corvette crankshafts are all forged steel.

To carry over the race engine theme: We knew the engine had factory aluminum heads because of the Winters Foundry mark, but when we ran the numbers (3946074) we found out that these heads were the same ones used on the ’69-’71 L88, L89, and ZL-1 engines. They have a great history but they’re dated in their technology. Don’t look for these heads on our engine when we assemble it.

The pistons were still with the engine, and they were definitely performance-oriented. 3947886 turned out to be the part number for the 12.5:1 compression ratio ’69 427ci, ZL-1 pistons. Once again, interesting parts, but not what we were looking for with this engine.

When looking for a used crankshaft, always check every journal for wear or scarring. Many machine shops will stamp on the crankshaft the amount if the crankshaft has ever been machined down. Usually this is listed on the first throw of the crankshaft. If it’s more than .010 inch, look for another crankshaft. Once you feel you’ve found a good one, you’ll still need to have it checked by the machine shop to see if it’s bent, cracked, or running untrue.

Of course, there would have to be 3x2 carburetion on this engine. We located a ’68 435hp 427 intake, casting number 3919852. This is an early ’68 intake with the large square-port intake runners. The heads we use will have to match up with these ports.

Nothing conjures up images of power and heritage like a Tri-power 427. This package in a Corvette is responsible for much of the respect and admiration that keeps the prices of 427 Corvettes beyond the reach of many. This financial hurdle, coupled with the errant view that these engines aren’t capable of keeping their cool, have relegated these powerful engines to only those who can “put up with them.” With the help of Chris Petris from the Corvette Clinic, we’ll take on the task of building a modern-day, Tri-power 427 that will keep its cool and have as much power on tap as when the General was building them. We’re assembling this engine for a driver, so the exact part and casting numbers will not be an issue. This will be a multi-part story, from the acquisition of the hard parts and what to look for, to finishing up with the compilation of the parts.

The General referred to the big-block engines built between 1965 and 1990 as the Mark IV (Four) family of engines. They were replaced in 1991 by the new generation of big-blocks titled Mark Five or GEN V. The earlier Mark IV engine blocks were offered in two- or four-bolt main caps, while the GEN V all have four-bolt mains. Interchangeability of the front four-main caps is possible if they’re align-bored, but the rear cap will not interchange because the GEN V uses a one-piece rear-main seal instead of the two-piece used in the Mark IV. We’ll be dealing only with the Mark IV engines in this story. Chris has been acquiring parts for this engine for some time and we wanted to show our readers what to look for when undertaking such a task.

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