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Chevy Small Block? Chevy V8 Specs & Options

Tech Q+A

As a "driveability tech" at the local Chevy dealer, Mr. Hassenboehler reprograms GM computers on the side as a hobby. For "off-road use only," he can "do anything from '86 to '95 chips," as well as reflash the chipless '94-'95 LT1 and LT4 computers. Those of you who need such work done in the New Orleans area might want to get in touch with him.

Crank's Got His Goat
I have a '69 400 Pontiac GTO with a four-speed. Recently, the crank I purchased and installed posed a problem. The boss for the new flywheel is only 2 1/2 inches in diameter. My old flywheel and crank used a 3-inch boss. What crank did I install? Is it a newer 400 or possibly incorrect? If I get the right flywheel, can I use it in my engine without any problems?
Eddie Maples
Saint Augustine, FL

There have been three different Pontiac V-8 crankshaft flywheel mounting-flange registers since 1961. A 2 1/2-inch-diameter boss was used on '61-'63 V-8 cranks, the '65-'75 had a 2 3/4-inch boss, and the '76-'79 boss measured 2 5/8-inches. Both 2 1/2- and 2 3/4-inch bosses were used in '64; the '76 455 retained the 2 3/4-inch boss while the other Pontiacs moved to the 2 5/8-inch boss. The Pontiac 326, 350, 389, and 400 share the same 3.75-inch stroke, so the smaller engines' cranks will fit the 400 subject to the following limitations.

The '66-and earlier 326 and 389 cranks require a corresponding harmonic balancer and front timing cover because the early crankshaft snout is about 1-inch shorter than later crank snouts. Very early cranks with the 2 1/2-inch flywheel boss are quite rare these days; using one in your '69 additionally requires either a corresponding original 2 1/2-inch crank-boss flywheel, a custom aftermarket flywheel, or the fabrication of a custom flywheel mounting flange spacer ring to interface with your existing '69 flywheel.

The '67 326 and '68-'75 350 cranks are a direct bolt-in (except for balance, see below), but you don't have one of these because of the flywheel flange differences. The crank definitely isn't from a 421, 428, or 455, either-regardless of any flywheel flange dimensional differences, these cranks had larger main journals, so they won't physically fit in the 400's main saddles.

Most likely you have a '76-'79 350 or 400 2 5/8-inch Boss crank. Using it in a '69 requires a corresponding late-model flywheel that is also compatible with your particular '69 clutch disc and pressure plate (Pontiac offered several different clutch options). Suitable components are available from GM or aftermarket sources.

Use of an early or late Pontiac crank, and/or a crank from a different displacement engine, usually requires rebalancing the crank because of varying stock piston weights. For example, a late Pontiac 350 crank is factory-balanced for 33-ounce heavier pistons than most '69 400 cranks were balanced for.

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