The New Math
Bruce Baxter; Missoula, MT: A fellow is trying to sell me a Gen I small-block Chevy that he says is 427 ci. Is that possible? Maybe our definitions of stroke are different. When I measure my 383, I get 3.75 inches stroke from top of piston at TDC to top of piston at BDC. His same measurements are 3.52 inches, so I told him he has a 357. He claims I should measure from the top of the piston at TDC to the wristpin at BDC. His math looks like this: a ‘69 350 block, 0.040-inch over, decked 0.100 inch, offset-ground GM crank with a 4.00-inch stroke, 6.00-inch V6 rods, flat-top pistons, and shaved double-hump heads. He says it is a 3.52-inch piston stroke, plus 0.100 decking, plus 0.250-inch to tall piston wristpin makes 3.87-inch stroke. How far can we safely deck a Gen I SBC block? Who is correct here?
Jeff Smith: I’ve heard of creative math before, but this is particularly innovative—flat out wrong, but innovative. The simple truth is that displacement is determined by measuring the bore of the cylinder along with the stroke, which is easily determined just as you said by measuring how far the piston travels from bottom dead center (BDC) to top dead center (TDC). All that other stuff about compression height has no bearing on displacement. We’ll even give you a simple formula for determining displacement in cubic inches: bore x bore x stroke x 0.7854 x number of cylinders. One way to remember that oddball 0.7854 number is that these four numerals are located in the upper lefthand quadrant of most numerical keypads. Start with the number 7 and work clockwise. Using the measurements of your buddy’s engine we get: 4.04 x 4.04 x 3.52 x 0.7854 x 8 = 360.9 ci or a 361ci small-block. It’s as simple as that. Everything else is just horse hockey.
It appears that he may have confused the calculation necessary to determine proper piston compression height when mixing and matching different crankshaft stroke and connecting rod lengths, since the definition of compression height is the distance from the wristpin centerline to the flat portion of the piston. This distance is used to determine on paper whether a given connecting rod length, stroke, and piston compression height will work in your engine. As an example, the engine in question has a 6-inch connecting rod length and a stroke of 3.52 inches. The formula for engine deck height is: rod length + piston compression height + 1⁄2 stroke = combined height. This can then be compared with the block’s actual deck height to make sure the piston does not protrude from the top of the deck (unless that’s what you desire) or that it falls way short of the actual block’s deck height. Let’s say the engine is assembled, and we don’t want to remove the piston to measure its compression height. The first thing to do is measure the deck height of the piston in the cylinder. Let’s say we measure this engine at a 0.015-inch height below the block deck surface, if we assume our block is at the standard small-block Chevy deck height of 9.025 inches. To get an idea of the compression height, subtract the rod length, half the stroke, and the amount the piston is short of the deck from the normal deck height: 9.025 – (6.0 + 1.76 + 0.015) = 1.25 inches. The one thing we have to assume here is the block deck height. It may not actually be 9.025 inches. The only way to know for sure is to completely disassemble the engine and then measure from the crankshaft centerline to the block deck. Usually too, you quickly learn that the block is not consistent and not square to the crank centerline, but we’ll save that discussion for a later date.
On a production small-block Chevy, I would hesitate to deck a block more than 0.020 to 0.025 inch. Typically, a production small-block Chevy will measure the piston-to-deck clearance at around 0.020 to 0.025 inch in the hole. So it’s quite common for machine shops to machine the block to an even 9.00 inches of deck height. You can take a block farther than that, but it begins to drastically reduce the block deck thickness, which will reduce its ability to seal combustion pressure with the head gasket. You can use the above math to determine if the deck has been machined, but I would be highly suspect of an engine that has been decked 0.100 inch if that is indeed an accurate number. A stock small-block deck thickness is only around 0.375 at best, so removing almost 1⁄8 inch would be excessive. That would remove almost a quarter of the deck thickness. This is clearly an issue because Dart builds all its small-block iron castings with 0.500-inch deck thicknesses, as deck strength is a critical component of sealing the head and block from cylinder pressure. If the block has been decked 0.100 inch, that’s a great reason to avoid this engine.
Dart Machinery; Troy, MI; 248/ 362-1188; DartHeads.com