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Engine Boring and Engine Stroking Fundamentals

By , Photography by , Illustration by Patti Paulk, Steve Amos, Crane Cams Inc.

There are only two ways to increase an engine's displacement: You can bore it (engine boring increases the cylinder diameters) or you can stroke it (engine stroking increases the crankshaft stroke).

Engine Stroking offers the potential for significantly larger displacement increases than those obtained from typical engine boring, but it also requires greater sophistication when selecting and integrating components. Like you, we want to find out how to get a good stroke (as opposed to getting stroked), so we consulted three of the most experienced crankmen in the country-the legendary Hank "The Crank" Bechtloff, his son Scott, and his brother Allan. While Hank and Scott are still active trick crank grinders at HTC Products, today Allan specs Winston Cup cam applications for Crane.

The info in this story can be applied to get more cubic inches from any brand of engine.

Engine Stroking & Engine Boring: What Do You Gain?
Just how much is a good stroke worth? Using the standard displacement formula...

Displacement = bore2 x 0.7854 x stroke x (No. of cyls.)

...we see that on a Chevy 454 V-8 (4.25-inch bore x 4.0-inch stroke), a 0.060-in overbore (4.31-inch final bore size) yields a 466.9ci engine, but keeping the stock 4.25-inch bore size and increasing the stroke by 1/4 inch to 4.25 inches results in 482.3 ci. (If the engine was both bored 0.060 and stroked by 1/4 inch you'd get 496 ci).

Engine Stroking & Engine Boring: Component Stack-Up
Rod length or piston compression height (piston-pin location) do not affect the bore size or stroke length, and hence do not alter displacement. However, rod length and/or piston-pin location changes may be required after a change in stroke to properly locate the piston top in the block for a desired deck height clearance at Top Dead Center (TDC).

To determine the total height (H) of a given reciprocating assembly:

H = Piston compression height (pinhole centerline to deck) + Piston deck height (deck to top of block) + connecting rod center-to-center length + 1/2 Stroke

You only use half the stroke because the crank spins in an arc, and half of the increase in total piston travel occurs at the bottom of the cylinder. The required change in the combined connecting rod center-to-center length, piston compression height, and piston deck height is inversely proportional to one-half of any change in stroke. You can make up the difference by changing any combination of the three variables (rod length, piston compression height, or deck height) the required amount to maintain dimension "H"-whichever is cheapest or easiest for the application.

For a practical example of factors that must be juggled when stroking an engine, suppose a Ford engine builder installs a 3-inch-stroke 302 crank in place of his stock 289 engine's 2.87-inch stroke crank. He wants to maintain the same 8.206-inch overall assembly height, 0.016-inch deck height, and 1.60-inch piston compression height, changing only the rod length (R). What length rods are needed?

8.206 = 1.60 + 0.016 + R + (1/2 x 3)
8.206 = 3.116 + R
8.206 - 3.116 = (3.116 - 3.116) + R

It so happens this is exactly what Ford does on standard (non-Boss) 302 engines-use shorter 5.09 rods in place of the normal 289's 5.155-inch rods.

Even though the same piston and deck height is retained, the overall static compression ratio goes up due to the increase in swept volume that is compressed to the same preexisting clearance volume at TDC. That's also why even though a short-rod (5.565-inch center-to-center length) Chevy 383 small-block can use 350 pistons, the compression ratio ends up higher compared to the ratio obtained with the same piston and 5.7-inch rod in a standard 350.

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