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Engine Boring and Engine Stroking Fundamentals

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

Custom Cranks
Once reserved for professional racers, custom forged or billet stroker cranks are now increasingly common in higher-end street/strip cars. Universal "econo" raw forgings are available for the more popular engine families. Their quality and metallurgy are acceptable for most uses. Typically, a universal raw forging is made with an elliptical rod journal so almost any stroke can be ground into it. The drawback is that grinding an elliptical journal into a finished round journal interrupts the continuity of the forging's grain structure, effectively negating its supposed advantage over a billet crank. Assuming you can afford it, you big-arm boys may as well buy a custom billet crank.

Limiting Factors
So many possibilities, so little room. With all the stroker options these days, you'd think the sky's the limit when it comes to building giant engines. But real-world stroke increases are limited by the physical constraints of the cylinder block. We've already discussed the problems with reciprocating assembly stack-up, but there are also other clearance issues: Big stroker cranks may hit the oil pan rails, and the rails can sometimes be trimmed, but there's danger of breaking into an oil passage or water jacket. Clearance problems with the bottom of the cylinder bores or with the camshaft are also common.

There's also the problem of the overlap between the main and rod journals. To find overlap (O):

As stroke increases, the overlap in the areas of circles defined by the main and rod journal diameters decreases. Less overlap reduces crank strength and rigidity. The amount of acceptable overlap is determined by the strength of the crank material, the engine's power output, and its intended use.

When regrinding a finished crank into a stroker, care must be taken not to run into the internal oil passages. Attempts at welding the original passage shut and drilling a new passage usually prove unsuccessful; eventually the crank cracks in the fillet area.

Piston Problems
As we've seen, increasing the stroke and making no other changes usually causes the piston to stick out the top of the block. Shorter connecting rods are usually not the best solution; rather, pistons with raised pin heights help move the top of the piston back down below the deck. The pistons can be made shorter but only to a point-there must be room for the ring package above the piston pin. Even using thin rings (1/16 -1/16 -1/8 inch or metric equivalents), figure on a minimum total piston deck-to-pinhole top dimension of about 0.750-inch (more if a valve relief extends below the piston deck, as is the case on big-block Chevys). Various devices allow running the oil ring through the pinhole area, and there are even two-ring pistons; while acceptable for regularly torn-down race engines, these solutions aren't recommended for long-term street use. You could also go to a smaller diameter pin and bush the rod-but "a smaller stick breaks easier."

Also, the bottom of the piston must clear the crank throw at BDC. Counterweight height (C) is determined primarily by the rod length for a given amount of stroke:

C = Rod center to center length

-Piston thickness below the wrist pin hole -Clearance value (usually 0.100)

Rod journal dia. + Main jouirnal dia. - Crank stroke
Stroke + Main journal dia. + Wrist pin dia.
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