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Engine Boring and Engine Stroking Fundamentals

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

Offset Grinding
Our hypothetical Ford engine builder installed a longer-stroke crank and shorter rods from the same engine family. But the big displacement increases that stroking makes possible are more often accomplished using modified and/or nonproduction parts. There are several ways to change the stroke of an existing crank. Tradi-tionally, the simplest and easiest method is a process known as "offset grinding." Normally, when a rod journal is reground to compensate for wear, the machinist reduces its diameter to the next standard available undersize bearing while maintaining the journal's existing centerline location. But when a crankshaft is offset ground, the rod journal centerlines are moved farther away from the main-bearing-journal centerlines.

Rather than minimally offset grinding the journal down to the next standard bearing undersize (which yields only a small stroke change), performance offset grinding typically reduces the final journal diameter to a smaller size used by a different (but still commonly available) connecting rod. For example, when a 2.10-inch large-rod-journal small-block Chevy rod journal is offset-ground, the journal is commonly reduced to 2.00 inches, the size of a '55-'67 small-block Chevy rod journal. In theory, going from a 2.10 to a 2.0 journal should allow a 0.100-inch stroke increase if the throw is reground with the maximum possible offset (relocating its centerline by 0.050-inch). However, in the real world this much of an increase isn't practical: p (Pi, the math operator used to define the boundaries of a circle) is an infinite figure, so trying to exactly intersect the tangent point of two different diameter circles is impractical; there is tool-chatter to overcome; and usually you're starting with a used crank that requires normal undergrinding to make up for wear. For these reasons, crank grinders typically allow a 0.020-inch "safety factor"-so the real obtainable stroke increase from a 0.100-inch change in journal diameter is about 0.080-inch.

The small-block Chevy guys are fortunate that their favorite engine design has two different rod journal sizes available. That's not the case for many other engines. In fact, the vast majority of engines built with offset stroker cranks use rods from a different model engine or aftermarket specialty rods. This may require the use of pistons with a different than stock (for the original engine) pin diameter.

Engines with large rod journals (such as Buicks, Oldsmobiles, and Ford 351 Windsors) can achieve fairly decent stroke increases via offset grinding and nonstock rods. But in most cases, really radical stroke increases require other solutions. Welding is a traditional procedure for obtaining big stroke increases. This process involves adding material to the top side of the rod journal, then regrinding to the original rod journal size but with the journal centerline moved outward in relation to the main-bearing centerline, thereby increasing the stroke. Forged cranks are more suitable welding candidates than cast cranks, but in any case, traditional welding induces tremendous heat which adversely affects the crank's metallurgical strength. In recent years sophisticated submerged arc-welding and reheat-treating processes have been developed that address these issues, but the cost of a welded stroker has correspondingly increased to the point that for popular engines like the small-block Chevy, a custom-ground stroker crank made from a universal raw forging is actually cheaper in many cases. Still, welding remains a viable alternative on those off-brand engines for which universal raw forgings aren't available.

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