Crankshaft Stroke

For most automotive applications, the stroke of a crankshaft is not necessarily a critical dimension. Most engines produced by General Motors, Ford and Chrysler are assembled with the piston well beneath the deck of the engine block. However, high performance engines typically run their tolerances much tighter and often calculate pistons to reach zero deck for added compression. Because of this, it is important to understand what a crankshaft's stroke is prior to ordering connecting rods, pistons and before any machining is performed on the deck of the block. Let's take a closer look at what the crankshaft stroke actually is and how you can determine the stroke of your crankshaft.

The stroke of the crankshaft is the total distance that the connecting rod journal travels up and down. Without special measuring tools, it is difficult for an individual to precisely ascertain what the stroke of their crankshaft is. However, if you are good at reading a steel rule then you should be able to get a rough idea of your crankshaft's stroke. Pictured below, you will see an image of me measuring the stroke of a crankshaft using only a steel rule.

Crankshaft Stroke Measurement

In the illustration above, you will see that I am measuring the distance between the centerline of the crankshaft main and rod journals. By multiplying this distance (1 5/8") by two, I determined that this crankshaft has a stroke of roughly 3.250". Since the crankshaft used for this demonstration is an aftermarket component, I was able to reverse the casting number to a Scat 3.250 small block Ford stroker crankshaft.

As noted previously, the stroke of the crankshaft will ultimately determine the piston height. Because this is a critical dimension, using a steel rule will not provide us with a definitive measurement that can be relied upon. Most automotive machine shops have a crankshaft stroke gauge that will provide a stroke measurement with an accuracy of .001". The stroke of a crankshaft can also be determined, with an even higher rate of accuracy, by chucking it up in a crankshaft grinding machine. The advantage of using a crankshaft grinding machine to determine the stroke of the crankshaft is that the rod journals may also be checked for a proper index (90° for this crankshaft).

Stroker engines, as they are often referred to, utilize a crankshaft that has a larger stroke then what was originally in the engine. New forgings can be made for stroker engines while welding or offset grinding OEM crankshafts may be performed to accomplish this same goal. Generally speaking, longer strokes will change the geometry of the rotating assembly but normally enables engine builders the ability to build higher compression engines that produce more horsepower.


 
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