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Text and Illustrations by Jon Mikelonis

More displacement and greater leverage means more torque. This concept is obvious when you compare the torque ratings between factory small block motors and factory big block motors. However, nowadays it is not necessary to suffer the time and switching costs of leaping to a larger block if you are only after more displacement. Displacement is just a factor of bore and stroke, by increasing the stroke of your current motor
you can enjoy the satisfaction of more torque disguised in the same package.

Formula A. Displacement. Simply a factor of bore and stroke. Increase the stroke of your current motor and reap the benefits of more torque.
Widespread awareness of the facts above and an abundance of aftermarket stroker kits have made the stroker option extremely popular. If you are out for performance, a stroker is a wise alternative to building a motor that only meets the factory displacement. Whether you have already built a stroker motor or are simply researching them, take a little time to learn the basics and understand the benefits and possible compromises of the now popular engine building practice.

Stroker Motor (def.)
A motor that has greater than stock displacement due to an increase in the factory crank throw. An increase in crank throw increases stroke (the difference between the piston's top dead center and bottom dead center position).

The illustrations below show the difference between a stock and a stroked rotating assembly. Study the differences and you can see what makes up a typical stroker motor. Though a bit exaggerated for effect, the stroked cross section in Figure 2 incorporates:

Increased Crank Throw (distance between C and D)
Increased Rod Length (distance between B and C)
Decreased Piston Compression Height (distance between A and B)

Keep in mind that rod length does not affect the displacement of the engine, it is common to have a stroker motor that uses an increased crank throw, decreased piston compression height, and stock rod length to achieve additional stroke. We'll discuss why longer rods are often used in stroker motors later in the article.

Figure 1. Stock Cylinder
Figure 2. Stroked Cylinder

The animation below helps visualize the effect of increased stroke and rod length on piston travel and speed.

Figure 1. Stock Cylinder
Figure 2. Stroked Cylinder


Stroker Evolution
Stroker engines are nothing new, and in fact they are not even an aftermarket invention. If you look closely at factory engine offerings, you'll see that changes in displacement are often nothing more than a change in stroke. This was a cost effective way for the factory to increase power for larger vehicles, or future models, while reusing the same block and accessory components.

Figure 3. Offset Grinding. The rod journal is offset ground to move the centerline of the rod journal further from the centerline of the main journal. Result is increased stroke.
Performance enthusiasts then caught on and they found that creative machining and parts matching could yield more cubes while hidden in the stock block to fool fellow racers.

One of the methods used to increase stroke with a stock crank, is called offset grinding. By offset grinding the rod journal you move the centerline of the rod journal away from or toward the centerline of the main journal. This will result in increased or decreased stroke. Figure 3 above illustrates the case we are interested in, the rod journal is ground in a manner to increase stroke. Keep in mind that when the rod journal is offset ground it now has a smaller diameter. The motor will require special connecting rods with correctly sized bearing bores. Additionally, if the rod journal is ground too much it becomes weak. Unless you add material and regrind, you can only stroke a motor so far with a stock crank.

Due to a demand for more stroke than offest grinding a stock crank could achieve, many aftermarket companies developed specialized cast and forged cranks with relocated rod journals. The specialized stroker crank has dramatically increased the amount of stroke you can add to your stock bottom end. Stroker cranks require a shorter piston to keep the factory sized piston from extending beyond the deck surface, it is also shortened to accommodate a longer rod. In the past the only way to complete a stroker motor was to find the right combination of rod lengths and piston heights. This often meant researching other factory motors for the right dimensions. It was not uncommon to have a Small Block Ford stroker motor consisting of Pinto rods and Chevy pistons.

Longer rods are often required to increase leverage and minimize the high degree of rod angularity created by the increase in stroke. The longer rod also prevents the piston from being pulled out the bottom of the cylinder bore. Rod Ratio and rod angularity are especially important issues to consider before simply choosing the stroker kit that yields the largest displacement for your application. We will discuss these topics in the following section.
(Rod Ratio Explained)
In This Article:
An illustrated review of the geometry behind the increasingly popular "stroker motor". This article covers the fundamentals and makes special note of the benefits, compromises, and limits of stroking your block.
Recommended References
How to Build Big-Inch Small Blocks” by George Reid, covers the theory behind increasing displacement via bore and stroke modifications and then dives right into stroker kit selection and assembly. The book has pictorial build-ups on 302, 351W and 351C based stroker engines. Reid also discusses proper cylinder head and camshaft considerations for the bigger displacement engines.

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