A Stroker kit is an aftermarket assembly that increases the displacement of a reciprocating engine by increasing the travel of the piston (that is, the piston moves farther up and/or down in the cylinder). This is done by using a different crankshaft where the crank pin is moved farther away from the center of the axis of rotation of the crankshaft. While this increases displacement and torque it can potentially lower the limit to which the motor can rev safely compared to the stock configuration.
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Differences from original equipment manufacturer (OEM) configurations
Two key characteristics differentiate stroker kits from OEM rotating assemblies.
First, and most important, the crankshaft's rod journals are physically farther from the main journals than on the original equipment. A given extension in the distance between the rod and main journals results in twice that distance in increase of piston travel. For example, in a Chrysler LA (1971-1992) 360 cubic inch (5.9 l) V8, the rod journal centerline is 1.79 in (4.55 cm) from the main journal centerline. The piston travels 3.58 in (9.1 cm) in this configuration. When this engine is modified with a 402 cubic inch (6.6 l) stroker kit, the distance between the main journal centerline and the rod journal centerline is increased to 2.00 in (5.08 cm), which results in the piston travelling 4.00 in (10.16 cm). So, the modified crankshaft with 0.21 in (0.53 cm) further distance between the main and rod journals results in 42 cubic inches (.7 ) of displacement greater than the engine had before.
The second key characteristic of a stroker kit versus OEM is the required changes to allow a stroked crank to 'fit'. This is accomplished in one of three ways:
▪ Location of the piston pin (and the top of the rod) within the piston, or the compression height is shortened.
▪ Length of the rod is shortened.
▪ Height of the cylinder is shortened.
Option One - Shorter Piston: The stock Chrysler LA engine in the example above has a main to rod bearing centerline distance of 1.79 in (4.55 cm), a rod that is 6.123 in (15.55 cm) long, and a piston that is 1.67 in from piston pin centerline to top, depending on the application. As the engine stroke is increased in the example above, the 402 cubic inch (6.6 l) configuration would place the piston 0.21 in (0.53 cm) higher in the cylinder at top dead center, and with a distance of 9.585 in (24.36 cm) from crankshaft main centerline to deck, the piston would protrude about 0.21 in (0.53 cm) higher than the top deck, or would collide with the cylinder head combustion chamber. To offset this, aftermarket pistons relocate the piston pin higher within the piston itself. This change in compression height leaves the piston with 1.462 in (3.714 cm) of material in the top portion of the piston, instead of 1.672 in (4.247 cm) as is stock.
Option Two - Shorter Rod: The original piston is kept and a new, shorter rod is fitted so that the original piston will not protrude above the deck.
Option Three - Shorter Cylinder: As viewed from below the wrist pin this option is the same as option one - the skirt length stays the same. However, since the height of the piston is changed,the distance from the wrist pin to the top of the piston is decreased.
There are some engines such as the small block chevy engine(350 Gen I) in which to install a stroker crankshaft the block itself needs to be machined to fit.
Advantages of a stroked crank
A stroked crank increases displacement, and also uses leverage to produce more torque.
An engine with a stroked crank will usually produce more overall horsepower than an engine with a bore kit with the same displacement
Disadvantages of a stroked crank
Increased stroke makes the piston travel further up and down. Since the amount of time allowed for this movement is not increased, the piston speed increases with a stroked crank (piston must move a further distance in the same amount of time). This can sometimes cause pistons to wear more quickly.
Large increases in stroke can decrease an engine's ability to perform optimally at high rpm. Engines used at sustained high rpm usually will be better with less stroke and more bore (oversquare).
As the stroke increases so does the side loading of the pistons, which increases wear.
Obtaining a stroked crank
Purchase a new stroked crankshaft from an aftermarket or OEM source: A typical complete stroker kit is composed of the crankshaft, connecting rods, pistons, piston pins, main bearings, rod bearings, and piston rings. This assembly is also called "the rotating assembly" or "the bottom end". Many different types of each of the components can be used for different applications. Crankshafts, for example, may be either of forged or of cast manufacture. Rods may consist of I-beam or H-beam rods made of various materials, from steel to titanium. Rods may be stock length (requiring the use of either shorter pistons, or taller cylinders), or shorter length (usually utilizing stock height pistons and cylinders).
Modify a stock crankshaft for increased stroke Some stock crankshafts can also be modified to increase stroke. This is typically more cost effective for single cylinder motorcycle and ATV engines.