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post #1 of 7 (permalink) Old 08-29-2008 Thread Starter
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Stroker question

I have been reading on here where some people have stroker motor/stroker kit. Can somebody explain to me what the difference in the stroker is, or point me in the right direction of where to look?
Thanks in advance guys (sorry if I sound like an idiot, I'm not exactly a gear head ................... yet)


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post #2 of 7 (permalink) Old 08-29-2008
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Originally Posted by Bpcephus View Post
I have been reading on here where some people have stroker motor/stroker kit. Can somebody explain to me what the difference in the stroker is, or point me in the right direction of where to look?
Thanks in advance guys (sorry if I sound like an idiot, I'm not exactly a gear head ................... yet)
A stroker motor is an engine that utilizes a different crank to achieve more displacement with the same engine block. A stroker's greater than stock displacement is due to an increase in the factory crank throw. An increase in crank throw increases stroke (the difference between the piston's top dead centerand bottom dead center position), which translates to more displacement. Longer rods are used 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.


1984 Mustang 331 stroker
1990 Mustang 5.0L convertible
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Thanks for the explanation!

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post #4 of 7 (permalink) Old 08-29-2008
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Quote:
Originally Posted by dakine858 View Post
A stroker motor is an engine that utilizes a different crank to achieve more displacement with the same engine block. A stroker's greater than stock displacement is due to an increase in the factory crank throw. An increase in crank throw increases stroke (the difference between the piston's top dead centerand bottom dead center position), which translates to more displacement. Longer rods are used 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.
This is not correct. With a longer stroke in the same block, you would need a shorter rod or a higher piston pin location to account for the increase in piston travel. With an increase in stroke, the piston travel increases equally in both directions.

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Last edited by stlwagon; 08-30-2008 at 07:43 AM.
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This is not correct. With a longer stroke in the same block, you would need a shorter rod or a higher piston pin location to account for the increase in piston travel. With an increase in stroke, the piston travel increases equally in both directions.
I'm not trying to argue but you are 90% incorrect. I noticed you said "you would need a..." which I took to mean that you've never built a stroker motor before... The reason I said longer rods are used is because I've built a handful of strokers, including my current daily driver and in every single case, a longer connecting rod was used. Here's the differences between a stock (left side of picture) and stroked motor (right side of picture)

A stroker motor utilizes:
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)

The rod length does not affect the displacement of the engine. One way to build a stroker is to use an increased crank throw, decreased piston compression height, and stock rod length to achieve additional stroke. The big drawback to this method is reduced crankshaft strength. The best way involves calculating the Rod to Stroke Ratio to adjust the rod angle. This is achieved by dividing a motor's rod length by its' stroke. This is an important calculation to understand since it informs the builder about a motor's rod angularity which for this purpose is translated to cylinder wall stress. A low Rod Ratio yields a high rod angle. For example, a motor with a 5.400" rod length and a 3.000" stroke yields a rod ratio of 1.8:1. If we maintain the same stroke and shorten the rod length to 5.000" we get a 1.7:1 rod ratio. The rod angle has increased. A high rod angle or low Rod Ratio creates a greater potential for accelerated wear to cylinder walls, pistons, and piston rings. You were correct about one thing, by lengthening the rod, as stroke is increased, we can offset the increased rod angle. This makes it necessary to use shorter pistons but the shorter pistons prevent the premature wear caused by high rod angles.
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1984 Mustang 331 stroker
1990 Mustang 5.0L convertible

Last edited by stlwagon; 08-30-2008 at 07:43 AM.
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post #6 of 7 (permalink) Old 08-29-2008
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Quote:
Originally Posted by dakine858 View Post
I'm not trying to argue but you are 90% incorrect. I noticed you said "you would need a..." which I took to mean that you've never built a stroker motor before... The reason I said longer rods are used is because I've built a handful of strokers, including my current daily driver and in every single case, a longer connecting rod was used. Here's the differences between a stock (left side of picture) and stroked motor (right side of picture)

A stroker motor utilizes:
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)

The rod length does not affect the displacement of the engine. One way to build a stroker is to use an increased crank throw, decreased piston compression height, and stock rod length to achieve additional stroke. The big drawback to this method is reduced crankshaft strength. The best way involves calculating the Rod to Stroke Ratio to adjust the rod angle. This is achieved by dividing a motor's rod length by its' stroke. This is an important calculation to understand since it informs the builder about a motor's rod angularity which for this purpose is translated to cylinder wall stress. A low Rod Ratio yields a high rod angle. For example, a motor with a 5.400" rod length and a 3.000" stroke yields a rod ratio of 1.8:1. If we maintain the same stroke and shorten the rod length to 5.000" we get a 1.7:1 rod ratio. The rod angle has increased. A high rod angle or low Rod Ratio creates a greater potential for accelerated wear to cylinder walls, pistons, and piston rings. You were correct about one thing, by lengthening the rod, as stroke is increased, we can offset the increased rod angle. This makes it necessary to use shorter pistons but the shorter pistons prevent the premature wear caused by high rod angles.
Actually, built somewhere around 5-6 hundred of them. In some cases it is possible to use a longer rod with a stroker crank, but that is also dependent on the stock rod length, the current compression height of the piston, and the amount of the stroke increase. The length of the rod used has no bearing (no pun intended) on the strength of the crank, only the amount of stress applied to it. The crank is what it is, strength wise.

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Last edited by stlwagon; 08-30-2008 at 07:44 AM.
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[quote=jgard07;1153295]
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Originally Posted by dakine858 View Post
Actually, built somewhere around 5-6 hundred of them. In some cases it is possible to use a longer rod with a stroker crank, but that is also dependent on the stock rod length, the current compression height of the piston, and the amount of the stroke increase. The length of the rod used has no bearing (no pun intended) on the strength of the crank, only the amount of stress applied to it. The crank is what it is, strength wise.
I never said the rod length had anything to do with the crank strength. I did make a mistake with how I worded the first method of stroking a motor. That was supposed to say that you can increase stroke with a stock crank, by offset grinding. Offset grinding the rod journal moves the centerline of the rod journal away from or toward the centerline of the main journal. This will result in increased or decreased stroke. This method weakens your crankshaft.

1984 Mustang 331 stroker
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