The term 'Stroker' means that someone has increased the stroke of their engine.  Displacement is determined by BORE (the size of your piston) and STROKE (how far that piston travels up and down).  There are two ways to increase displacement:  You can increase bore (with a larger piston), and/or you can increase stroke by moving the crank pin (the pin holding the rod to the flyweights) further outwards on the flyweights.

If you aren't familiar with general engine operation, check out some diagrams:

WHY STROKE INSTEAD OF JUST GO BIG BORE?

Okay, so we've increased the stroke.  You now have more power from the displacement increase, but you've also gained more torque.  Why?  Because moving that pin location outward on the crank is just like using a longer wrench to remove a stubborn bolt.  You've given the engine more leverage.  This allows the engine to generate power at lower rpm much more easily.

Another good thing about increasing the stroke is that with any engine that you plan on really going big, it makes more sense to increase bore and stroke, rather than just stick in a whopping large piston.  In most cases, keeping the bore and stroke increases relatively equal, you will end up with a powerhouse that's more reliable and delivers power more evenly.

GREAT, WE'VE STROKED THE CRANK, AND THE PISTON'S HITTING THE HEAD, NOW WHAT?

Ok, there's a few ways to solve the problem:

1. You can make the cylinder taller using a base plate (spacer).  This works very well on 2 strokes, but can create more problems with 4 strokes since you'll have the timing chain to deal with.

2. You can have a piston made that is shorter, or modify a stock piston.  On minor stroke increases (totaling 2mm or less), you can sometimes modify the stock piston to fit.  More than 2mm would require a special piston - that can get expensive unless you plan on talking 3 of your buddies into doing the same thing.

3. You can have a shorter rod.  Here's where the famous Powroll engineering comes into play.  Back in the stone age (okay, 1962), Paul Olmstead (founder of Powroll), created an ingenious method of shortening rods.  The 'Shrunk Rod" process uses heat and pressure to create a rod that's the proper length for our stroker cranks and actually increases the strength of the rod.

Powroll doesn't use our patented method for rod shortening on every crank we stroke.  Our development team works with each engine design to determine the optimum rod length to stroke ratio.  If the stock rod length is the most advantageous, we don't shorten it.  We use a piston with a shorter pin-to-deck height to keep things working right.

There are also some engines which have a weak rod design.  We replace those stock rods with our own heavy duty rod which is the correct length.

CAN'T I JUST REPLACE THE ROD WITH A LONGER ONE?

Rod length has nothing to do with displacement.  Really.  Trust us on this one.

Here's the best way to figure out why this is true.  You've probably got a glass of your favorite beverage sitting in front of you right now, don't you?  Okay, lets say that glass is 1/2 full and has 10 oz of liquid inside.  Now, pick up the glass and hold it up in the air.

Is there more liquid in the glass?  Nope.  Same amount, it's just in a different location.

Now, put the glass back down.  Same amount of liquid?  Of course -- unless you cheated and took a swig on the way down.

The rod does the same thing that your arm just did.  It only changes the location, but not the displacement.

Changing rod length can change the way an engine produces power, but it will never change the actual displacement.

Okay, take a drink of the stuff in the glass, you deserve it!

WHAT IS DISPLACEMENT?

Displacement is another term for engine size.  It means 'how much volume is displaced by the piston travel'. 

	Figure 1 - SINGLE CYLINDER CRANK The crank is what transfers the power of the 'explosion' of fuel and spark to the wheels. Every time your engine fires, the piston is forced downward in the cylinder.  The PISTON is connected to the ROD by the PISTON PIN, the ROD is connected to the CRANK by the CRANK PIN. The crank is held in the engine cases by main bearings located at the CRANK CENTER. This means that all the downward force of the explosion is turned into rotation, and the crank spins.
STOCK STROKE	Figure 2 - STOCK STROKE PISTON TRAVEL
STROKED CRANK STROKE	Figure 3 - STROKED CRANK PISTON TRAVEL The piston and cylinder circumference remain the same, but the piston now travels further up and down in that cylinder. So, if you put the piston at BDC and poured fluid in there - you'd be able to fit more fluid, right? That is how a stroker gives you more displacement.
	So, what is a 'Shrunk Rod', and how does Powroll make them? The photo at left shows a standard rod, then the same rod after processing.  Powroll uses a patented heat-and-shrink process which shortens the rod to the proper length, and also strengthens it.

HOW TO DETERMINE THE CC'S OF YOUR ENGINE

(WITHOUT FILLING IT FULL OF WATER)

Since most people don't want to go around pouring fluids in their cylinder to determine cc's, there's a formula you can work to figure the displacement of any engine.

BORE (in cm) X BORE (in cm) X STROKE (in cm) X .7854 = Cylinder cc's

Of course, if you have an engine with more than 1 cylinder, you X's the number by the amount of cylinders.

Here's the formula at work.

You've got an engine with a piston size of 47.5mm, and a stroke of 50mm.  Converting mm to cm is a cinch - just move the decimal point to the left one spot (heckuva lot easier than fractions, huh??).

4.75 x 4.75 x 5.0 x .7854 = 88.60 cc's

If this happened to be a V-twin engine, then we'd times that number by 2.

88.60 x 2 = 177.20cc's

And yes, you can use this formula to determine the volume of any cylinder.  Go measure your beer mug or Starbucks thermos - the formula's the same - as long as it's a straight cylinder - tapers are different!