Increasing Motorcycle Engine Size
TAKING AIM, by Chris Maida, Editor
Want more power? Get a bigger engine. It’s as simple as that. And though increasing your engine’s displacement isn’t the only way to get more power, it’s a very effective one. Since an engine’s displacement is the result of two factors — the bore of its cylinders and the stroke of its pistons — increasing either one (or both) will make your engine larger and more powerful.
The displacement of a piston engine is the total swept volume of all the engine’s cylinders. A cylinder’s swept volume is the area that the piston travels (sweeps) over as it moves up and down its cylinder, called its stroke. The displacement (area) of a cylinder depends on its diameter (bore) and length (stroke). To compute a cylinder’s displacement, you plug these two measurements into the formula: bore x bore x 0.7854 x stroke. The number 0.7854 is a constant value that’s needed to make it all work out correctly.
Once you have the swept volume of a cylinder, you multiply that number by the amount of cylinders the engine has, which for V-twins is two, to get the total cubic-inch (or cubic centimeter for you metric lovers) displacement of the engine. If you use the bore and stroke measurements for a TC 103, you’ll get (3.875″ x 3.875″ x 0.7854″ x 4.375″) x 2 for a total of 103.19″ (1,690 cc).
The purpose of a big-bore cylinder kit is to dramatically increase the diameter of an engine’s cylinder bores. There are two ways to do this: buy bigger bore cylinders or machine the bore of the stock cylinders larger. The stock TC 88 and TC 96, and 883 Sportster cylinders have walls about 1/4″ thick, so they can be bored out to the same diameter as some big-bore kit cylinders and still thick enough for safe engine operation.
Changing the bore of an engine to increase its displacement should not be confused with an overbore, which is done during an engine rebuild to remove imperfections in the cylinder’s walls. While an overbore does change the bore size slightly, it’s a very small amount – 0.005″ or so, depending on the amount of damage to the cylinder walls. Once you’ve changed the bore of the cylinders, even if it’s just 0.005″ over for a rebuild, you need to
install new, equally larger pistons and rings.
As for engine longevity, there are no problems associated with increasing an engine’s bore, other than the additional stress and strain that the increased power will put on all the engine and powertrain components. However, the size of the cylinder holes in the crankcases, called the cylinder spigots, limits how much you can increase the cylinder bores. That’s why builders who want very large displacement engines must also increase the engine’s stroke. And that’s what we’ll cover in another issue.
See you on the road,
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