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Understanding cubic inches, bore & stroke

I am Joe Biker Malloy, from H&L Performance. I have been working professionally on motorcycles for over 17 years now. I have been a motor builder for many years, and  my partner Tom Haner and I have been fortunate enough to provide motors for many, many Orange County Chopper creations such as; the Comanche Bike, The Statue of Liberty Bike, Lugs, Livestrong, Jorge Posada build, and Chris Angel. What people may not know is that we also provide the engines to rival builders Eddie Trotta, & Jessie James, Redneck, and NYC Choppers, just to name a few. We know our motors. We have two of our own style motors out now ranging from 113 cubic inch all the way up to our 131 cubic inch. We know power, we know speed and we understand the feeling of squeezing every single inch of power from your machine. So, after much prompting from Krit (she is so relentless sometimes I want to kill her!) I have been “recruited” as the Tech Guru for NY Rider Magazine.  

So, throughout the riding season send in your questions to: [email protected] and I will do my best to answer them! Now, I am not professing I know every single thing about everything- but what I do not know, I will find a colleague in the industry who does and we will find out the answers together.

So, to start off this series let’s discuss some motor basics. The next two months I will explain what bore and stroke are from a mechanical perspective and what they mean to the rider. 

So as not to confuse, I will use a TwinCam 96” motor as the basis of our discussions in all of these articles. I will use different bore and stroke configurations as examples to explain how we wind up with the larger displacement engines that are common now versus a few years ago.

There are three primary ways to increase the cubic inch displacement of an engine. The first is increasing stroke, Stroke is the distance the piston travels from top dead center to bottom dead center. This distance is determined, in large part by the design of your engines flywheels. The second is by increasing the size of the bore. Bore is the diameter of the cylinder or piston. And the third, which is my favorite, is combining a longer stroke with a larger bore.  

In the case of a TwinCam engine, the stock cylinders are 3.750”(3 “”) bore. If we were to use a 4” bore cylinder this 96” motor now becomes 110”. Now let’s add some stroke to the engine configuration. Let’s put a 4.625”(4 5/8”) stroke, keeping the 4” bore, you will now have a 116” motor. If we then take away the 4”bore, going back to the 3.875”(3.7/8”) bore, and just stroke the motor. Again, using the 4.625”(4 5/8”) stroke, this would give you a 109” motor. Much like baking a cake, we need the right ingredients (stroke & bore) to wind up with the displacement and configuration we desire.

H-D engines have a considerably long stroke when compared to most modern engines, and have inherently poor valve train geometry. It is important to remember we are still using a pushrod motor designed for the most part in the early 1900!s. Although it has been improved upon, this engine design is not optimal. The combination of long stroke and pushrod under-head cams, limit the ability of the V Twin engine to maintain or for the most part reach high RPM!s. 

Harley!s usually peak power at around 6000 RPM.  

But (and isn’t there is always a but), the benefit of this configuration is, that with such a long stroke, these engines make lots of low end torque. As any of you gearheads know, some of the fastest Metric bikes can be taken light to light by a beefed up and built H-D powered bike. Keep in mind, the longer the stroke, the more low end torque a motor will produce. Torque is exciting and as a result, strokers are fun.

A shorter stroke motor will not produce as much low-end torque but will spool up through the RPM range quicker. Riding style is a very important consideration when making a decision on how you are going to design and build your motor. A well built motor will last an acceptable amount of time for the money invested, but there is no doubt when considering a very long stroke, usually more than 4.750”(4 “”), you start reducing the life of the motor drastically.  

Ok, pocket protector types, here are the equations to figure out the c.i.d. (cubic inch displacement) of any motor, whether it be a lawn tractor, a Harley Davidson, or a top fuel dragster. Multiply bore, times bore, times stroke, times .7854 (this is your constant), times number of cylinders.

EXAMPLE:  3.875 X 3.875 X 4.375 X .7854 X 2= 103.191  (or 103 cubic inches)

I hope this light overview explains some of the mystery regarding c.i.d, bore, stroke and how they affect the longevity and “seat of the pants” feel of your bike’s engine.

Next installment: we will talk about Stroke in greater depth.

Ride safe-

Joe Biker Malloy