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Calculate Displacement PDF

How to Determine the Displacement of a Kart Engine
(or any other single cylinder engine)

Here’s what you will need.

This picture illustrates the items that are needed to measure and calculate the displacement of a kart engine. On the right side from back to front; 3” travel dial indicator, 2” travel dial indicator, digital 6” caliper, 6” dial caliper, in the middle back to front; bridge for holding dial caliper on to cylinder, socket for turning some engine crankshafts, notebook and pen. left side; calculator, 5/16 hex key (allen wrench) for turning crankshaft.

Step 1 Remove the cylinder head from the engine.

The engine should not need to be removed from the kart to do this although the
illustrations show a Rotax Max FR125 that has been removed.

Step 2 Position piston at bottom dead center (BDC)

To accomplish this use insert the 5/16 hex key in the starter nut or place a socket on the clutch nut. Turn the hex key or socket wrench until the piston is all of the way down to the bottom of the cylinder. Another way to get the piston to bdc is to simply push the piston down with your finger.

Step 3 Measure and record the cylinder bore.

Use the digital or dial caliper to carefully measure the cylinder bore. Be very
carefully to hold the caliper straight. It is also a good idea to measure it several times to ensure an accurate measurement. The measurement can be in millimeters or inches but you must use the same system for measuring the stroke in the next step. Record your measurement. This measurement is the bore of the engine.

Step 4 Measure the Stroke of the Engine

A. Make sure that the piston is pushed all of the way down to the very bottom of the cylinder.
B. Mount the bridge on to the top of the cylinder.

C. Insert the dial indicator into the hole in the bridge just enough for it
to touch the piston. Snug up the locking screw.

D. Turn the face of the dial indicator so that the needle is pointing to zero.

E. Slowly turn the crankshaft with one of the wrenches while watching the dial.

F. Count how many full times the needle revolves plus how many additional
hundredths or thousandths to reach top dead center (TDC). It’s always a good idea to repeat the above step just to be sure.

G. Record the reading. This measurement is the stroke of the engine.

Step 5 Calculate the Displacement of the Engine

A. Plug your measurements into the following formula.

____________ X ___________ X _____________ X .7854 = Displacement
bore measurement ---bore measurement---stroke measurement

B. If the measurements were made using the metric system, in other words, in millimeters, the result will be the cubic centimeters of the engine. You now know what you need to know to calculate the entrant’s minimum weight.

C. If the measurements were made using the customary system, in other words, in inches, the result will be in cubic inches. This willneed to be converted to cubic centimeters.

D. To convert cubic inches to cubic centimeters;

__________ X 16.39 = _________ CC
cubic inches

Questions or comments about these instructions should be directed to;
Mark Bergfelt (mark@bergfeltracing.com) (724) 847-3677