How To: Measure and Calculate "Squish"

The elusive "squish." What is it and how do you quantify it?
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What Is Squish: When the piston is at top-dead-center (pushed as far into the cylinder as it will go), the gap between the top of the piston and the edge of the cylinder-head is known as the "squish." Knowing your squish gives you some idea of how much compression you are getting in your engine. Furthermore, knowing your squish will allow you to optimize your compression so that you too can milk the sweet teats of masterful destruction like the pros!
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What You Will Need: You will need a calculator, a roll of 0.05 inch solder (from Radio Shack), a pair of dial calipers, a pencil, and a pad of paper. The solder is shown in Figure 2 and also in the middle of Figure 1. The dial calipers are shown on the right in Figure 1.
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Measuring Squish:
1. Remove your fairings to allow access to the cylinder-head. Having done this, remove the spark plug.

2. Unroll about 6 inches of solder from the roll and bend it slightly to one side about 1 and 1/2 inches back from the end as shown in Figure 2.

3. Lean across the bike as you see me doing in Figure 4. Insert the bent end of the wire into the cylinder via the spark plug opening (See Figure 3). Direct the tip of the solder to the right as indicated in Figure 3 by the red arrow. If you like, you can direct it to the left. You want the tip of the solder to touch the cylinder wall just at the point where it joins the perimeter of the cylinder head (See Figure 8). Note: It is important that the measurement is made on the right or left side of the piston (See Figure 8), directly above the wrist pin. Rocking of the piston in the cylinder can cause measurements made elsewhere to be unreliable.

4. Continue leaning across the bike and steady it with your chest just as you see me doing in Figure 4. As you continue to hold the solder in place inside the cylinder, slowly pull on the pull-starter with your free hand. You should be able to feel the piston squish the solder against the cylinder-head. Also, as the piston reaches top-dead-center, you will hear a hiss as the reed valve opens in the case.

5. Remove the solder from the cylinder. It should be squished as is seen in Figure 5. If you look at it from the side, you will see that it is not uniformly squished. That is, it will be more squished in some areas than in others. Using your dial calipers, measure the thinnest section of squished solder (See Figure 6).

6. Write this measurement down on the pad.

7. Using a pair of wire-cutters, cut off the squished end of the solder. Reform the bend as you did in step 2.

8. Repeat steps 2 through 7 until you have taken and written down 10 measurements. For better accuracy, alternate your measurements from the right side of the cylinder to the left. This process will not feel natural at first. However, in time and with enough measurements, you will gain confidence.
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Calculating Your Squish: (See Figure 7) If your dial calipers are metric (they measure millimeters), then add up all of your measurements and divide the total by 10. This is your squish in millimeters.

If, however, your dial calipers are giving you measurements in inches, don't panic. Begin by adding up all of your measurements. Divide the total by 10. This gives you your average squish in inches. Now, take your average squish in inches and multiply it by 25.4. The resulting number is your squish in millimeters.

For a stock 47cc cag, your squish should range from 0.5mm to 1mm.

MISSION ACCOMPLISHED!