Model Engine Company of America
12-26-2021
The Squish Band
The squash or squish band is the clearance between the head and the top of the piston. First you need to know the different types of combustion chambers to understand this.
The Wedge and Full Hemispherical were used on many of the earlier engines. There is no Squish band to adjust so just be sure to maintain clearance between the top of the piston at top dead center and the bottom of the head.

Wedge chamber.

 

Full hemispherical.

This was used on many
of the earlier engines.

 

These types of heads have the squish band...

  

Hemispherical (bowl)
with squash band.

Schnuerle Ported Design

Hemispherical (bowl) with squash band.

"Looped Scavenged" or "Baffle" type design.
Cone chamber with squash band
 
 
You must measure the distance from the top of the cylinder to the top of the piston at top dead center
This is how to check and adjust the Squish Band
Position the piston at top dead center. You will have to keep the cylinder firmly seated as you rotate the crankshaft to the TDC (top dead center) position. ABC engines will usually push the cylinder out of the crankcase due to the tapered fit. See also ABC info. An easy way is to turn the engine up side down and press it firmly down onto a table top while you rotate the crankshaft.
The piston must be at TDC and the cylinder liner must be firmly seated into the crankcase to obtain the correct measurements. If it is not the final result will be incorrect.

Measure the distance from the top of the cylinder to the top of the piston. Using a depth micrometer as shown is the preferred method. Be careful not to allow the measuring instrument to push the piston off TDC. You need an accurate measurement to .001 of an inch. You can use a depth micrometer for the rest of the measurements shown here however we omitted photos and reference to it as this instrument is less common for many people.

This example measures .062"
Dial calipers can be used as they are more common in the modeler's workshop and are less expensive. Use the depth end of the caliper and it is a good idea to check the reading at zero. By placing the depth end of the caliper on a flat surface and pulling the moving slide of the caliper downward you can see if your caliper is accurate to zero. You should read zero with this test but in many cases a .001 or .002 reading will show. This will not affect our result but you should just be aware of it. As you can see we have the same .062" measurement with the dial calipers. It should be the same as with the depth micrometer or something's wrong.
Now you must measure the portion of the head that protrudes into the cylinder. Many people do not realize that there is another set of surfaces on a dial caliper that can be used for measurements. It is on the back side of the jaws and is perfect for measuring these types of surfaces.
Here we have a .041" measurement on our sample. Depending on the quality of the calipers it is a good idea to so double check the measurement in the fashion shown in the next photo.

Using the calipers in this manner will provide an accurate comparison if your caliper is older or of lower quality.

A note on cheap or junky calipers. If you measure both the depth of the piston in the cylinder and the protrusion of the head in the same fashion (the same end of the calipers), it actually doesn't matter if the caliper zeros properly. Our result will be the difference between two measurements, so if both measurements are off .003" the result will be the same.

Here our example again measures .041"

 

Now do the math...

.062-.041= .021

The squish band is usually set between .016" and .022"

This at the upper end of the squash band width and the lower end of the compression spectrum and since this is a larger engine .021 is a good setting. No head shim is required. This setting can vary with Nitro content of fuel, exhaust systems, and air density. See also compression ratio

If you need adjustment of the clearance the proper amount of head shimming can be added.

For example if our result was a .056" piston depth and a .044" head protrusion we would have .012" difference.
At this point we can add a .005" or .008" or .010" shim or two .005" shims to achieve the desired clearance.

See also compression ratio
The above information is provided as a guide. Since MECOA/K&B has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.