Technique: what is the squish and how do you measure it?
Most motocross riders rarely or never encounter the word “squish,” it is almost exclusively in the vocabulary of motorcycle tuners. You will never come into contact with it if you drive a standard motorcycle and do not make any major adjustments to the engine block.
What is Squish?
The famous squish is the distance measured between the top of the piston when it is at its highest point (TDC) and the bottom of the cylinder head measured at the edge of the cylinder. In two-stroke engines, the squish is very important because the tight space between the piston and the cylinder head must ensure a swirl of gases and the removal of heat due to the combustion of those gases. If the squish is too tight, the combustion temperature will rise high, with all its consequences, such as a seizure.
Again, you don't need to change your engine's squish if it remains stock. It is a basic setting that is known to the manufacturer of your engine and is rarely communicated in a technical manual. If you experiment with this fact, you will fiddle with many other (fixed) values such as the temperature of the spark plug. Changing the squish to its simplest form is done by changing the thickness of the base gasket. A direct consequence is a changed position of the flush ports relative to the piston and therefore has a (negative) influence on performance. Advanced users modify the squish by changing the dimensions of the cylinder.
This is how you measure the squish.
Motorcycle and moped forums wrote half bibles about this fact, but one is more incorrect than the other. We asked the Dutch company VHM for a correct measurement of the squish. What do you need? A wire of solder and a digital sliding compass. The tin wire is quite a bit thicker than the expected measurement. If you expect to measure a squish of 0,80 mm, it is best to use a tin wire of 0,9 to 1,0 mm. Below you can see the measurement explained step by step.
- Cut four pieces of tin from the roll of solder. One or two pieces is not enough because otherwise you might take a measurement with a tilted piston and therefore get an incorrect measurement.
- Disassemble the cylinder head and place the four pieces of tin at a 90° angle from each other. To prevent the tin from shifting during the measurement, you can apply a lick of grease to the tin.
- Reinstall the cylinder head on the cylinder and tighten the mounting screws to the correct torque (refer to your technical manual).
- Turn the crankshaft until the piston reaches its TDC (top dead center).
- Remove the cylinder head again.
- Now measure the flat areas of the four tin wires with a digital sliding caliper or a micrometer. The four measurements should be approximately the same. This is your squish size!
Photos: VHM
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