Squish measurement and adjustment on 2 stroke engines
by Had Robinson & Gale Tyler
updated February 4, 2020
Squish is an effect in internal combustion engines which creates sudden turbulence of the fuel/air mixture as the piston approaches top dead center (TDC). In an engine designed to use the squish effect, at top dead center (TDC) the piston crown comes very close, (typically less than 1mm, to the cylinder head. The gases are suddenly "squished" out within the combustion chamber, creating turbulence which promotes thorough fuel/air mixing, a factor beneficial to efficient combustion. – from Wiki
If the cylinder is removed from the engine, the cylinder base gasket should be replaced. The new gasket must be of the correct thickness for the squish to have the right value. Squish is the clearance between the piston and the squish band inside the cylinder (see illustration). It is a critical value because high squish velocity shortens combustion duration, staving off detonation (knocking/pre-ignition). If it is not set correctly, the engine can be damaged. The squish is adjusted by changing the thickness of the gasket that is between the base of the cylinder and the engine crankcase.
Typical for the Top 80 engine is the loosening of the cylinder head nuts caused by detonation from the squish being set incorrectly, namely, having too little squish which effectively increases the compression ratio.
There is no reason to check the squish unless the cylinder has been removed or you suspect a problem (pre-ignition/detonation/knocking).
It is always prudent to order additional gaskets one size larger and one size smaller. Measure the thickness of the gaskets with a digital caliper (under $20 from Harbor Freight) and write the values on the gaskets with a pencil.
The motorcycle, kart racing, Jet-Ski, and dirtbike guys are experts on this topic. Here is helpful article that gives more information on squish and how to measure it very accurately. Because most pilots will not modify their engines per changing the piston size or stroke length, I do not think that the steps given to measure squish in the article link above are necessary.
Cylinder gasket squish values
Top80 0.60mm-0.70mm (0.024"-0.028") Most common gasket sizes are .30mm & .40mm. Top 80 engines made before June 2003 require a squish of 0.80mm-0.85mm.
Minari 180/200 1.30mm-1.50mm (0.051"-0.59") – Here is Minari's instructions on how to measure the squish
For low octane fuel increase the thickness 0.10mm - 0.25mm. Excessive gasket thickness is always better than too thin. Too thin a gasket can cause engine damage i.e. raise the compression and cause knocking/pre-ignition.
For other paramotors, check your owner's manual for the correct squish value. Some engine manufacturers e.g. Polini may not need to have the squish set and supply only a single gasket size.
A. Special tools and parts needed
- Rosin core solder is best – available at Radio Shack or Harbor Freight.
- Caliper – The cheap digital calipers available at Harbor Freight are sufficient.
- Cylinder gaskets – If you are going to do this project, order in advance a selection of the common sizes for your engine. (For the Top 80 they are 0.30mm and 0.40mm. If you are unsure
what size to order, 0.40mm would be the safest.) You should always measure the thickness of the received gasket and write the value with a pencil on the gasket itself. Gasket thickness can
vary up to 0.06mm either direction. Gaskets are available from Miniplane-USA.
- A new cylinder head O ring or gasket. The pressures inside the cylinder are very high (over 1,000 psi) and using RTV or an equivalent to seal the gasket that is leaking will not work. Leaking head gaskets may damage the engine.
B. Measuring the existing squish
- Make a (2) strand "rope" of the solder about 4" long.
It must be tight (10-25 turns/inch). Making a rope makes it
easier to measure the thickness of the squished solder.
- Remove the spark plug.
- Insert the solder so that it touches
the wall of the cylinder, where the squish band is.
- Gently pull on the starter so that the piston
goes past TDC at least twice. Never pull on the starter quickly unless
the primary wire is grounded with a jumper wire or the spark plug is connected to the secondary wire and is GROUNDED to the engine because you might burn
out the ignition coil.
- Remove the solder and measure the solder thickness (the squish) with a caliper. Note: The very tip (0.1mm) of the squished solder will be thicker because of the clearance between the edge
of the piston and the cylinder wall. Do not measure the tip but just inside it.
- Reassemble the engine.
C. How to determine the correct size of gasket to use
- Remove the cylinder. If a new cylinder and piston is used, do not put the ring on the piston. This will save trouble
when taking the cylinder on and off.
- Remove the old cylinder gasket, if necessary, and then reassemble the cylinder WITHOUT THE CYLINDER BASE GASKET. Properly torque down the cylinder head nuts to the correct value for your
engine. (Top 80 is 9 Nm.)
- Measure the squish per section "B" above steps 3-5.
- Subtract the measured value from the specified value. Use the higher specification (e.g. 0.70mm for the Top 80) because it is always safer to have too much squish than too little. The
difference will be the thickness of the gasket needed.
Note: the gasket will compress when the cylinder head and cylinder are torqued down so a thicker uncompressed gasket must be used. Gaskets compress to about 70% of the uncompressed (unused) value i.e. they compress about 30%. This must be calculated when determining the gasket thickness. For example, if a 0.30mm gasket is needed, install a gasket that is 0.43mm uncompressed (0.30 ÷ 0.70 = 0.43). Choose a gasket of the next greater thickness if you do not have the exact sized gasket that you need.
- Reinstall the cylinder (with the piston ring if it was removed) with the correct gasket. Using sealant on engine gaskets is unnecessary. Install the cylinder head WITHOUT the O ring
or head gasket,
torque in a cross pattern in stages to the correct torque and measure the squish again. If it is within specifications, then remove the cylinder head and install it with the O ring or
gasket. O ring installation: if it is gently
stretched, it will tend to stay in the groove in the cylinder head much better.
Remember: do not reuse an O ring or a head gasket because it will eventually leak. Glopping RTV on old gaskets and O-rings does not work.
Torque the cylinder head nuts in a cross pattern in stages to the correct value.
Replace the spark plug with the spacer (if one is required) and torque it down to the correct value. (Top 80 is 21 Nm.) You should be good to go.