Squish measurement and adjustment on 2 stroke engines
by Had Robinson with assistance from Gale Tyler
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 gasket must 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). You should NEVER reuse any of the engine gaskets which is why you must check the squish if you remove the cylinder from the engine.
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). 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")
For low octane fuel make the thickness 0.80mm-0.95mm. Engines before June 2003 require 0.80mm-0.85mm. Most common sizes are .30mm & .40mm. Excessive gasket thickness is always better than too thin. Too thin a gasket can cause engine damage i.e. raise the compression and experience knocking/pre-ignition.
Minari 180/200 1.30mm-1.50mm (0.051"-0.59") – Here is Minari's PDF on how to measure the squish
For other paramotors, check your owner's manual for the correct squish value. Some engine manufacturers may not need to have the squish set supply only a single gasket size.
Special tools and parts needed
- Rosin core solder –
available at Radio Shack or Harbor Freight. DO NOT USE SOLID CORE SOLDER!
- Caliper – The cheap digital calipers available at Harbor Freight are sufficient
- Cylinder gaskets – If you are going to do this project, you can order in advance a selection of the common sizes (for the Top 80). The common sizes are: 0.30mm and 0.40mm. 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. If you are unsure what size to order, 0.40mm would be the safest.
- A new cylinder head "O" ring or gasket.
Measuring the existing squish
- Make a (2) strand "rope" of ordinary rosin-core solder about 4" long. The rope 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 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!
- Reinstall the spark plug.
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 nor install both circlips that hold the wrist pin in place. This will save trouble
when taking the cylinder on and off.
- Remove the old cylinder gasket, if necessary, and then reassemble the cylinder WITHOUT A CYLINDER GASKET. Properly torque down the cylinder head nuts to 9 Nm.
- Measure the squish per the above.
- 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. However, the gasket will compress when the cylinder head and cylinder are torqued down. 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 and circlips if they were removed) with the correct gasket. The opening in the circlips should face down. Do not use sealant these gaskets. Install the cylinder head WITHOUT the "O" ring, torque in a cross pattern in stages to 9 Nm and measure the spacing again. If it is within specifications, then remove the cylinder head and install it with the "O" ring. If you gently stretch the "O" ring, it will tend to stay in the groove in the cylinder head much better. Remember: do not reuse an "O" ring or a cylinder gasket because they will eventually leak. Globbing RTV on old gaskets and O-rings does not work. Torque the cylinder head nuts in a cross pattern in stages to 9 Nm.
Do not forget to replace the spark plug with the spacer and torque it down properly (21 Nm). You should be good to go. Remember that too much cylinder head/piston spacing is always better than too little.