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Squish measurement and adjustment on 2 cycle 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

squish band in a piston engine - from WikiIf 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 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 caliper (under $20 from Harbor Freight).  Write the values on the gaskets with a pencil.

The motorcycle, go-kart, 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, I do not think that the steps given to measure squish in this article are necessary.

Special tools and parts needed

  1. Rosin core solder – available at Radio Shack or Harbor Freight.  DO NOT USE SOLID CORE SOLDER!
  2. Caliper – The cheap digital calipers available at Harbor Freight are sufficient
  3. Cylinder gaskets – If you are going to do this project, you can order in advance a selection of the common sizes.  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.
  4. Cylinder head O ring.

To measure the existing squish

  1. 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.
  2. Remove the spark plug.
  3. Insert the solder so that it touches the wall of the cylinder, where the squish band is.
  4. 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.
  5. Remove the solder and measure the solder thickness (the squish).  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.  For the Top 80 it should be 0.60mm-0.70mm (0.024"-0.028").  Note that Top 80 engines made before June 2003 should be 0.80mm-0.85mm.
  6. Reinstall the spark plug and torque in a cross pattern in stages to 9 Nm.  Getting the spacing correct is important if you want your engine to reach full power but not experience destructive pre-ignition (detonation/knocking).

To determine the correct size of gasket to use

  1. 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 Gudgeon pin in place.  This will save trouble when taking the cylinder on and off.
  2. Remove the old cylinder head gasket, if necessary, and then reassemble the cylinder WITHOUT A CYLINDER HEAT GASKET.   Properly torque down the cylinder head nuts to 9 Nm.
  3. Measure the squish again per the above.
  4. Subtract the measured value from the specified value.  Use the higher specification e.g. 0.70mm 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 is torqued down.  Gaskets compress about 70% of the uncompressed (unused) value.  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 (0.43 x .7 = .30).  Choose a gasket of the next greater thickness if you do not have the exact sized gasket that you need.
  5. Reinstall the cylinder head (with the piston ring and circlips, as necessary) with the correct gasket.  Sealant is NOT needed on the cylinder gasket. 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.  It is prudent to use a small amount of RTV "blue" type gasket sealant on the new O ring.  Put the sealant between your fingers and pull the O ring through your fingers.  Do NOT use too much sealant.  You only need to just coat the O ring.  More is NOT better.  Sealant should be used because there may be slight deformities or nicks in the top of the cylinder or head and the RTV sealant will take care of it.  Remember: do not reuse the O ring or cylinder head gasket.  Torque the cylinder head nuts in a cross pattern in stages to 9 Nm.

Let the engine sit (8) hours for the sealant to cure.  Replace the spark plug WITH the spacer.  You should be good to go.  Remember that too much cylinder head/piston spacing is better than too little.

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