Rotax Squish Gap Check - 587/657/717/787/947

Whenever rebuilding the top end of a Rotax 2 stroke, a very important step is setting the "Squish Gap". It involves assembling the shortblock, inserting a piece of solder in each plug hole, turning engine and seeing how thick solder is. If not in tolerance, motor is disassembled and thinner/thicker base gasket is installed. If you don't check and the gap is out of spec, serious engine damage can occur. Specifically, if the gap is too large, a loss of power will occur and too small will lead to detonation.

I check squish gap before installing the rings to make it a little easier but that’s your choice. If you check it with the rings installed and using the same thickness gasket as was removed then assemble it properly before checking gap, you can just torque everything down if you find it’s in spec instead of taking it back apart to install the rings. Also, remember to tell what thickness base gasket you have, count the holes. A 6 hole gasket corresponds to 0.6mm.

Tools needed:

Procedure:

  1. With base gasket installed (start with a 0.5 or 0.6mm), cylinders bolted down, and head bolted on, insert a piece of solder so as to position the end over the piston pin at the edge of the cylinder wall.
  2. While holding the solder in place, rotate the engine past TDC so as to compress the solder between the edge of the piston and cylinder.
  3. Remove the solder from the cylinder and measure it.
  4. Repeat the procedure for the opposite side of the same cylinder.
  5. Repeat all above for the other cylinder. You now should have 4 measurements logged.
  6. Check the chart below to see where you’re at. The lowest and highest values observed for both cylinders should be within the specs for your engine model. If your gap is too small or large, change the base gasket to compensate.

    Example – if I was checking a 587 with a 0.4mm base gasket and got a low to high measurement of 1.0 to 1.4 and the spec was 1.2 to 1.8, I then would need to add 0.3mm to the base gasket to get the range to 1.3 to 1.7 which is in spec. That would be a 0.7mm base gasket.

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