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How to Measure Squish Clearance

The squish clearance is the gap between the top of the piston and the head when the piston is at TDC. It is important to set this gap to the correct measurement to ensure efficient combustion and maximum power.

In order to setup your head or cylinder accurately we need to know what your engines current squish clearance is. The process for measuring the squish clearance should only add 10-15min to the job of removing the head.
The steps are as follows:

  1.  Cut a length of resin core solder about 100mm long. This solder can be purchased from most bearing suppliers, plumbing suppliers, tool shops and many hardwares. If you cannot find solder anywhere we can post out some pre-prepared strips for a small charge. In general it is best to use solder approx 2.5-3mm thick on a stock 250 or 300, and solder about 1.5-2mm thick on a stock 85 or 125, however in some cases you may need to use thinner solder than this to get an accuate result… more on this later…
  2. Bend a curve about 10-15mm high into the middle of the length of solder and lay it flat on a bench. Measure your bore (250’s are 66.4mm, 300’s are 72mm, etc) and then cut the solder to length leaving the bend centred in the strip. It is very important to cut the ends of the solder vertically (viewed from when the solder is lying flat on the bench)… if cut horizontally the wedge shape that is left in the solder from cutting wont crush properly and you will not get an accurate squish measurement at the bore wall.
  3. Remove the stator/flywheel cover and rotate the engine by hand so that it is approx 5mm below TDC. Make sure you remember which direction to turn the flywheel to make the piston move up to TDC from this position… it will be important later.
  4. Lay the prepared solder strip sideways across the piston so that the ends are touching the bore wall both sides. If necessary push/pull the bend in the solder to lengthen or shorten it to the correct length (see picture below).
  5. SONY DSCReplace the head and tighten all head bolts to the required torque.
  6. Using a socket, slowly rotate the flywheel nut by hand, moving the piston up to, and then past TDC. Do not use the kickstarter to rotate the engine. As the piston approaches TDC the engine will get much harder to turn as the solder is being crushed. Once the piston is past TDC the engine will suddenly get easier to turn. STOP at this point, do not turn the engine further as the solder may get caught in the ports.
  7. Remove the head and take out the crushed solder. Make sure that both ends of the solder are crushed evenly and that the bend in the middle of the solder is NOT crushed. If it is, repeat the process with a smaller bend so that it does not touch the squish band.
  8. Check the ends of the solder… if they appear to be crushed to well below half of their original thickness you may need to repeat the process with a smaller thickness solder. Using a thick solder to measure a thin squish clearance may result in the solder ‘bouncing back’ after being crushed and often gives an inaccurate result.
    SONY DSC
  9. If you are happy with the solder test strip then repeat the process twice more so that you have THREE properly crushed strips.
  10. Send all THREE strips to us… we will then measure each strip and use the average measurement to determine your squish clearance.

 

If requesting head modification or cylinder porting you MUST take an accurate squish clearance measurement and send it to us with your job otherwise we will not be able to proceed.