Technical Update

This weekend I set about measuring as accurately as possible the squish clearance of my cylinders, and I learnt some stuff in the process.

I started out using 2mm diameter solder which I bent into "L" shapes with roughly the radius of a piston, I then inserted the solder into the combustion chamber via the spark plug hole, and trying to get the solder pointing along the gudgeon pin axis, and touching the cylinder wall, you turn the engine over and the piston squashes the solder. In principle you remove the solder and measure your squish clearance. You then do it again in the same cylinder but on the opposite side of the piston, and then you have two readings which you add together and take an average.

The problem of this approach is that you can not be sure that your solder is really at the edge of the piston, and therefore it is not as accurate as hoped (good for a quick check though!).

A more accurate method, is to remove the cylinder head, and then "stick" some solder along the axis of the gudgeon pin with some grease, ensuring that as you move the piston the solder is not moved by any ports, once you are convinced that the solder will not move, you position the pistons some where in the middle of their travel ie. not at TDC or BDC, then you replace the cylinder head, and torque it down, and then using the kick start you turn the engine until both pistons have come to TDC (one after the other) and squashed the solder, if you have put too much grease to hold the solder in place, the solder may stick to the cylinder head, so care should be taken to only do one revolution of the engine - no more as the solder will have moved, and the results will be spoilt. You then remove the cylinder head, and remove the solder, and measure the squish clearance. Repeat for the other cylinders!

This method proved far more accurate although the next challenge is where to measure the thickness of your solder! When the solder is pushed up against the cylinder wall, and then squashed, the very tip of the solder is not squashed, because it is in the clearance gap between the piston and cylinder wall, so you end up will a small ridge of solder at the very tip, which should you measure here, will give incorrect results. You need to measure just behind this ridge at the very smallest width - I have tried to represent this in the following diagram.

 

Of course the other problem is the width of your vernier caliper, which induces some error in the measurement although mine has a quite sharp edge which I used to insert just after the ridge of solder.

On a 125cc cylinder the squish clearance is given as an optimum of 0.8mm, mine were all measuring around 0.78 or 0.75 mms, which is just about within tolerance. So I will not be changing them, which is somewhat of a disappointment, as it means there is no additional power to be had there!