Setting the Squish!

Ok, so you only need a couple of tools to check your cylinder head squishband. A spark plug spanner, some digital calipers and some solder will do it. If you can get the thicker plumbers solder then great, but if not simply twist some thinner solder into a little ball on the end.

Start by removing the spark plug if fitted and then poke some solder down the hole until you are in the area marked below with the red circle. This is the area we are trying to measure. When you think your solder is in this region turn the engine over. You can use a spanner or socket if you have the chaincase off or use the kickstart. The solder will get compressed when the piston moves up and down.

Remove the solder and measure its thickness using a pair of vernier calipers. As you can see my squish was 1.52mm. This was a little high for my liking as you need to aim for around 1-1.2mm on engines of between 125cc and 250cc such as a Lambretta. When the squish is too wide you will lose some power and the engine will also run hotter, if the squish is too low then you run the risk of the piston hitting the cylinder head when running!

So as you can see i needed to lower the squish on my machine slightly. As a quick experiment I placed the head gasket in the calipers and then zeroed them using the button shown in the picture below. This would have the effect of showing what the new squish will be if I were to exclude the gasket on reasembly. 

I then checked the solder again. As you can see my new squish should be around 1.16mm if I reasembly this time and exclude the head gasket. As long as the gasket faces are in good condition it is fine to run without gaskets as many people do this already. Simply use a small amount of silicone based sealent on the faces when you assemble.


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