Squish

[Peterp]

Me again,
Checking the squish, I know I’ve asked questions about this before. But since then I’ve bought a new piston, 63.4mm and had the barrel bored out with a 3thou’ tolerance.
I’m attaching a picture of my squish check which is 3mm, I’ve read so much about the benefits of tighter squish so, as I’m able to do something about it I thought I’d ask.
For some reason, I cannot attach because it’s too big, why

[Warkton Tornado No.1]

The squish is very important & something that knowledge has been gained of due to the quest for reliability as much as power. The application of a ‘safe’ minimum by which an engine builder should conform is perhaps somewhat debatable but the fundaments of why a minimum squish is important still apply.

When we measure the squish - often by rotating the crankshaft to move the piston up to TDC & beyond with a piece of solder in place to record the resultant thickness - the figure achieved should conform to an optimum we have in mind fundamentally to avoid the piston hitting the cylinder head when the engine is revolving at its highest rate.

Any minimum squish opted for must take into account the state of the bottom end & to whether any of the bearings are worn as well as the con-rod, gudgeon pin & it’s fit in the piston as well as the con-rod bearings. These ‘variables’ are often taken to be in first class condition as checks such as squish are only usually applied to a fresh engine build or by A N Other checking out an engine.

Obviously, squish varies with the physical size of the bore & stroke as well as the design of the bottom end & crankcase. Consequently, a small capacity racing two stroke engine of, say, Japanese origin will have a much tighter squish than those used in drones to embarrass war criminal leaders with a surname something like a fart…..

Back to the plot. Squish should be as tight as we dare because the closer the piston crown can get to the cylinder head, the better the heat transfer. Squish is not a product of the compression ratio, though varying will have an effect. Poor quality gaskets that compress during running will affect both & that is yet another good reason to never use head gaskets & only the alloy base gaskets/packers available nowadays.

Unless an engine has a flat top piston - as utilised in Quattrini’s M210 - the cylinder head should have a concave squish area to match the piston crown. I know some tuners just modify cylinder heads with a straight cut rather than using a form tool, but if a cylinder head is being machined on a lathe anyhow, I don’t understand the reason for compromise.

[Peterp]

Thank you Warkton,
An impressive reply, which in an ignorant way is wasted on me, sorry.
However, the measurement I get is 3mm, by way of three pieces of solder, should I reduce it to 1.5-2.0mm?
I genuinely appreciate the time you take to give such an in depth explanation, thank you again.

[hullygully]

due to modern fuels Pete & in trying to keep the engines heat down, defo try to bring the squish down to @ least 1.5mm, but no less than 1.1mm

[Warkton Tornado No.1]

Thank you Warkton,
An impressive reply, which in an ignorant way is wasted on me, sorry.
However, the measurement I get is 3mm, by way of three pieces of solder, should I reduce it to 1.5-2.0mm?
I genuinely appreciate the time you take to give such an in depth explanation, thank you again.

I can only advise you an actual minimum squish based upon the scenario - as previously described - of everything that affects the ‘play’ within the crankshaft, con-rod & piston assembly which is 1.0 mm. Some very successful rider/self tuners in Group Four have applied even tighter squishes, but they have almost certainly benefitted from the experience of building many more engines besides their own.

Given an engine with first class components, I wouldn’t settle on anything larger than about 1.4 mm & would most likely endeavour to improve even that. If I were you, I’d probably settle at 1.2 mm presuming an absolutely concentric bore to squish face. Now, that is a ‘given’ within all of my responses & in itself an important consideration & probably the subject of a separate discussion