Piston clearance

[nevhunter]

A flat top piston would absorb a lot less heat. (Less area). Make it a bit easier to Kick start as well. The cast in pushrod tunnels don't help cooling. I agree with your suggested Piston clearances. It's the clearance they have when running that counts..My VCH is still running the Diecut Piston it's had since I first rebuilt it in 1973 and it's not had an easy life. Nev

[allan.walker]

Could someone please confirm exactly what is meant by 'piston clearance?'

My understanding is that a specified piston clearance of, say 5 thou, means that there is a 5 thou gap between the piston and the barrel, all the way around the circumference.

However, I have also heard (and read) that 5 thou clearance means that the i.d. of the barrel is 5 thou greater than the o.d. of the piston. If this is the correct definition, then there would only be 2.5 thou clearance, wouldn't there?

Allan.

[david.anderson]

Alan
.005” clearance means that the piston at its largest diameter (bottom of skirt) is .005” smaller than the bore. Pistons are tapered toward the top ie they get smaller toward the top of the skirt (ie more clearance) and there is even more clearance above the bottom ring groove. The reason is that the top of the piston gets hotter and expands more than the bottom of the skirt. So in theory the piston will expand at the top of the skirt, when hot, to be the same diameter as it is at the bottom. . The piston will also have a lot less clearance when hot than cold as the piston expands more than the cylinder.
A piston is also ground oval so that there is more clearance around the sides than at the back and front. That is because the thrust faces are front and rear, so with more clearance at the sides there is less friction. So if you have .005” clearance then there is .0025” clearance either side bottom of skirt when cold. The above only applies to 4 stroke pistons.
David

[allan.walker]

Thank you David.
That clarifies it very nicely.

So, 'piston clearance' is a term referring to a static situation when measuring the gap at one side of the piston, with the piston skirt in contact with the opposite side of the liner.

I've always tended to think about 'working clearance,' i.e. the gap all around the piston when the engine is running. Every day is a school day, even at my age!

Allan.