piston dimensions
Discussion
Hi anybody out there an ace on piston dimensions? I recently bought a set of 'as new ' pistons. When I came to measure them although they are round at the top edge they are slightly tapered on two sides of the skirt. This is NOT due to wear, and I am wondering if it is a built in clearance for oil supply etc. Any ideas?
I *Think* it's because different parts of the piston run at different temperatures, and so expand by different amounts; the idea is that at working temperature, it's round. I think.
Another possibility might be, if it's narrower measured in line with the gudgeon pin than it is at right angles to it, that it's been done to keep more of the non-wearing parts of the piston away from the cylinder wall, to reduce friction.
Another possibility might be, if it's narrower measured in line with the gudgeon pin than it is at right angles to it, that it's been done to keep more of the non-wearing parts of the piston away from the cylinder wall, to reduce friction.
Pidgeon is correct, the piston is deliberatly designed the way it is so that when hot, expansion deforms it to a more perfectly round shape.
FWIW forged pistons tend to have less well controlled expansion than cast pistons, so the cold clearances are usualy larger (and hence piston slap is common of engines running forged pistons when cold).
FWIW forged pistons tend to have less well controlled expansion than cast pistons, so the cold clearances are usualy larger (and hence piston slap is common of engines running forged pistons when cold).
Yeah, the guys above are correct. A typical piston has an ovality of say 600uM (0.6mm) if measured across the pinhole/non pinhole axes and what we call a 'barrel' of around 300um to maybe 1mm down the sides. If you put two together they will 'rock'.
I spend half my life tuning the machines to get piston shapes to drawing..
I spend half my life tuning the machines to get piston shapes to drawing..
chrisx666 said:
Yeah, the guys above are correct. A typical piston has an ovality of say 600uM (0.6mm) if measured across the pinhole/non pinhole axes and what we call a 'barrel' of around 300um to maybe 1mm down the sides. If you put two together they will 'rock'.
I spend half my life tuning the machines to get piston shapes to drawing..
Out of interest, how to they work out the required dimensions to give a nice round, straight piston at working temperature? I'm imagining a perfectly machined piston must be very carefully measured at high temperatures to see where the distorion is, and this amount taken off the cold machining dimensions?
The guys in design use computer modelling to predict the expansion of a given shape of alloy along with any other features they may want, such as reduced contact patch etc. Some pistons are seriously complex, with 20 different asymetrical profiles down the skirt.
When the theory is complete they do the testing and make changes to eliminate any problems they may have (noise or uneven wear etc).
Chris.
When the theory is complete they do the testing and make changes to eliminate any problems they may have (noise or uneven wear etc).
Chris.
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