Are Pistons Round? - lezer
I took a VW piston which had failed to the Engineer/owner of a engine services company in the Bath area to have it measured, because the piston measured .8mm less than the measurement stamped on the top. If this was correct, it would be like putting a standard piston into a cylinder that had been rebored twice. whereupon I was told "We dont measure across there, we measure across there, pointing to the other side of the piston, the side that was damaged. It was at this point I asked him if the piston was not round, whereupon he said in a much louder voice " We dont measure it there, we.....
Are Pistons Round? - L'escargot
It is quite normal for pistons to be not round when cold. This is to ensure that the piston is round when it has reached its running temperature. Because the piston is not a regular shape it will not expand evenly in all directions as it warms up. Similarly, it is normal for pistons to be not cylindrical when cold, i.e. they are invariably barrel-shaped. Another factor that is taken into account in designing a piston is that different parts of the piston run at different temperatures.
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L'escargot by name, but not by nature.
Are Pistons Round? - Garethj
If a piston is out of round by 0.8mm there's a big problem! Overheating?

Gareth
Are Pistons Round? - stackman
Of course, if you are riding a Honda NR750 your pistons will be oval.

The V4 motor bike engine featured pistons shaped like a spam can, each with 2 con-rods and 8 valves per cylinder.

Clever chaps those Honda techies.
Are Pistons Round? - Garethj
Oops, schoolboy error forgetting about some motorcycle engines. However, Passat pistons....

Gareth
Are Pistons Round? - AlanGowdy
I always thought pistons were cylindrical, so that they'd fit neatly into cylinders which are...er... cylindrical too.
Are Pistons Round? - Aprilia
Piston is designed to be round and cylindrical at normal operating temperature - it may not be so at lower temperatures.

Piston will wear non-uniformly (i.e. more on 'thrust' side). 0.8mm out-of-round sounds a lot under any circumstances - I think you may have a problem with that piston.
Are Pistons Round? - lezer
Thank you for your thoughts so far.

Further to my report about the failure of the VW piston. If the piston is oval with both axes in spec., large axis 79.5mm small axis 78.7: large axis in line with thrust, small axis at 90deg. to it: there must be another reason for its failure, the other three pistons being undamaged.
How about lack of lubrication? Ther was no bore washing by the injector for all four were tested and found to be OK.
Looking inside the piston, four oil ports can be seen in the piston wall leading to the bottom ring, and these are clear. The bottom ring is partly seized in its groove. The two top rings have no oil ports at all and so must pickup lubrication from the cyliner wall. Can this be correct?
The two top rings are seized in their grooves, the lower one off centre. Original machining is visible on much of the undamaged area of smaller diameter.
There is evidence of overheating in the damaged areas of the piston but two observers noted that no coolant was missing either before or after standing overnight in the workshop.

Your observations would be appreciated
 

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