>I don't know what dimensions new pistons are supposed to be. Am I
>observing an excess wear condition? OR are pistons suppossed to be tapered
>like this and oval shaped?
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Actually, yes they can be. Pistons are designed this way because of the
differences in operating temperature, and the forces involved while
running. Most of the parts in an engine are designed for the temperature
at which they will be operating at, not the temperature that the temp gauge
says. That is why it is important to let the engine warm up to normal
operating temp before pushing it. The pistons, in this case will be much
hotter at the combustion surface or in the ring area and expand much more
than at the skirt which will expand less and doesn't run as hot, and
therefore to operate with the same overall design clearances the ring area
of a piston will typically be of slightly less total diameter than the
skirt, so that as it expands it will be at the same dia as the skirt at
it's normal operating temperature. Also pistons are oval to allow for the
same expansion in the area of the piston that is not cooled by contact of
the piston pin and the forces exerted on them by the combustion process
itself. If they were not oval, then you would be scraping the sides of the
piston on the cylinder walls at 90 deg from the piston pin. And you
thought they were just round :-). At the operating temperature that the
piston is designed for they are round. If it meets the specifications in
the manual, at the points specified, and are not otherwise damaged, then by
all means re-use them
Barry Schwartz (San Diego) bschwart@pacbell.net
72 V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (project)
73 Ford Courier (parts hauler, rain vehicle, getting a V6 soon!)
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