sidney raper wrote:
>
> Marc,
>
> I would have to disagree with the notion that just because pistons are
> relatively cheap they should be replaced. I am of the opinion that the
> serviceability is the issue. If the bores are still within spec and the
> pistons are still within spec, what is to be gained (other than helping the
> machinist's kid go to college) in boring the block and buying pistons? Now,
> I will agree that if the pistons are dished, it is a wise move to get
> flattops or even popups for more compression (to a point), but if all is ok
> otherwise, I would reuse the pistons and have no qualms about calling it a
> rebuild.
>
> Sid
How will you determine the condition or serviceability of the pistons? Obviously
the gross physical dimensions can be checked, but what about ring lands? Are
they still perpendicular to the bore centerline? On both the top and bottom? How
will you check this? Are the surfaces of the lands correct, i.e. will the rings
bed properly? (you did know that rings bed not only to the bore but also to the
top and bottom of the lands, didn't you?) Again, how will you check? Is the pin
bore still straight and perpendicular to the bore centerline? Again how will you
check? What sort of hardening or softening of the metal has taken place? Is the
metal hardness uniform? Again how will you check? Pistons are heat treated and
all heat treated parts that go through significant heat cycling in operation are
supposed to be thoroughly checked or replaced. Why? Because changes take place
that can not be observed with the naked eye. Will you have the pistons checked
for cracks? (No machine shop I know of does this normally, yet a piston can be
cracked in ways you can not see. Why not? Cost. It's cheaper and much smarter
with most engines to just buy new pistons). Where will you get the specs for
metallurgy, and heat treating, and all the minute and exacting dimensions that
need to be checked? Car makers do not make that info available, though it does
exist. Why not? Because they expect you to renew parts during a rebuild. My
point? Just because a part does not exhibit symptoms of a catastrophic failure,
does not mean it is still good. And when it comes to some sensitive or mission
critical parts, such as certain critical fasteners, pistons, valves (at least
exhaust valves), etc., unless you have to reuse, it is just plain foolish to
risk it.
A *rebuild* means new parts, otherwise its just a freshen up. If I put in new
bearings and rings, and clean things up, it is NOT a rebuild, it is a freshen
up. Drag racers don't claim they rebuild the engine between runs, they say they
freshen it up (and they usually replace most parts with new). A rebuild *by
definition* means new parts. Anyone says otherwise is really misusing the term
and blurring the line between good work and shoddy work.
As I said, with *some* engines (1500 for example) you may reuse parts because
you have to, or because checking the parts out *properly* is still less
expensive than buying new. But with a 302 it would cost you much more to
properly check out the pistons than to buy new, so there really is *no excuse*
for reusing them.
There is a reason most new motors get 2-3 times as many miles as "rebuilds."
--
Marc Sayer
82 280ZXT
71 FJ510
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