Dick Taylor wrote:
> Stuart---Grade "B" pistons are slightly larger than those stamped with
> an "A". It is odd to find them mixed, but we're only talking about a
> difference of .0005 or so.
>
> Dick
The other significant difference is that it is obvious the pistons are an
unmatched set, so that their individual masses may not be equalized either.
AE / Hepolite pistons used to come in matched sets according to their size,
as Dick stated differing by very small amounts, but uniform across a set of
matched pistons. AE / Hepolite piston sets also had matched weights to
within 1gram of each other. Quite nice for 60s and 70s piston sets.
AE pistons are now produced by Federal Mogul, the casting technology they
use has improved to the point that they no longer grade their pistons and
final machining for batches of pistons is consistent enough that they still
have relatively equal weights. I have not seen any actual piston "sets"
from AE in the past 4 years, pistons are all boxed individually, and when
you order a "set of AE pistons" you get 6 individually boxed AE pistons.
It is still good practice to have your machine shop check the piston
diameter at the first ring land and the bottom of the skirt to ensure
uniformity across all 6 pistons. Weights should also be double checked as
well. But then again, I am a member of the Obsessive Engine Builder's
Association ;-)
The difference in grade in one cylinder may also indicate a piston that was
previously replaced, rather than being fitted at the factory, which is
highly unlikely. Blocks were graded during production, so if you had a
block with Grade B cylinders, Grade B pistons were fitted to achieve the
correct piston to wall clearances. As blocks are machined on an individual
basis, the tolerances per block should have been tight during the final
machining that only a single grade of piston would be needed... I have never
heard of a mixed set being fitted.
Kai
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