RE: Pistons for MC motor needed...

["Michael Pederson" <mlped@concentric.net>]

Bruce -

I don't know how "authoritative" a data point it might be vis-à-vis the Audi
engine, especially the Turbo motors, but FWIW, Greg Raven's publication
"Water-Cooled Volkswagen Performance Handbook" MBI Publishing (c) 1999
opines on Pistons,

	"There are two basic kinds ... cast and forged.  The stock [VW] is cast....
Then why use forged?  In some high-stress applications such as in full-race
engines or engines with a turbocharger, it can be difficult to control
thermal expansion of a cast piston.  A forged piston will give you greater
strength without a weight penalty.

	"The drawback is that the forged piston expands much more that a cast
piston, so it must have more clearance...the more clearance ...., the more
the piston can rock from side to side, upsetting the seal....  When forged
pistons are run loose, they are noisier and can use more oil than cast
pistons.

	"Piston-to-wall clearance for a CAST PISTON is 0.001 inch {@ .024mm.}  You
can run a forged piston as tight as 0.0015 inch {@ 0.0381mm} , although
0.002 through 0.005 inch {0.0508mm to 0.127mm} are much more common
clearances.

He goes on to state the "...spec for Mahle and Kolbenschmidt forged pistons
is .0025 inch {0.0635mm}; TRW pistons are often run at 0.0045 to 0.0055 inch
{this would be 0.1143mm to .1397mm}.

Else where I've also come across a reference to possible piston alloy
choices (high silicon aluminum vs. a lower silicon content aluminum) that
may allow some "tailoring" a piston to fit tolerances.

Would like to get a figure for "stock" piston weight (piston, rings, gudgeon
or wrist pin and circlip) for comparison to the J&E et al forged
alternatives.

Mike Pederson

-----Original Message-----
From: Bruce Bell [mailto:bbell04@ibm.net]
Sent: Thursday, November 11, 1999 10:17 AM
To: mpederson@bigfoot.com
Subject: RE: Pistons for MC motor needed...

Data point: the "cast" pistons for an 85 Jetta GLI has an 80.985 diameter
stamped on top. I know technology is getting better, but, I understand
forged pistons need considerably more clearance than cast. Also, with Audi's
luxury car image I would expect them to use the quieter, more oil friendly
cast pistons.

Bruce


FWIW I had an "AAN" motor piston and rod assembly briefly.  There is a
recessed dish in the top of the piston if that's what you are referring to
as an "...offset dish."   Scribed in the top of the piston, right above the
point where the two intake ports look like they must dump their charge & the
plug fires (i.e. marked by a dark carbon stain) are the numbers "80.98"
which I believe are the stock piston diameter.