Rebore one pot only?

[isham-research.freeserve.co.uk at pop.pol.net.uk]

>>Has anyone ever tried to repair an engine with _one_ badly
>>scored cylinder bore by reboring just that one pot?

> Yes, this can be done. Caveats: Match weight of new piston to weight of
> removed piston as closely as possible. You might get very minor vibration
> at high RPMs. (the overbored cylinder is making .0001 HP more than the
> stock ones) Keep the difference between overbore and stock as small as
> possible. (+.010" will be OK, +.500" won't be.)

Mahle tell me the oversize piston is adjusted (different depressed crown
and wrist pin location) to modify the compression ratio.  They say
horsepower is largely related to inducted fuel/air mass, and this
doesn't change significantly with overboring because most of the
limiting factors are in flow.

You can check this out by going to the Mahle site (www.mahle.de) and
downloading the catalogue - about 10MB.  It's a bit idiosyncratic and
German, but it lists 0349200 as the 'standard' AAN piston, 0349201
as the 0.25mm oversize, and 0349202 as the 0.50mm oversize.  The
critical dimensions given for each (wrist pin location, crown dimensions,
etc.) _ARE_ different.

> This sounds like you're trying to bail someone out who needs to fix their
> car as inexpensively as possible. The "standard" solution, as you are well
> aware, is to do all five, and of course the dealer's answer will be to buy
> a new A6 at full retail and junk the old crock.

Absolutely right.  However, the more I discover about the technology of
oversize pistons, the more I'm beginning to think this might be a
perfectly acceptable repair technique in _all_ cases of single pot
scoring.

--
 Phil Payne
 UK Audi quattro Owners Club
 Phone +44 7785 302803   Fax: +44 7785 309674