Rebore one pot only?

[Ameer Antar]

Well, didn't think this would be much of a controversy. The price you're 
getting for pistons is insane. I can get custom forged pistons w/ pins for 
about $100 each for 5kT. Check out a custom piston place, they might be 
able to help you, and forged pistons are much stronger and knock resistant 
than OE cast ones.

Oversize pistons aren't the same weight, they're bigger. And the 
counterweights are there to balance each other, not the piston and rod. If 
the counterweights are not balanced, you will have vibration, b/c gravity 
pulls one end of the crank down more than the others. The others go down on 
different strokes, causing a vibration and maybe bearing damage. The weight 
of the piston and rod is not balanced w/ the counterweights.  In fact, you 
can balance an engine by balancing all the parts separately in sets. All 
pistons must be the same, all rods must be the same, all counterweights 
must be the same. Imbalance only occurs when one of a set is not the same, 
b/c that one rod or piston will be pulled down more by gravity than the 
others and since the rod or piston goes down at different times, vibration 
occurs. Maybe find a new block. Short blocks from Audi might be avail. I 
was quoted $500 for an '84. Do some searching, instead of compromising. All 
I'm saying, is that if you went to the trouble of pulling out the engine 
and rebuilding it, you might as well get everything done right. I really 
hope no shop is rebuilding engines this way. I don't understand how they 
would be able to guarantee anything. good luck.


At 03:27 AM 9/27/00, you wrote:

> > no one will sell you a single oversize piston. it affects the balance of
> > the engine since one piston will be slightly heavier and not move w/ the
> > same force as the rest. That bore will have more volume and affect the
> > compression ratio. These are all minute problems, but an engine can destroy
> > itself down the road. Your Audi engine was built and designed w/ fine
> > tolerances which is why many of their engines last so long. No competent
> > machine shop will bore only one cylinder. They all must be done, b/c the
> > savings is negligible, esp. compared to engine rebuilding.
>
>Both Mahle and Audi will sell single pistons.
>
>The savings are NOT trivial.  Oversize pistons for the AAN engine list
>at GBP135 - about $190 - _EACH_.  The saving if only one cylinder is
>rebored is thus about $800.
>
>Since the counterbalance weights on the crank are not touched during a
>rebore, I assume oversize pistons weight the same as standard ones.
>The compression ratio delta is negligible compared with the 2 bar
>discrepancy allowed in the service documentation.
>
>--
>  Phil Payne
>  UK Audi quattro Owners Club
>  Phone +44 7785 302803   Fax: +44 7785 309674
>
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