[Vwdiesel] Piston fit in block ---- ( when reboring ).

LBaird119 at aol.com LBaird119 at aol.com
Tue Sep 28 15:20:15 EDT 2004

Gavrik was concerned about ring seating and oil consumption  and I heard the
German say that VW diesels are not a straight bore.  It is like cone or 
The top part is smaller than the bottom. ---Is that true ?.

  Seems odd that it would be.  It'd be difficult to set a boring bar to 
bore bigger at the bottom than the top.  You could hone that way a bit 
but all in all, it'd be a tough do, it'd have to be very minor and the only 
purpose I could think of is to even out the bore as the top wears.  But 
then the top isn't going to wear evenly into a taper, the bottom really 
doesn't wear, so there'd really be no reason to do it. 

> Val Christian mentioned to be careful when doing compression test  -- she 
> may
> fire if you do it wrong. ----I did hundreds of tests wrong .  Yes I turn off 
> the fuel
> but I was in the habit of squirting lots of AD-100 in there to see if the 
> rings were
> the problem.  ----- bang there goes a perfectly good gauge ?. (he is right).

  Had that happen on a warm Ford 302 even.  Only tried it once on a diesel 
and then thought about what I was doing!  :-o

> Now for the pistons they are special --- and do not expand according to
> Shepherd math.    What is a camground piston ?   And one member mentioned
> that his piston was marked 10 on top --I think that means oversize. Not fit.

  Usually the overbore is indeed stamped into the top, in mm on these so 
it's .25, .50, .75, and 1.00 but only .5 and 1.00 are available aftermarket.
1st and 3rd are dealer only.  They do have the piston size stamped into 
them or most anyway.  There's also a marking for size group, listed in 
Bentley.  A couple ten thousandths difference between each of 3 groups.
  Dunno, maybe a cam ground piston is the same as a hyperutectic?  I 
can't remember what the claim to fame was for those.  Something like 
less slap on bigger clearances or tighter clearances possible, just don't 
remember.  James or Roger??

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