Thanks to all who replied.
On Wed, 5 Jul 2006, Navarrette, Vance wrote:
> 0.020" overbore is a routine, conservative rebuilding technique,
> and does not imply a race prepared engine.
Right, let me clarify. My belief is that the cylinder is bored larger
than .020 oversize making too much side clearance for the .020 pistons.
I was told that overboring the cylinder for any specific piston was common
race preparation technique, and could be creating my problems.
I don't currently have the proper inside mic to confirm the bore size, but
a new .020 top ring squared in the bore has .042 end gap, and Bentley says
that it should be .012 to .017. As there's no ridge in the cylinder and I
can still see cross-hatch, I'm assuming that this is not due to wear but
rather the wall is very close to the bored dimension.
> The TR deck value of -0.017 was typical for its day, and would
> be difficult to see without measuring. Unless you actually measured it,
> I would hesitate to conclude the block had been zero decked. In any
> event, it is not as insane as it seems, because there is the thickness
> of the head gasket, typically around 0.030" so there is plenty of margin
> for piston to head clearance unless you use a steel head gasket, or some
> such.
It was the slightly shinier and harder-packed carbon on the piston tops
that led me to question the piston-to-head clearance. I agree that .017
isn't easy to see in this context, but I was specifically looking for any
negative space at all, and concluded that there wasn't any. This was
admittedly before I discovered the broken pistons, and the carbon
obviously had some dimension (approx .030 if there was contact?) so this
may be a spurious question on my part.
> Because of your cracked pistons, I would measure the deck
> clearance to make sure this is not a causative factor. Cracking implies
> cyclical overloading of the piston, and its cause should be determined
> and corrected. If someone did over deck the block, your machinist should
> be able to recommend a fix. I don't know what it might take to fix,
> other than just replacing the block (they are cheap and readily
> available).
Yes, my thought was that I might be able to go .030 with the block and
just put it all back together. The Bentley book says that .020 is max,
thus my concern.
> Some other possible causes of piston cracking, some of which are
> unlikely:
>
> 1. Detonation
> 2. Bent rods
> 3. Rod big ends out of round
> 4. Defective pistons (seems unlikely)
> 5. Camshaft mistimed (piston to valve contact, again seems
> unlikely)
> 6. Rod bearing failure.
> 7. Pistons installed backwards (Yes, pistons can be put in
> backwards).
>
> If I had to guess, I would guess mis-installed pistons, but that
> is just a WAG. Have your machinist check for a root cause.
Hmmm, these are AE pistons with the stamped FRONT toward the front of the
car and the rod bolt heads all oriented toward the left side of the car.
The engine didn't ping and while there was a fair amount of aluminum from
the broken piston skirt embedded in the bearings they certainly were not
excessively worn.
Having said that, I am not an experienced engine rebuilder, although I am
getting more experience on this particular engine than I would wish for.
Pinging was never a problem with this engine but the head measures 3.45
inches. Is there a chart somewhere to get an estimate of compression
ratio assuming zero deck height?
Regards,
-Don
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