Don---I will assume that your suspicion that the block has been decked,
as part of my answer.
Note that the area above the top ring land on the pistons is about .030
smaller than the main diameter of the pistons. This should allow plenty
of clearance, even on oversized pistons, to clear the head gasket's
O-rings. The gasket itself would have to be seriously misplaced to make
contact with the pistons. An examination of this area of the gasket
should be a clue here.
As Shawn mentioned, even if the pistons rise above the deck height, no
contact should be made with any stationary object.
But something is breaking your pistons, so I would suspect that it is
detonation. Either from excess compression for the level of octane being
used, or ignition timing being too far advanced for the above. Other
factors enter here, such as air/fuel ratios being too lean, fuel
distribution, etc.
When you get the engine back together, measure the height of this #6 at
TDC compared to the others. It could be that the rod length may make
this cylinder's compression higher than the rest, pushing it over the
edge.
An interesting subject!
Dick
From: Don
Hi, this is my initial post and I have some questions about "zero
decking". I pulled the head off of my 74 last week to chase down some
stubborn tapping noises, and found some problems. The most shocking
thing was my #6 piston: http://zeni.net/~derick/brokenpiston.jpg but #1
is also cracked.
The deck appeared to be zero-decked by eyeball, but I didn't confirm
this before removing the pistons. The piston tops show compressed carbon
where the solid part of the head is above them.
Does it seem like the fact the the pistons were essentially hitting the
head with a "cushion" of carbon be a factor in the state of the pistons?
It was suggested to me, and I currently believe that the block must have
been race-prepared with the cylinder walls overbored for the .020
pistons that were in the car when I got it. This would explain the fact
that this is the second and third broken piston since I acquired the
car, and the strange wear pattern on the rings. The only history on the
car when I bought it 15 years ago was that it had spent the previous 8
years in a field.
The Bentley book says .020 over is maximum on these engines, but I've
seen pistons advertised as high as .040 over.
So, I guess that my non-deck related question is, is the .020 maximum
bore conservative, or is the .040 option crazy? Or both?
Thanks for any attention or advice,
-Don
|
