Deck height variance in pistons?!

[cstreit]

I installed my Mahle pistons and cylinders # 1 and #4 on the engine tonight and noticed that from top to bottom or from top to bottom there was a deck height variance by as much as .007" on them.

Is this normal?

Thanks,

[turbo6bar]

Case height and rod lengths OK?

.007" doesn't sound horrible, but you need to run the numbers and see if 7 thousandths is enough to change your CR significantly. My brother and I are building up a Rover 215 V-8 for his MGBV8 conversion, and deck heights varied approximately 0.005" from tallest to shortest. However, we felt it was not enough to justify extra work, since this was a lower compression engine (8.5:1). To remedy it, you might have to machine the piston tops.

That is just my amateur opinion.

Jurgen

[HawgRyder]

If the cylinder heights are OK....then the pistons would need to be cut to bring them even.
However....I agree with Jurgen...the difference of .007" would not change the ratio by more than .07 in the affected cylinders.
I base this on the numbers I use to increase the compression ratio....to cut the cylinders...or shim the bottoms....about .020" for each change of .2 in compression.
Bob

[cstreit]

As it turned out, the pistons were just rocking slightly in the bores. Once I figured this I could move them slightly and get an even deck height measurement top to botom. The deck heights are all within a 2 thousandths on a side.

[turbo6bar]

Chris, one things you can do is measure the deck height directly above each piston pin. This will help to eliminate error due to the piston rocking in the bore. 0.002" sounds pretty darn close. Without running the numbers, that probably does not affect your CR very much. In a perfect world, all numbers would be spot on, but...

Jurgen

[HawgRyder]

Chris...the rocking in the bore is what normally does in the rings.
This is why we used to Teflon button the pistons..so that they rode up and down in a straight line (kept them from rocking).
It helps to keep the rings flat against the bore so that the ring outer edge doesn't become rounded.
I believe that autothermic pistons do the same thing because they are ground like a cam...out of round...so the side to side in line with the wrist pin is smaller than the side to side at 90 deg to the pin.
This negates thee rocking motion also.
I don't know if anyone has tried the Teflon trick...I might do mine that way in the rebuild to gain some insight in P-car engines and how they react to teflon.
I am going to use Total-Seal rings when building...to reduce the blow-by and gain some compression.
Bob

[911pcars]

I thought I'd contribute a few $.02.

The deck height for each cylinder depends on the many variables inherent in an engine with individual cylinders and cylinder heads:

- Are the crankcase spigots machined parallel to the crank center line?
- Are the cylinder heights the same?
- Were the con. rods rebuilt so the distance from small-to-big end the same?
- Are the piston heights the same? (pin-to-piston crown)

If so, then the deck height should be within spec. Another important factor is to have the same clamping force between the cylinder heads and the three cylinders on each side. You can only achieve this if the individual cylinder heights are equal and the thickness of each cylinder head are the same (distance from head gasket surface-to-cam housing mounting surface).

Best wishes,
Sherwood Lee
http://members.rennlist.org/911pcars

[cstreit]

THanks all... These are brand new Mahle's and I'm going to hazard a guess that they're within spec height-wise... I'm wondering though what the tolerances are for varying heights.

I'll see what I can devise to check the heights of the cylinders against each other, because obviously you need that to be very close.