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Originally Posted by ajwans

I am a bit confused about this. Is this the piston edge to cylinder top measurement or the piston to head clearance?

I'm putting together a 2.0S and it seems I have tons of clearance above the piston dome, maybe as much as 4mm while the deck height is close to zero (but will increase with base gaskets.) I can't see how the factory would have ever got a 1mm deck height given the 0.25mm base gaskets I removed during disassembly.

The cylinder to head gaskets may add a little but if I understand correctly they should squish down to allow the cylinder top to meet the shoulder of the head dome adding no additional deck height. If otherwise, how is this additional deck height measured?

andy

Andy,

Let's go back to your original question before this thread took a semantic turn.

Your 2,0 S pistons can be thought of in two pieces. A perfect cylinder, for the bottom piece, and a dome, on the top. If you were to take a bandsaw and cut the dome off, so that there was exactly 34mm of distance between the center of the pin bore and your cut, you would end up with a flat-top piston and then a dome. If you measure the volume of the dome, you can use that measurement in computing compression ratio.

Now, in practice this is not what we do. Some pistons, JE being a good example, actually have a flat ring around the outside of the piston top, to make it easier to measure deck height. You can actually set up a dial indicator on the edge of the cylinder, zero it out, then move the indicator over to this ring and measure deck directly. On a typical 911 engine, the deck is "positive" which is a bit misleading-- positive deck height is when the piston's theoretical deck is below the top of the cylinder.

This is the measurement that Steve is referring to when he says you want 1,0mm deck. This 1,0 mm deck is a good place to start, you don't want the piston sticking too far out of the hole, as you will end up with very high compression indeed AND will begin to impinge on a healthy piston-to-cylinder head clearance.

Interestingly, but not relevant, is the fact that 964 pistons actually have negative deck, they slide up into the head.

All right, how do you measure deck when you have a 2,0 Mahle piston? Look at the edge of the piston where the cylindrical wall stops and the dome begins-- it's just a smooth radius, there's no ledge to measure from! If you look in Bruce's book you will see a photo he took where he uses a height gauge and a test indicator to measure the distance between the height of the dome and a spot on the piston wall that is 34mm up from the center of the pin bore. (He probably subtracts half the diameter of the pin from 34mm and measures from the top edge of the pin bore.) Not perfect but it works.

A couple other points-- the 3,2 motor used a 32,8mm compression height- I guess they moved the pin up when they lengthened the stroke. Not a factor for you.

Also-- we think that Mahle used a ZERO deck to compute the stated compression ratios. This would make sense because deck height varies, so if you zero it out it gives you a reference for everyone's engine, and the actual static compression works out lower in practice. I credit Kenik with discovering this.

SO- to answer your original question- the factory achieved 1,0mm deck with normal crank, rods, spigots, spec cylinder height and 0,25mm base gaskets. S pistons have something like a 45cc dome, this is how they got to 9,8 to 1.

Hope this helps! Email me if you need the calcs.