Deck height dilemma
 

I am in the middle of my first engine rebuild and I have a deck height of 2.2 mm. I am thinking this is way too high and seem to recall reading something Steve said about too big a deck height being as big a problem as not enough deck height. Any advice?

http://forums.pelicanparts.com/uploa...1285328615.jpg
http://forums.pelicanparts.com/uploa...1285328776.jpg

Engine specs
3.0 L
Pauter rods
Mahle 95P30 W2 10.5:1

Thanks,

Andy

I built a motor very similar to yours. The experts suggested a deck ht around 1 mm.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/224742-advice-deck-ht-comp-ration-shims-my-3-0-a.html

I found the easiest and most reliable measurement was with a vernier caliper. Those Mahle piston have a nice flat around the perimeter for measuring the height.

When using the caliper it's critical to make the measurement exactly at TDC. I used a dial gauge on the piston crown to determine exact TDC.

Steve's advice is right on (as ever)

Too much deck can promote stagnation of the flame front out at the edges, you want a good squish.

What copper spacers are you using under the cylinders?
Did you modify the rods?

Tom's right: you have the advantage of a nice concentric shelf on the edge of the dome. After you are absolutely certain that the piston is at TDC, don't just go by the Z1 mark on the crank, actually use a dial indicator on that shelf, then you can measure the deck between the cylinder edge and the shelf.

Install the Dial indicator in your z-block and put some nuts on the top to hold it firmly to the cylinder. Locate the point of the indicator on the shelf. You may want to source a different indicator tip, these are cheap. Z

While supporting the other rods (some use rubber bands), turn the engine slightly past Z1 until the indicator has a positive reading, then zero the indicator i.e. push it down so the indicator reads 3mm and then turn the bezel to line up zero with the pointer. Or if you are using a digital indicator just hit the damn zero button (yeah!)

Then turn the crank the other direction until the numbers increase, stop and then begin to decrease. We're talking a couple of crank degrees here, don't overdo it. The point at which the numbers STOP, that is TDC. Make your measurement at that point.

If you're measuring deck height with vernier calipers make sure they are a) vertical and b) measuring on the side of the piston parallel to your wrist pins (not the top and bottom).

Looks like you have your flame ring installed there? You aren't measuring to the flame ring top are you?

Oh, also noticed you don't have the cylinder bolted down there! Are you bolting it down before you measure?! I use sections of pvc pipe with the head nuts.

Thanks for all the help so far. I used .25 base gaskets and crushed solder when taking measurements, i will recheck all six deck hights on Monday using vernier calipers as I am thinking the solder might be slipping away from the edge of the piston. Both pistons and heads are polished so this might be what the problem is.

Cheers,

Andy

FWIW I never had success trying to use the solder method and always got overly large measurements. Never sure why.... Interested what your veneir caliper measurements come out to me. Hopeful that it will be better news.

Andy, are you measuring deck height or piston to cylinder clearance? Deck height is the theoretical measurement between the flat top of the piston (and sometimes the "shelf" if it is present) and the top of the cylinder. It is the measure of how far below the cylinder top the theoretical deck is, both for determining combustion chamber volume and for consistency of measurement.

Piston to cylinder clearance is something different, that is the difference between the top of the dome and the cylinder head. Two completely different things that are often confused, even in Wayne's book.

"Piston to cylinder clearance is something different, that is the difference between the top of the dome and the cylinder head."

If that statement is worded correctly then what do you call the clearance between approximately 1/2 inch up from the bottom edge of the piston skirt to the cylinder wall 90 degrees from the wrist pin?

Sorry, piston to head clearance. Piston to cylinder clearance is something different entirely. . . :)

Well what's the veneir caliper verdict? Inquiring minds want to know!

Ok have rechecked with vernier callipers and all deck heights are 1.2 -1.3 mm, i totally misunderstood Wayne’s book about using solder for the measurement.

Thanks again for all that gave advice.

Andy

Aha. This is what I suspected. That's exactly what happened to me every time and why I use the calipers. Great news!