got engine back from machine shop and assembled the short block. I have a .009 difference in piston to deck height from highest to lowest. .007 average difference left side to right side. Engine will be a street motor. mild cam, 4bbl headers, 9.4 static compression.

Is this going to be an issue? should I tear it down and have the left side milled another .007?

I'm assuming that the pistons are in the hole (below the deck). Is that correct?

It will be fine but it definitely won't hurt to get them as close as possible. If you're using composition head gaskets, it will increase the compression and increase the detonation resistance of the engine. If you're using steel gaskets, you might actually want to cut the pistons. The machine shop may charge you to cut it again, which is understandable. Chalk it up to a learning experience. Next time assemble first to see where you're at then have the shop cut the deck. Once this is done you may need to cut the intake flange on the heads or intake but wait until you have assembled it to that point to determine if it is necessary.


Lawrenceville, GA

Jepito.  Those deck variances you bring up are not unusual for a block that has never been cut on before.  But you can take your assembly a step further in measuring each of the rod lengths and also the stroke on each rod throw to see exactly what those particular variances are.  The strokes can be measured without taking the engine back down but the connecting rods will need to be removed from the pistons to accurately measure those.  If there is any notable variance in either of these two areas, then the rods can possibly be juggled between the cylinders to help equalize finished deck height clearances.  If you just leave it alone, you can rest assured it’s no worse than what the factory originally put out the door.  Getting all cylinders closer to being the same in regards to deck height simply makes the engine hit more equally on all the holes and makes for an overall smoother running engine.

Lorena, Texas (South of Waco)

I have been following this discussion on deck height and I have a couple of questions. The original poster said his motor was going to be a street driven, mild cam and have 9.5 compression( static).

The questions are

1 At what point approaching zero deck (piston top to block top) would a .009 difference make any measurable difference?

2Would balancing the crank etc be of more benefit for the same cost?

3 With a 9.4 static compression ratio and a mild cam wont the ignition timing have to be retarded so much that performance is compromised

4Even with a true zero deck you still will have to use a cylinder head gasket of some thickness so how does that figure into your assumptions.

Thanks Pete

While Jepito doesn’t mention in this thread what bore and stroke he’s working with, a run of the mill bored 292 with a 0.009” variance between piston heights accounts to about 0.16:1 variance in compression ratio assuming all the combustion chamber volumes are the same.  That’s not much in the grand scheme of things but it is a variance that’s worth reducing if concerned about overall performance or the long term bearing wear.

Pete.  Balancing is in a separate category from engine blueprinting and is equally important.  It’s not one of those things that can be substituted for an engine build area that’s not up to par.

No.  But 9½:1 is a safe limit for 91-93 octane fuel with the smaller cam grinds before ignition timing constraints come into play.  As the intake durations get longer, then the compression ratio can also get larger than this and still optimize the ignition timing.  The EMC Y engine was still running at 38° total timing although the static compression ratio was 10.8:1.  This worked because the dynamic compression ratio was 8.5:1 and that’s where the camshaft that was being used was a critical part of the low octane fuel combination.  There were zero detonation issues with this combination on a fuel that was 86 octane (MON method).

As long as a composition head gasket is being used, then zero deck simply reduces the quench area which as a general rule should be no less than 0.035”.  That means you could potentially run the piston 0.005” out of the hole with a 0.040” thick gasket and get away with it.  Reduced quench allows for higher compression ratios while also reducing the propensity for detonation.  As a general rule, quench areas up to 0.065” are tolerable before detonation comes to the forefront with all else being equal.

Lorena, Texas (South of Waco)

Its balanced and the highest piston is .001 out of the hole. Thanks for the info. Didn't think it would be a major issues but it doesnt hurt to ask. Did get a bit lucky though. Measured the heads and one was deck .004-.005 more than the other. Will match them up and call it good. Will be trying a different machine shop next time though.

We checked the stroke on a stock crank ground by a local machine shop. There was .010" total variation on the crank. Also, every time a connecting rod is resized it gets shorter, Not much, but 1-3 thousands.

I had to replace a couple of rods in a set of C2AE's a while back and there was .012" difference in lengths.

I decked a Y-Block that needed .018" just to even up the decks sided to side and front back.

Put all this together and I laugh when the magazine builders assemble an engine and claim the deck is EXACTLY where expected!

Everything is machined to a tolerance and sometimes they stack up against you. "An Accumulation of tolerances" is a good way to describe it.

http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

working with .007 variation and 3.8 bore i guesstimate 1.3cc difference

another variable is combustion chamber volume ... i've measured 4 sets of heads and found 2cc variation

potentially 3.3 difference but i lack the experience to say what such a variation would feel/drive like

stuey

England