Opinion on deck height

[pampadori]

My heads had .016" machined off. Based on measurements EBS took, .010" during a previous rebuild and another .006" this go around. Knowing this, I was thinking I should shoot for a larger deck height. Maybe around 1.5mm. Does this seem like a good idea or will the machining on the heads not really make that much difference?

I'm using 9.3:1 factory Alusil CIS pistons with nikasil coated cylinders if that matters.

[Tippy]

The taller the deck height, I believe your chances of detonation go up (less squish) and also lose some compression resulting in less power.

[pampadori]

thats true but since the heads were machined a little on the heavy side, wouldn't the squish be much smaller given its a hemispherical shaped head? Does that make sense?
Maybe i'm over thinking it but i thought there might be some difference from having so much material removed.

[Tippy]

I just hung on your 1.5mm deck height. Seemed pretty big?

[Eagledriver]

I think your normal 1mm deck height is fine. You will just have slightly higher compression ratio. 9.3 pistons only give about 9.0 anyway. Just check your piston to valve clearance when building.

-Andy

[WERK I]

Deck Height should not be altered to compensate for head milling. Valve clearance is probably OK, but doesn't hurt to make sure.

[Trackrash]

How are you measuring your deck height?

In theory you should shim up the cylinders to compensate for the amount removed from the heads unless you want to raise your compression.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/937294-rebuilding-my-83-3-0-a-2.html See post #38

[pampadori]

i'm measuring deck height with a vernier caliper with two nuts hand tight securing the cylinder to the case. I'm getting lots of variation from cylinder to cylinder and I'm shocked it isn't recommended in Wayne's book to check all 6 cylinders.

I guess next up is pre-assemble and check clearance. WHat is the percentage of folks that don't have proper clearance? I can't say i've ever read a post where clearance wasn't ok.

Thanks for all the replies and help!

[WERK I]

Percentage is low with original cams and pistons. High lift cams with wide lobe separation and/or aftermarket pistons with higher compression (higher dome and modified valve pockets) are a different matter.

[Henry Schmidt]

Maintaining the engine dimensions is always something to shoot for.
.060" deck height will function just fine.
With .016" removed from the head, I would try to run a base gasket combination that most closely replicates the original engine dimension.
If a single .020" base gasket gets you close, run it.
If you really want to make it "exact" you can trim the barrels (at the base) to match a thicker base gasket (IE: .040") which are available.

[m42racer]

You are in that awkward position of how to move forward from here, I guess.

First off, there should have been checks done before getting to this stage so as to determine where the differences may be generated from.

Case deck surface parallel to main housing bore? Same deck height both sides from Crank CL?
Crankshaft stroke. Same on each throw?
Rod length CCL, same for each rod?
Piston Comp height, same?
Cylinder Heights same?

If these checks were done, then the only question will be the amount of deck height you have , not any differences. Make sure you are measuring across the pin direction so to eliminate any piston rock. If its too late to go back and check everything, or you cannot be bothered, then take the least measured piston deck height and use that as your minimum clearance dimension and the maximum piston deck height and use that as your CR number.

As for deck height number, this is a safety thing. This can be as tight as you wish it to be. There are two factors that should be considered here. Valve to Piston clearance and final static compression ratio. So before you do any anything, check both of these. Then either one or both will give you some idea if you need to increase or decrease the deck height. Unless you have some custom piston, the piston design should be OK and not an issue.

If you have differences in the deck height, I suggest you determine what is causing this now, fix that first, then do these two checks.

As for deck height clearances, there is no reason why you cannot run with the piston just cleaning the carbon off the chamber roof without hitting the head. Also, check the plug clearance as well. If the P/V and the CR numbers are OK, then run it as close as possible. There is no magic number here. As long as the P/V and the CR #'s are good, fear not. For years there has being some number put out there that you need "X" deck height. This along with many other myths, have being carried over for years and engines are continued to be built the same way "George Washington" built his Porsche engine.

Don't be scared of deck height and don't be driven by this number. Do the other checks first and the deck height will probably take care of its self. If you have to make a deck height decision, then go with the maximum of 1.00mm. Unless you are rev'ing this engine to some high number you will not experience any "stretch" of the internals.

Just make sure any measurements you do are correct and eliminate any error.

[Henry Schmidt]

The reason for a generic "deck height" number is consistency of the build process.
When you shorten the stack (head, cylinder, base gasket and cam tower) you effect the relationship between cam, chain, chain box housing and tensioners.
It also important to remember that "most" pistons are designed to function with a specific deck height. Valve pocket depth, dome shape, piston height and static compression are all a function of theoretical deck height.
Of course you can shorten chains, chain boxes and calculate static compression ad nauseam but why if all these measurements are simple when you keep the engine dimensions (stack) consistent?

[m42racer]

Absolutely. No question about the design built in on Pistons etc.

The chain tension has to be considered when shortening the engine width as well, which I did not mention.

My point was to show that the deck height should be one of many factors to consider and that the deck height should not be the end all of how the engine is built. That the CR and P/V checks should be the determining factors.

To add deck height and not consider these could render a poorly functioning engine.

[boosted79]

Ignoring the cam drive consequences I would think on these engines with aluminum cylinders and steel rods you could go tighter than 40 because of the growth of the cylinders when hot vs the steel rods. On an iron block you have to run more with al rods vs. steel because of expansion of the al rod and because they stretch more at high rpm. The tighter the better as long as you don't hit. You can always deepen the valve pockets if you have room piston to head.

[Aircooled4evr]

I have slowly started on my 3.3 70.4X100mm engine project.
First: I am completely free to choose as I want, as I "do" everything. Longer connecting rods, machining the pistons, my own base gasket. The engine wide remains org. My question is, is there experience out there, with low deck height? On steel connecting rods, and factory pistons (964) and rpm limit of 7000.? (or in the neighborhood of)
Right now I am planning 0.4 - 0.5mm deck height. to achieve 11: 1 final static compression ratio.
(with the org. heads and pistons, and only 70.4 ... not 76mm stroke, I have a long way to go)

[KTL]

When you asked about reduced deck, the first recollection I had was Grady Clay (RIP, such a good guy in so many ways) mentioning how he chose to reduce deck height on his race engines.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/173331-so-how-light-ti-valve-retainers.html#post1421771

His engine formula was much more aggressive than yours in terms of compression ratio and peak rpm. I recall Grady saying his engines ran well into 8000+ rpm. Do a search of his name and you'll find some other threads with his mentions of deck height, factory Porsche valve springs, and other stuff. He really knew his craft.

My search terms were "deck" and "shadow" with Grady's username, because I recalled by memory how he mentioned he reduced his deck height incrementally until he could see a shadow of the valve on the piston. That said, you also have to make sure you look at the piston-to-head clearance at TDC and also measure your piston-to-valve clearance along a range of the timing curve (not just one point that you believe is the "peak" valve timing event)