Deck Height

[zcoker]

I’m only starting this thread because there are published figures out there on 911 deck height yet many disagree and always have some form of specific reason to justify their measurement, whether it hard core experience or basic air cooled engine knowledge. Okay, so what is it? Many on this forum say around 1mm. Wayne, however, publishes that a .050-inch or LESS deck has the potential to damage the engine. The recommendation in his book is 1.25mm to 1.50mm. That’s not 1mm. Then there are those other pro engine builders who claim it to be 0.035-inch to 0.040-inch, which is in the danger zone, accordingly. Should not we clarify this?

[PFM]

Zcoker,

I will not place myself with the master builders here but the question you pose is not cut and dry. What deck on what motor? What bore? What stroke? What RPM? What rods? What rod bolts? What piston? What piston to bore clearance? Quite simply all of the above have affect on what deck height is "safe" and or even recommended.

Just my two cents worth,

Stay tuned,

PFM

[zcoker]

Let's keep it cut and dry and start with stock engines, 911 or 911 turbo.

[Steve@Rennsport]

.040".

Beyond .043", you have a recipe for detonation.

Many of our race engines have less than .037", however those contain very carefully chosen components and are assembled with care,...

[MBruns]

I have looked at the factory dimensions for a long time on virgin engines and have found 1 mm to be the target, The setups we design on custom rod lengths,custom pistons etc.. all are designed for .040 in neg. deck. I have seen some unusual deck hgts. on the 935 and early 962 air cooled engines that fall into the .070 range, but they are a different animal. If you reverse engineer the stock factory combos pour all the vols. and change the deck hgt. values you will find that the range of .040 gives you the C/R that is in the spec books, now if you add another .015 plus to that deck it can change the C/R by 1/2 a point.

Mike Bruns JBRacing.com

[Wayne 962]

In the book, I discuss following the guideline laid out by Walt Watson at Competition Engineering (1.25mm to 1.50mm or .050" to .060"). For higher performance engines where you have checked all of the clearances and can be assured of no conflicts, I recommend running 1mm or about .040". If you run it really tight on a mag case that has not been properly machined, then you might run into problems.

-Wayne

[911TES]

Hello All:
Question on deck height.
Background Information:
1969 911T Stock 2.0L motor
Re-assembling top end with one new piston. New rings and reconditioned cylinders. Piston number 3 had both skirt "tangs" broken off and recovered intact.
Cases never split. All original components 67k miles.

To my question: When I measure the deck height in accordance with the pictures shown I get a value or 0.40mm with the 0.25mm copper gasket and predictably 0.65 with the 0.50mm gasket.

Basically, I'm re-assembling just as it was prior so the ~1mm deck height suggestion is not making sense to me.

The 2.0 T engine has the head gasket that is "flat" between the head and the cylinder. I.e., it doesn't fit in a groove. Additionally the used (compressed) gasket is approximately 0.65mm.

I'm trying to pick the "best" thickness of copper cylinder to case gasket. and I'm thinking it is the original 0.25mm thickness - correct?

Note: Although I want the right number I don't think it's critical within the range in question. The piston will fit several millimeters "up into the head without interference."

Thanks, Bill

[snbush67]

Use the .50mm gasket or even stack a .50 and a .25 to get close to that 1mm. Otherwise you are losing volume and elevating your CR which could mean detonation.

Looks like you know what you are doing, make sure the piston is TDC and not wobbling, Also have you torqued the cylinder to spec?

Shane

[brads911sc]

At the edge of the piston (like you show in your pics) I measured 1.0 mm. When using the Flux Core wire like Walt at Competetion and Wayne outlines.... the same piston is slightly over 1.25, almost 1.5 mm. So I am not sure that the flux core piston measurement is going to always be exactly in synch with the edge of the piston measurement. The measurement you show is generally for Flat Top pistons. Just my understanding of it. I am by no means an expert...

[911TES]

Thanks Shane and Brad.

Yes, the piston is at TDC and I believe the wobble is eliminated. The confusing part to me is that the "stock-from-factory" set-up that I've dismantled had the 0.25mm gasket.

The torque on the cylinder is a good point. I don't really have the correct set-up with the nylon washers and all, but I believe I eliminated the tolerance build-up that would occur.

At this point, my intuition is to rebuild it just as it came from the factory, but there will not be 1.0 mm deck height as commonly measured.

I believe the 2.0 T is different from most in that the head to cylinder gasket is additive to the stack. The gasket when compressed is approximately 0.65mm which coincidently, or not, adds to approximately 1.05mm.

Note: While this is not a flat top piston, I'm measuring it as an approximation of one. As a last step, I'll probably measure the depth that the piston can "go into the head" Just from eyeballing - It appears to be several mm's.

[snbush67]

Originally Posted by 911TES

The 2.0 T engine has the head gasket that is "flat" between the head and the cylinder. I.e., it doesn't fit in a groove. Additionally the used (compressed) gasket is approximately 0.65mm.

Your are right, that head gasket is supposed to be included in the deck height measurement on a 2.0. With the .25mm gasket you should be good to go. So as you are measuring above, now include the thickness of the gasket to your deck height.

You are measuring now at .40mm then you add the .65mm of the head gasket, you end up with 1.05mm deck Height which is perfect!

You should stilll dry build the #1 cylinder and head to check deck height and piston clearance.

2.2 liter engines and later do not have a gasket that adds thickness in this measurement, only the 2.0.

Reference: Page 146, How to Rebuild and Modify Porsche 911 Engines 1965-1989, DEMPSEY, W.R.

Shane

[911TES]

Thanks Shane. Good catch on the reference in Wayne's book. I'm sure I read it several times and just missed it. Wayne - good catch to include the reference.
Cheers all.

Bill

[snbush67]

I see that you figured it out first. But it is good to have it referenced.

I guess we have a pretty decent question for the next round of Porsche 911 engine building Jeopardy.

Good luck with your build.