Help with Deck Height measurement

[Dauner]

Hi everyone,

Car is 1989 911 3.2. I am struggling a bit with deck height repeatability. Cylinders bored to 3.4 but no machine of cylinder height. No machining done on Heads affecting height. No machining done on case. When I measured deck height with no cylinder base gaskets last night on cylinders 1 and 4, I was getting average of 0.020" on #1, but closer to 0.016" on #4.

My question is, are the cylinder heights in tolerance groups on the 3.2 engines similar to how things were in earlier engines? My cylinders have markings like 3B, 4B, 4A, etc. on them.

What is the tolerance for deck height variance cylinder to cylinder?

I know the ultimate goal is to have flat surfaces to ensure no torquing of the camshaft carrier. I'm just not sure what's going on.

I'm measuring deck height using a standard Mitutoyo digital caliper from the cylinder top to the flat on my JE pistons. I'm pretty sure I'm at TDC, but will bring a dial gauge and mag block home tonight to be 100% sure before measuring.

In talking to my two machinists, there shouldn't have been a need to do any work on my block seeing as car ran well with no major issues except for leaky valve guide seals, and everything was pretty clean and orderly on disassembly.

Any advice greatly appreciated.

thanks,

mark

[Dauner]

Quick update. I measured all of my cylinders from spigot mating surface to head mating surface. All 6 cylinders pretty darn repeatable at 3.365". That's not my source of variation.

I need to stare at my shoes a bit and think about how to remove other variables. Or, install all 6 pistons and cylinders and see if deck heights are consistent on each bank, and if I have flat plane between all three cylinder tops per bank.

Mark

Thx

[Catorce]

As someone who actually makes cases, let me tell you that the Porsche cylinder spigot mating surface is rarely as flat as you think it is.

I measured cylinder spigot surfaces that range in flatness up to 5 thousandths and even more. The machining on these motors is very sketchy, and not flat at all.

So knowing that you are starting with a "not flat" surface, you need to make it as flat as possible with cylinder base gaskets and get yourself in the ballpark. It's never going to be perfect. My case has a spigot flatness of about two tenths, which is very good, but still not perfect.

Oh, and there will be height differences in your cylinders as well. And you won't properly be able to measure either of these with your caliper, it;s just not the right tool for this.

[Eagledriver]

The cases are not all machined perfectly. My deck height was different side to side. Sometimes you can get it closer with different thickness base gaskets.

-Andy

[Dauner]

Thanks guys,

I was on a short break due to accidentally pulling a cylinder off of a piston (again). I ended up buying a Wiseco piston ring compressor (tapered cylinder), which worked great.

Last night installed all pistons and cylinders and this morning measured several times. I have same .5mm base gaskets on all, with PVC bushings and barrel nuts "snugly finger tight".

before measuring each piston deck height, I am establishing TDC with a dial indicator.

in the pics below of my notes, you can see I have measurement repeatability issues, but am using a digital 4 place micrometer with last digit being 5 or 0. JE pistons have a flat so deck height measurement easier. for the engineers, disregard the significant digits issues...

My questions are as follows:

1. do I need to torque down to some level before i measure deck height? can I just torque the barrel nuts on the PVC bushings? Not sure if this will improve my repeatability.

2. Is it normal to have this amount of variation from left side to right side? I am measuring over the wrist pin at 3 and 9 o'clock. averages work out to all be pretty close.

3. If i do torque things down, will I need to buy new gaskets as these will have been crushed once?

4. From other people's experience are my measurements similar to what you have gotten with similar setup? I have a window of roughly 0.040" to 0.043". Don't know how to know where the variation is coming from, measuring or machining.

I'm struggling with the fact that I disassembled a perfectly good working engine with decent compression, just a lot of miles and shot valve guides, and I don't know that I wouldn't have gotten the exact same measurements and repeatability before disassembling. It worked fine before, and I haven't machined block, heads, or cylinder heights, so why would it not work now? Only variable I can think of would be the machining of the rod bores, which I had done at a reputable, well known shop. all of my rod and main bearing clearances were right in the nice zone for a street daily driver.

I really want to keep moving on and start assembling the engine to measure piston valve clearance but am stuck on this step.

a course I have considered is keep going, torque down heads and camshaft carriers following published patterns, and if I can easily install camshafts and they spin freely, I should be in good shape. Correct?

Sorry for the long post.

thx

mark

[Eagledriver]

I’d be concerned with why you have different numbers between the 3 and 9 positions. That implies that the pistons are cocked. Are the wrist pins loose in their bushings on the rods? Could the bushings be at an angle? As far as the variation from one side of the engine to the other, it’s normal and small. I’d be happy with that deck height assuming it gives you the compression ratio you want.

-Andy

[Dauner]

Hi Andy,

Everything seems very snug. I did find that one time I altered the process of how I slid the pin thru the rod. I wiggled the cylinder/piston assembly back and forth a bit to allow the pin to more easily make it's way thru the path. When I did this, the sides were more even on measuring. After I get my piston valve clearance OK'd I have to go all of the way back to lifting cylinders off of the block again as I plan to put a small amount of loctite on the base gaskets. I may do some experimenting at that point. The only axis of motion that moves very freely is the rod on the crankshaft. Even though my clearances were right in the sweet spot, I can move the rods through their opening on the crank fairy easily.

With respect to deck heights, I do have a follow up question. If i don't get an answer here I might start another thread. When I dissassembled the motor, I did not check deck heights. I wish i would have. Original base gaskets were 0.25mm. I am using 0.5mm new base gaskets, and have the numbers noted below. So the only thing I can conclude is there was much less deck height originally. Has anyone ever measured deck height on dissassembly, and was it less than 1mm?

I ended up building out all the way to torquing camshaft carriers and heads down. Camshafts spin smoothly in their bores. I do wonder what the clearance is there tho. Boy do you need to have the shaft absolutely perfectly aligned before it will slide thru the next bearing surface. Both sides feel about the same. No binding, and with a small film of oil, spin freely.

[Neil Harvey]

Andy,

The only way to eliminate doubt is to check. But checking starts before you do any assembly.

It starts with the crank, is it straight and are the journals correct. Then the case, is the main housing bore straight, round and are the decks parallel to the bore and equal both sides.

Are the rods straight, are both bores parallel, and are they the correct sizes. Are the cylinders the same height and are the two faces parallel. Is the bore in the piston the same each end to the piston deck? So many possibilities, especially in street parts. The tolerances are a lot bigger.

Maybe some of this is out of sequence, but all these checks and others need to be confirmed before you assemble anything. If you don't bother to do any of this, and you find yourself trying to understand why the piston deck heights are different, take your pick where the problem may be.

It also could be in how you are measuring the deck height. As for any difference in the deck height, by how you installed the pin, it must be considered that there is some error in your measuring.

As for what the deck height was before, it is always a good idea to check the spec's before you disassemble the engine, Checking the cam timing is always a good idea.

Why do you have to have 1.0mm piston deck height??? Is there a rule that says you have to have 1.0mm??? If so, I have broken that 'rule" so many times. Just because someone wrote in a book something, doesn't mean it has to be.

Years ago, my boss used to say to me, " they advertise tea on the sides of buses but that doesn't mean you can get a cup of tea inside".

I think I will write a book about building Porsche engines. It will be called, " all the things you can do and get away with". Get the point!! Forget about what is written. Understand what you are doing and why.

I have never seen or read "the book" and do not intend to. It may have a lot of info in it about how to build one of these engines, but it seems to "trip up" a lot too. Does it ever state anything about any sort of tolerances and allowances that can be had, and why???

Deck height is a function of piston to head clearance and the desired compression ratio. All three are tied together.

Where to go from here? Unless you wish to disassemble and start again you don't have a lot of choices. Re check the measurements and confirm them. Then take the smallest and use that as your base line. CC the head chamber and the piston and calculate your CR. If its where you want it to be, leave it. But if you decide to go with more deck height, you can then re calculate your CR number.

[Inkblot]

@Neil, I’d buy your book in a second!


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[Inkblot]

@Andy I really don’t have enough experience to offer you any meaningful advice, but I’ve been going through a lot of similar questions with my rebuild (see my thread in this forum). I can say that it’s really easy to get hung up on minute details, while losing sight of the big picture. It’s natural... these machines are wonderful, but imperfect- despite being way more precise than many of their other European counterparts, there is still a fair bit of ‘slop’ in there. At some point you just gotta go with it! Do your best, make sure your pistons aren’t gonna ram into your valves, that your compression ratio is pretty close to what you expect, focus on getting your cam timing right, etc etc then let ‘er rip

Jake


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