Question about the deck height

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tpubert
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Question about the deck height

Post by tpubert » Wed Jan 11, 2017 5:20 am

Hello,

I am rebuilding my engine, a 2175cc with 94mm bores and a 78,4 crankshaft, after to have broken a piston.
But when I measure the deck height between #3 and #4 cylinders, I find a difference of 0,2mm (1,10mm and 1,30mm) ... some guys say that this dimension must be under 0,15mm.
The 2 cylinders have the same height, and they are perfectly aligned on their top when they are pressed to the engine case, so I think that this one has been well machined.
The alignment of the crankshaft line seems to be correct because the deck height of the cylinder #1 is correct (1,19 mm) and the difference with the #2 is also correct (1,24mm)
I have tried to exchange the #3 and #4 rods, thinking one can be longer, but no change.
Actually, the first time I assembled my engine, some years ago, I think I have measured the deck height only on one cylinder and I haven't saw this difference ... so, I have this problem from the beginning.
So, what do you think about that ? Must I modify something to have better dimensions ?
In that case, don't you think the only one solution is to machine the top of the #3 piston, remove about 0,1mm and increase its deck height ?

Please, excuse my poor technical english, I'm french !
Thanks a lot for your help !
Thierry

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Marc
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Re: Question about the deck height

Post by Marc » Wed Jan 11, 2017 6:29 am

If swapping about rods & pistons makes no difference and the cylinders are perfectly even (checked with a straightedge across the tops) and the crank is parallel to the case, then the only thing that would explain the discrepancy is the crankshaft being machined with different strokes on #3 than on #4.

The difference in unswept volume is only 1.4cc which you could probably compensate for by opening up one combustion chamber, but it would be more elegant to shave the piston top and have the pistons rebalanced (you might find that the wrist pin weights are slightly different and be able to get the weights closer by simply interchanging the pins)....or have the crankshaft reground.

Personally I wouldn't use much less than 1.4mm piston deck height at 94mm bore (on a Type I engine) so if this were mine it'd be getting .010" cylinder base shims.

Right now the difference in compression ratio between the highest and lowest cylinder is about 0.2 which isn't terrible - you might just want to lower your standards a little for now and deal with it if/when the crankshaft needs to be reground.

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tpubert
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Re: Question about the deck height

Post by tpubert » Wed Jan 11, 2017 9:22 am

Thanks Marc for this precise explanation !

Yes, I agree with you, it's preferable to shave the piston top and regrind the 3 others to align the weights !

Just some other questions about your answer ...

About the solution you talk about concerning to add shims under the cylinders, I understand that it would be for approaching the 1.4mm deck height mini that you preconize ... but, must I put a 0,10mm shim under the 3 cylinders #1, #2 and #3 and nothing under the #4 one ?
Isn't there a risk to have the top of the 2 cylinders #3 and #4 at a different level and to have a bad support and a leakness of the cylinder head ?
Unless if this 0.10mm gap is compensate by the pliability of the aluminium of the cylinder head when it is torqued !

Sorry, I don't understand exactly what you want to say in your last paragraph !
Do you mean that, in fact, if the real cause is from the machining of the crankshaft, the lastest of the solution would be to leave this difference of 0.2mm between the 2 cylinders the more distant (#3 and #4) because it's after all not so important for the engine operation !?

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Re: Question about the deck height

Post by Marc » Wed Jan 11, 2017 6:27 pm

Adjacent cylinders need to be identical in height, so both would receive the same thickness shims (unless you were compensating for uneven case or head surfacing or unequal-length cylinders). The head can conform to a slight disparity, perhaps .05mm.

If you've ruled out all other causes and the crankshaft is ground with unequal strokes, you can only partially compensate for it by shortening the piston. If the stroke is off, so's the displacement (but only by <1½cc) and that'll be true even after shortening the piston - so everything's still not going to be perfect.

So, if you're willing to accept less-than-perfection, the only question remaining is how much less?

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tpubert
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Re: Question about the deck height

Post by tpubert » Thu Jan 12, 2017 8:13 am

If I concentrate my attention on the deck height only, the only solution is to shorten the #3 piston, and effectively the displacement will always not be the good one ... my question : Is it so disturbing to have different displacements between 2 adjacent cylinders ?

On the other hand, if I more considere the displacement and I want to have the same for all cylinders, I think the only solution is to change the crankshaft ... that isn't at all my idea !

So, I think I'm going to cut off the #3 piston of 0,1mm (0,1mm or more, what do you think of that ?) to have a deck height of 1,21mm and the let the 3 others at their original height.
Never mind for the deck height of 1,4mm mini that you preconize !

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Re: Question about the deck height

Post by Marc » Thu Jan 12, 2017 6:18 pm

The difference in displacement due the uneven strokes would be on the order of one quarter of one percent in this case, it would not keep me up at night worrying about it.

Have you CC'd the combustion chambers and found them to be identical? It's somewhat pointless to obsess about making the pistons deck heights all identical if the chambers have different volumes. But let's assume that they're the same for purposes of this discussion.

Here's what I'd do.

Weigh all of the pistons and wristpins separately.
Put the heaviest piston/wristpin combination on #3 and the lightest on #1.
From the remaining two pistons & pins, put the lightest pin in the heavier piston and vice-versa, and install those as #2 & #4.
Remeasure all of the piston decks. If they're not essentially the same as before it's time to rethink this, but supposing that they are the same, have 0,15mm shaved from the top of #3.
Recheck the balance. If you can't match all four perfectly, at least find the piston/pin combination which is closest in weight to #3 for use as #1.
5 grams difference between opposing pairs and 10 grams difference overall should be the worst acceptable (hopefully you'll have less than half that difference), if you can't achieve that then the pistons will need to be rebalanced. There's not much material available for removal without weakening the piston so reassign the pins again if need be, to get the combinations as close as possible before you start carving on the pistons.

I still strongly recommend against such a small piston deck at this bore for a street engine and I would add a .010" shim under each cylinder, but that's your decision.

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tpubert
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Re: Question about the deck height

Post by tpubert » Sat Jan 14, 2017 8:54 am

Yes, when I've assembled my engine, some years ago, I've measured the volume of the combustion chambers. They were quasi equals but I'm not anymore sure (I don't remember) if they were perfectly identical !
So, I agree, if the chambers are not equal, it's not necessary to try to boil the ocean by tuning precisely the deck height !!

Thanks a lot again to have taken my problem into account, I think I'm going to follow your advices and, step by step, succeed to solve that problem !

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Re: Question about the deck height

Post by dragvw2180 » Sat Jan 14, 2017 12:28 pm

Me personally I would find the cause and either repair or replace the cause. You may want to make sure the crank is not bent , it would cause a differant deck. If you cannot afford to do that I would reassemble it back together just like you had it and it will be as good as it was when you ran it before . VW engines continually surprise me with what they will endure and still survive .

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tpubert
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Re: Question about the deck height

Post by tpubert » Sun Jan 15, 2017 1:19 pm

You are right, dragvw, the right way to act would be to look for this problem until I've found the root cause, but I miss of courage for dissassemble all the engine, and then, if I find that my crankshaft is bent, I don't think that I'll spend money to have another new one or to repair (if it's possible only) mine !

A mechanics friend thinks to compensate the lack of deck height by machining the inside of the cylinder head ... a machining of the flat surface just around the chamber, inside of the bearing surface of the cylinder, in fact, just above the cylinder bore ... I don't know if you understand where I mean !
I think it's a good idea, this allows to have the right dimension between the top of the cylinder and the flat surface around the combustion chamber.
I also think this allows to keep my new (and expensive) pistons in their original form !
If it's not good, it's always possible to rework a cylinder head, but not a piston on which we have remove raw material !

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Re: Question about the deck height

Post by dragvw2180 » Sun Jan 15, 2017 3:30 pm

A simpler way to accomplish what you are trying to do is to add a shim under your cylinders. I would measure the worst cylinder and then measure how thick of a cylinder shim it would take to get .060 inch for deck height ( distance from top of piston to top of cylinder at TDC) then add the same thickness shim to the rest of the cylinders. . You ran this engine before and were satisfied , you do not want to repair it , so repair it as cheaply as possible and enjoy the car for as long as it lasts. If you lived anywhere close to me I would have you bring it by so I could help ya.

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tpubert
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Re: Question about the deck height

Post by tpubert » Sun Jan 15, 2017 5:12 pm

OK, you think that if I was satisfied of my engine as it was before I dissassemble it, I increase just the deck heights values to 0,060" and continue to ignore this difference between the #3 and the #4 cylinders ! Why not !

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Re: Question about the deck height

Post by Marc » Sun Jan 15, 2017 5:58 pm

0,010" shims under the cylinders would yield .057-.059-.053-.061". If you haven't had #3 piston smack the head yet at 1.1mm (.043") then .053" should be safe for you.

You didn't post your chamber volumes, but for example if they're 52cc your individual cylinder compression ratios would be: 9.77 - 9.72 - 9.87 - 9.67 (average 9.757) which, although not perfect, is certainly close enough to run OK.

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Re: Question about the deck height

Post by dragvw2180 » Sun Jan 15, 2017 7:15 pm

Thierry , I did not mean to insult you or anything , I truly was just trying to give you a possible simple solution. The reason I suggested what I did was because doing machine work to your good parts only increases the amount of parts replacement or machine work you will have to do in the future. I cannot tell from here what the problem is but I would bet it is something simple . I would recheck to make sure that your cylinders are fully seated against the case. Case savers can turn slightly and raise the cylinder and prevent it from sitting all the way down flush with the case. Another thing is the clearance between the case nuts and the cylinders can be too small and cause the cylinders to be up slightly.

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