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some_german_guy's Avatar
Join Date: Oct 2016
Location: North Texas
Posts: 13
Need opinion on whether I need to check deck height

I'm in the (very slow) process of restoring my '79 SC and need you guys to tell me if I should perform a deck height check or not. I'm getting ready to put my motor back together and got my heads back from refurb'ing at Ollie's. They did not take any material off of the mating surfaces and I am replacing the current Kolbenschmidt P&Cs with a new set of Mahle 9.5:1s. The mating surfaces on the block haven't been touched or altered either.
The most prudent thing to do before bolting it all back together would be to do a deck height check as outlined in Wayne's book but do you guys think it is really needed since no meterials were shaved off the heads or the block and the p&cs are stock replacements?

Any input would be greatly appreciated

Old 03-03-2018, 08:49 AM
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Join Date: May 2007
Location: Fort Worth Tx.
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You should be good, but why not check during the trial assembly, then there is no doubt.
Old 03-03-2018, 10:43 AM
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Measuring things like deck height, clearances, etc. is inexpensive and builds confidence during the build. It eliminates questions like "what if..?" and "was that correct..?" that will pop up later if you don't do it.

Discovering unexpected interference during start-up is expensive.
Old 03-03-2018, 11:25 AM
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I vote yes to check it. And to make it easier, you can leave the rings off, assemble one from each side (#1 & 4 with base gaskets) and use cylinder hold downs on all four studs to simulate assembled height and measure. Ten to 15 minutes a side for your peace of mind.
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Old 03-03-2018, 12:19 PM
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First and foremost, I am not an expert but have rebuilt three (3) engines to stock specifications so far using Wayne's book, the Bentley and Porsche Factory manuals as well as the information found on this forum.

IMHO, I would say "Yes" that you should check the Deck Height. Why?

Because of a couple components which have changed in your particular build, one being the 1978/1979 factory pistons are 8.5/1 CR. Your new Mahle setup is 9.5/1 CR which means some significant difference with the shape of the tops of the newer pistons.
Also, you have changed the Cylinders so one question that comes to mind is: Are the newer Mahle cylinders of the same height group ? ( Hint: check the KS cylinders and look for a triangle with a number in it.) Is it height group 5 or height group 6?

I believe you would want to be careful and do the Deck Height checking because in Wayne's book he states that this procedure should always be carried out "when any of the components have been changed" with the build.

Just my opinion here, so if it were me "yes" do the measurement to be on the safe side. As mentioned above, the alternative could cost many $$ to fix should there be a problem with valves hitting pistons on start-up.

Hope this helps and "Good Luck" with your engine.
Ed Paquette
1983 911SC
1987 944 Manual (Donated to the Nat. Kidney Foundation)
1987 944 Automatic (Recently sold to another Pelican)

Last edited by TibetanT; 03-03-2018 at 12:37 PM..
Old 03-03-2018, 12:34 PM
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Easy to do. IMO, every aspect of these motors should be checked during assembly.
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Old 03-03-2018, 12:53 PM
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Join Date: May 2004
Posts: 93
Ask yourself why not, not why.

79 engine and the heads did not need the mating surfaces touched? Was that because they didn't need it or that they were not considered. Each head should be measured for their heights.

The case decks need to be measured for their dimension from main housing centerline. You should check the decks for flatness and parallel in both directions. Then you can measure the cylinder liner heights as well. This gives you a fixed deck height number from crank centerline, up to the cyl head. This can be considered your block deck height in any other type of engine. You need to establish both sides are the same. You can adjust any differences with the case deck, or by shimming with base gaskets. This is now your block deck height.

Now if you cannot measure the each rod crank for its stoke, you can now measure each rod CCL, and piston comp height. Assuming that the strokes are the same, you can calculate the piston deck height from theses parts. From this dimension you can establish on paper if you have a negitive or position piston deck height and how much that may be.

Now when you actually measure the piston deck height it should be what you calculate and they should all be the same. Remember, measure the deck height across pin axis, not as shown in the photo here. This takes any rock out of the piston.

Now if you measure the chamber volume and the piston volumes you can calculate your actual static compression. It is always a good idea to do this as often stated CR numbers are quite difference to actual. CR numbers can be adjusted here to with base gasket shimming.

But if you have not checked the case decks are flat and the same each side, you will have CR numbers all over the place along with differing piston deck heights. This is a street engine so you will see some differences, and so its up to you to decide what is acceptable.

If you need help in any of this, I can answer any questions.

Leave nothing to chance.
Old 03-03-2018, 03:06 PM
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