Deck height too large (?) Even without gasket
 

Hi all, I'm just in the process of checking my deck height. With the 0.25 gasket/shim, I'm getting around 1.70 - 1.80mm.
I understand I should be aiming for 1 -1.5mm
Without the shim underneath it brings the height down to around the 1.5mm mark.
The cylinders and pistons haven't been machined and are stock. The case sleeves also haven't been touched by the machine shop.
The rods were resized and re bushed. The heads were also rebuilt. I beleieve that includes flycut as well.
I've attempted on both cylinders 1 & 4 but get similar results on both sides.
Is it ok to run without any cylinder shim at all to keep the deck height as small as possible?
I'm using the vernier caliper method on the flat outer edge of the pistons. They are 911 SC pistons with the dome top from an 83 cabriolet. US spec.
I'm holiding the cylinder down gently with some sockets just so there's no movement when rotating the engine. Would love some advice on how to proceed here. Thanks! https://uploads.tapatalk-cdn.com/201...68107983f6.jpghttps://uploads.tapatalk-cdn.com/201...b644b1339c.jpg

Sent from my Pixel 2 using Tapatalk

I think you should keep the .25mm gasket at the cylinder base for sealing.

There is no real downside to too much deck height except some loss in static compression ratio.

1mm is 0.039”. 1.5mm is 0.059”. 1.7mm is 0.067”. So you are over by 0.009”.

I think you will be just fine.

You need the base shim to help seal, On the 3.0 bore/stroke .010 is a quarter point of compression. Were the rods rebuilt and possibly lost some length?
Mike Bruns

I went from 9.5:1 to 9.3:1 with the extra deck height in my build with a 0.25mm base gasket.

It is what it is.

Thanks team. I had read a couple of posts where people suggested using no gasket and instead using curil T. I wasn't convinced of that idea, so thought I'd ask.
I also did piston to cylinder head sanity check with some modelling clay. The smallest clearance that I could find on the clay was 1.70mm.
Are the recommended specs here the same for deck height?

I couldn't find anything in the Bentley manual about either measurement. Anybody know if it's in there?
https://uploads.tapatalk-cdn.com/201...d4a85cf41e.jpghttps://uploads.tapatalk-cdn.com/201...e26be182dd.jpg

Sent from my Pixel 2 using Tapatalk

I think so. The deck height checks the edge of the piston to cylinder top. And the head clamps right on to that. So any region less than 1mm is a problem.

Since you are at 1.7mm you have added margin.

Did you check the head volume and measure the piston dome volume to get the net combustion chamber volume? Bruce Anderson’s book has step by step method for doing it.

As was said earlier the larger deck dimensions hurt compression ratio. And the way to judge if it is a problem is to check the net combustion chamber volume including your deck height.

I thought I'd read that too much deck height is undesirable because it can encourage inefficient combustion propagation and detonation. So more isn't necessarily better safety margin.

Yeh, that's the only thing that's been machined here and the crank. Crank was within spec. I didn't measure the rods myself. But the machine shop had all the minimum specs required for matching the rods.
So, what are we thinking here, just run the .25 gasket? There's really no other option is there? I can't make the rods longer again if that's the issue.

Sent from my Pixel 2 using Tapatalk

That sir, is correct.

Excessive deck height leaves too much end gas at the periphery of the piston domes which can spontaneously ignite before the plug fires as the piston approaches TDC.

You REALLY want no more than 1.1mm-1.2mm

One lasts thing, I never assemble one without base gaskets since I cannot afford any oil leaks.

You can run without a base gasket.

My last motor went 30 years and 80K miles with no leaks from the cylinder bases with no base gaskets installed. Use Yamaha Bond sparingly on the bottom of the cylinders and be sure the mating surfaces are clean and scratch free.

BUT, you really NEED to calculate your compression ratio before moving forward.

Bugger! I thought that would end up being my next step. I was hoping I could just measure the deck height and just move on. I've been trying to do some reading on compression calculations - but I think I suffer from Dyscalculia! Whenever I see where calculations are required - my mind goes fuzzy!

I understand you need plexiglass to seal the top of the heads - but the smallest amount i can buy here easily costs $60 a sheet. :(

Hopefully a 100ml syringe is ok to use over a burette?

I also wanna information about this a lot. Please suggest me more.

Not sure where you are located, but hardware stores will have small pieces of acrylic for reasonable. Maybe check on line. Some members here may have a suggestion for a source. I bought a burette on line for something like $20.

Bruce Anderson's book has a good section on doing CR. I used a spread sheet to calculate mine. That allowed me to play around with different base gaskets and see the results with out using a calculator.

Thanks mate. I'm in Australia. I'm sure there's somewhere I can get some off cuts of plexi glass - it will just take time. The big hardware store here is easily accessible but $$ when it comes to this stuff.

Appreciate the advice. I'll see what I can track down.

Where in Australia?
 

Where in Australia are you? My wife is leaving in mid February 2019 for Sydney and I’m not coming with her this time. I could ask her to mail a piece of plexiglas that I used for CC’ing the cylinder head dome volume. PM me your shipping address.

Tony

The measurement you want is between the ramped area of the piston and the head dome. Minimum 1mm. Obviously you don't want the small shelf on the piston to contact either, but that is not the significant measurement concern.

Oh wow. Thanks so much for the offer mate. That is so nice of you! I've actually found some plastic sheet at a local hardware store that I'll go check out tomorrow - so I should be ok. Thanks again for the offer!



Sent from my Pixel 2 using Tapatalk

Oh... I wonder if I'm measuring the correct part of the piston? I'm measuring closest to the wall of the cylinder. This is using the calipers. There's a tiny little flat section. These are standard 83 stock pistons.

Sent from my Pixel 2 using Tapatalk

I find it strange that nowhere that I can find in Bentley does it mention deck height measurements or even compression measurements. It just says that if your heads have been machined, add a 0.50 base gasket. https://uploads.tapatalk-cdn.com/201...4617a5ca74.jpg

Sent from my Pixel 2 using Tapatalk

If the heads are machined, it has to be very accurate as the exact same amount on all heads. Otherwise the cam housing will become bowed and bind up the cam. Also, it may cause an overhang shelf protruding into the bore and create interference with the piston. So once machine work has been done, a lot of checking has to be performed. I use clay with soft solder. If they were cut, need to find out how much.

For sure! I was just highlighting the fact that it's strange that given the comprehensive information in bentley - it doesn't include something like deck height or calculating compression. I assume that's possibly partly because they give the exact specs for all the parts, so they assume that yours will also be within those specs.

So, back to the measuring technique - am I measuring in the correct spot on the piston? See the white dot on the photo - that's where i'm measuring - on the flat lip.

https://lh3.googleusercontent.com/12...g=w424-h565-no

You are correct. There is some discussion here. http://forums.pelicanparts.com/911-engine-rebuilding-forum/575057-deck-height.html

and https://rennlist.com/forums/964-forum/289949-3-8-rsr-deck-height.html

FYI, you can make your rods longer. The small end bushing can be offset bored to make the rods slightly longer. Cost is listed as $250 from Ollies. Might get you were you need to be.

john

The detonation that guys mention is at the periphery of the piston. Here's what a 98mm piston looked like in a 3.2L short stroke. See all the tiny little speckles around the left periphery? That's detonation. Eventually it gets covered up by carbon buildup. But it still doesn't stop the detonation.

http://forums.pelicanparts.com/uploa...1334036363.jpg


Here's what caused the detonation- too much deck height. I thought the engine was a 3.0L, as that's what I was told when I bought the car. Engine was made of:

• typical SC 3.0L 70.4mm crank
• Pauter 3.0L rods (which was a very nice discovery but.......)
• Rods were not offset bored at the pin end because........
• The pistons pictured above are designed for Carrera 3.2 crank and rods with 23mm pins and different pin height

So my measured deck height in the cylinder was fat by over 1mm at a total of 2.4mm :rolleyes:

This is a good thread I recall trading responses with Henry Supertec about the offset bored rods

http://forums.pelicanparts.com/911-engine-rebuilding-forum/755709-upgrading-80-sc-w-88-3-2-p-cs.html

Anyhow, the kicker here is that the long deck height also affects your compression ratio. If my pistons and combustion chambers actually measured up to the advertised 9.8:1 compression ratio, that loss of deck height would have resulted in a theoretical compression ratio of like 8.5:1 Knowing that the published compression ratio of the Mahle pistons is typically a bit optimistic, it was probably quite a bit less than 8.5!

Can she bring a whole bunch of suspension parts from Pelican Parts for me too????
hha.. only joking.. or am I ????

Well... I managed to have another look at the deck height today.
Turns out that the deck height isn't as big as I first thought!
When I sat down to think about why the gap was so big, I looked at the bottom of the venier calliper and noticed that the probe that slides in and out doesn't actually sit on the edge of the bottom of the caliper when they are closed (at zero)! So, when I was closing the calipers completely and zeroing them out - they had to slide that extra mm before it would even touch the piston... which is what led me to my big numbers. That will teach me to buy cheap measuring tools on ebay.

So, with this new information, I got to measuring again. This time with no shim/gasket underneath I was getting 0.80 on one side of the piston and 0.90 on the other side - both measured at the location of the pin. Is it unusual that one side would be 0.10 higher than the other side? I wouldn't have thought it would be a big deal? possibly just how the cylinder was cast. They're not new pistons, only cleaned up and being re-used.

The Victor Reinz shims that I have measure on a micrometer at 0.283 instead of the 0.25 they are sold as. These shims will take my deck height (only measured one cylinder today) to somewhere between 1.08/1.18.

https://lh3.googleusercontent.com/nH...w1054-h1404-no

I tried CC'ing the heads, but the plastic i purchased ended up being a little bit too flimsy - it was easy to cut though! :)

How concerned do I need to be now about even measuring compression now that my deck height measurements are much more within spec?

Thanks everybody for your help and advice so far.

I'm not surprised.. when I saw the photos, it didn't look too far off.

Yeh that's what led me to look closer as the gap was tiny. One less headache. Now onto the next headache!

Sent from my Pixel 2 using Tapatalk