Deck height too large (?) Even without gasket

[Adrock]

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!

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[VFR750]

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.

[MBruns]

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

[VFR750]

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.

[Adrock]

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?


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[VFR750]

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.

[stownsen914]

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.

[Adrock]

Originally Posted by MBruns

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

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.

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[Steve@Rennsport]

Originally Posted by stownsen914

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.

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.

[Trackrash]

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.

[Adrock]

Originally Posted by Trackrash

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?

[brindhujay]

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

[Trackrash]

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.

[Adrock]

Originally Posted by Trackrash

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.

[boyt911sc]

Originally Posted by Adrock

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 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

[r lane]

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.

[Adrock]

Originally Posted by boyt911sc

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

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!



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[Adrock]

Originally Posted by r lane

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... 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.

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[Adrock]

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.

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[r lane]

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.