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Measuring Deck Height
I am in the process of rebuidling a 1983 911 SC to address broken head studs. While I was there, I had the heads reconditioned, which included a fly cutting (0.25 mm of material removed from heads) to clean up the mating surface between the cylinder and head. I am now in the process of installing the pistons and cylinders. Initial base gaskets were 0.25 mm, so I installed 0.50mm base gasket to make up the difference. I would expect that this would take me back to the clearance that existed before the teardown. I have installed P&C #1 and 4 and have begun the deck height check. I started with the acid core solder (2 mm in diameter) at the 3 and 9 o'clock positions on the piston. Installed and torqued the heads and rotated the crank. The result is that the solder did not compress at all, indicating a clearance greater the 2 mm. I tried the same thing with teflon putty and this indicated a clearance at 20-2.25 mm. I then took a vernier caliper and measured from the top of the cylinder to 3 flat spots on the piston head for each of the cylinders and came up with a reading of 0.045 to 0.05 in (approx 1.25 mm).
What gives? Which number is correct? Does this type of piston not lend itself to the solder measurement Any help is greatly appreciated. Rick 
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07-15-2006, 06:25 PM
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Join Date: May 2003
Posts: 3,346
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I think that what you are seeing is the difference between deck height and piston to head clearance. A deck height (what you measured with the verniers) of 1.25mm is fine from what I've read and is about what I've seen on the engines I've built. The clearance from the head can be more (as in your engine) or less depending on the shape of the piston and head. I think it has to do with the chamfer on the head inside the sealing surface. I'd put it together with the .5mm gaskets like you measured with and call it good.
-Andy
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72 Carrera RS replica, Spec 911 racer |
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07-15-2006, 09:04 PM
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Join Date: Mar 2005
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IMHO, head to piston clearance is a power robber.
And I'd reduce the clearance as much as you dare. Solder etc works fine. What cams are you running? you need plenty clearance on the exhaust valve with hot cams and high revs.. Kind regards David |
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07-16-2006, 01:02 AM
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Andy/David,
Thanks for the thoughts. I was thinking about this further last night and decided, given the geometry of the piston and the head, that the only place where I would get corroboration between the two reading was at the point where the solder touches the cylinger wall. This would only occur in the first fraction of a mm at the wall. When I measured the solder, I didn't notice this, as the solder was slightly crimped from cutting. I thought hard about the potential gain in compression with the 0.25 but I am going to put it back together with the 0.5mm shims and call it a day. Thank you both for your help. Rick |
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07-16-2006, 06:36 AM
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Join Date: Mar 2005
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Hello again.
No, the compression gain is negligible. Its the combustion which improves..especially imprtant with single plug of course. Combustion is the biggest downside of the 911 head..anything you can do to improve it is golden.. Not boasting, nor recommending, but fwiw I have sucessfully run 25 thou.. Kind Kind regards |
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07-16-2006, 06:48 AM
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David,
Thanks for the clarification. I've already installed the P&Cs with the .5mm shim. What is the effect if there is too much clearance? 25 tousands = 0.635 mm. You've got more guts than I do. Thanks Rick |
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07-16-2006, 05:22 PM
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