deck height question
 

Can anyone help me with where I am to place modeling clay to determine my deck height? My car is an ‘82 SC. See attached picture of piston.


Thanks,


Billhttp://forums.pelicanparts.com/uploa...1692036985.jpg

Deck height is the distance from crank center line to top of block. For Porsche, center line of crank, case mating surface, to top of cylinder (head mating surface). Porsche case is 112.5 (112.4) from crank center line to top of cylinder spigot cylinder is 85.3 (85.4) total 197.8 That is deck height. To figure piston below (or above) deck simply measure. 3.0 crank is 70.4 take half of that 35.2 add that to rod length 127.8 then ad compression height of piston 34. That stack or length is 35.2+127.8+ 34 = 197 so if block hasn’t been machined and your running stock piston rods etc then there is built in a .8 below deck piston clearance. Which is what you want Measure your case and cylinders and compare to your rod length and compression height of the piston. Done. Now clay is a good idea for valve to piston clearance

Thank you. I am all ears until I turn another wrench. Waynes book doesnt crawl deep into exactly where I place clay for my domed cis piston setup on my ‘82 SC. Everything is bone stock, except my heads were fly cut .14mm


Bill

Before I did the clay or solder test, I loosely assembled the cylinder to the head, and pushed the piston down in (without rings) so see where it contacted the inside of the head. I rocked it around a little bit to see the witness marks. When I test assembled the engine, I put the solder at that point. That contact point was about the middle of the back side of the dome of my Mahle Sport 3.0/3.2 "10:1" pistons. I ended up with a bit more than .030"/.75mm of clearance between the pistons and inside of the head, without the .25mm copper base gaskets. I try to keep the squish clearances tight.

I never clay or solder to check piston clearance.
1st, I measure piston dome with piston edge flush with cylinder. This base measurement will be used again and again, for dome volume and deck.
2nd, with rings installed for tension in the cylinder, I align the piston in running position and shove it into the head. Again measure the piston dome in that position. Subtract the dome height and the results are piston to head clearance with “0” deck.
3rd, install piston and cylinder one the engine with the base gasket u might use, snug down the cylinder and measure the piston dome again.
Using the dome measurement and arithmetic u now know your deck clearance and total piston to head clearance when u add it all up. I also use the Bruce Anderson compression ratio measurement. I use a digital caliper for my dome measurements.http://forums.pelicanparts.com/uploa...1692311148.jpg
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That's a more elegant method.

The deck height is measured at the edge of the piston. Notice the flat edge at the perimeter of the piston. I measure at four places around the perimeter.
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BTW, depending on your heads clay may not give you a good reading.

This is exactly how you measure the difference between the block deck height and the piston deck height, in theory. However, ALWAYS recheck on the engine. In this Porsche work, as in almost all the production engine world, repairs are done to repair and often never repaired to spec. As an example, I have never seen a set rods that have been repaired in the past, having the same CCL. Case deck heights may not be as they were originally either and Cylinder heights do differ as well.

Measure the piston deck height across the pin axis so you remove any piston rocking motion.

OK, I'll answer the question you asked:
Place solder on the domed portion of your piston; 2 o'clock and 8 o'clock in your photo. This measures piston to head clearance once engine is rotated with cylinder head in place and cam timed.

Deck height by definition is distance from piston deck (not considering the dome) to top of cylinder when at TDC.

Edited above post...

I never use clay or solder either.

Physically measure stuff with dial calipers, micrometers, and/or depth gauges instead, then verify odd shaped stuff with the burette and colored alcohol.

Of course, there's a difference between a street engine where the only thing that will happen is you'll need to run a different grade of fuel or give up a few hp. In racing, the sanctioning body could throw you out if the compression ratio is off, potentially costing you $$ or points.

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OP asked where to put clay to measure for deck height. That’s not how you calculate it. Deck height is simple.
Porsche cases are 112.4from crank center to cylinder mount surface. Cylinders are 85.4 totaling 197.8. That is standard Measure measure measure
If you are trying to figure piston to valve clearance that is totally different. Put play dough across valve relief in piston fully assemble engine rotate engine at least two full revolutions . Disassemble and check clay thickness that is valve to piston clearance PITA on these engines.
Now if you have bored the cylinders like 95 open to 98 and you have not opened the cylinder head combustion chamber to match then measuring with clay like the above picture. ok. But that is not deck height. That is just poor work and no attention to detail
When building an engine any engine takes planning That’s why piston manufacturers ask what cam lift and duration want size valves combustion chamber timing etc. that’s why you have to pay close attention to stroke rod length piston compression height. That way if you have to add a base gasket you know how thick.
I feel the only thing putting clay in the combustion chamber will show is poor planning or poor execution. Take some veneer calipers and measure you cylinders the measure the combustion chamber should be the same if not fix it
Deck height is simple