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Slumlord
 
Porsche_monkey's Avatar
 
Join Date: May 2001
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Deck Height Numbers

My deck height (with the .25mm shim) measures only .030" (0.75mm). I want 1.5 mm total, so I need to instal 1.0 mm shims.

The other side of the case measures 0.5mm, 0.020". I need 1.25 mm of shims on that side.

Do these numbers seem high? To the best of my knowledge there have been no mods to the case or cylinders.

I guess I don't have any choice but to stack a combo of 0.5 and 0.25 mm shims do I?

Old 12-03-2003, 11:46 AM
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If in fact your current deck heights are .50 and .75 with the .25mm shim, you will need to stack a combination of .25 and .50 shims but a deck height of 1.25mm is better as the chance of detonation decreases with a smaller deck height. You can also go smaller, not decreasing a deck height of 1mm if your piston to valve clearance allows. Unfortunately, you won't be able to check this until the cams are timed, but 1.25 should be safe for a stock build.

Some things to look at:

1) Are you sure the cylinder bases and/or case was not trimmed?

2) Make sure the cylinder is seated all the way down in the case bore.

3) Make sure to check the deck height on the wrist pin axis as the piston does 'rock' a little in the bore

4) Make sure the tip of the instrument is physically able to sit on the piston ridge. I used a gauge whose end was a bit thicker than the piston ridge and the slider would become lodged between the piston crown and the cylinder wall, not sitting totally flush on the piston ridge. I eventually used a gauge that had a skinnier slide.
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Last edited by kstylianos; 12-03-2003 at 12:32 PM..
Old 12-03-2003, 12:24 PM
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Charlie:

1) Not sure of anything on this engine. It appears to me that something was done given these numbers. Stupidly, I did not check what was there when I tore the engine down.

2) Cylinder is clamped tight on .25 mm shim.

3) Yes I did this. I also did the solder crush as a double check and got similar numbers.

4) On the picture above I checked the near side with the micrometer depth gage (this is a large flat area). I checked the opposite side with a vernier, and it is similar.

I am a bit concerned because it means either stacking shims (not receommended in Wayne's book) or making my own (a pain). Pelican only sells half and quarter mm shims, which worries me a bit...
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Old 12-03-2003, 12:33 PM
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PBH,

I have a set 1mm shims if you need. These should work on one side at least. PM me.
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Old 12-03-2003, 12:56 PM
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I asked a similar question a while ago, and the concensus was stacking is OK if it is necessary... I found .25 0.5 and 1mm shims available. Here is the old thread:

Cylinder base shim, how thick is too thick
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Old 12-03-2003, 02:49 PM
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Deck height: I wish I could find it in my notes but I spoke to Steve Weiner about deck height once and he said that he doesn't like more than 40/1000" (1mm) because detonation starts out in the "pocket" near the edge of the piston. I hope I'm remembering that right. Hopefully Steve will chime in.

Base gaskets/shims: You don't mention what the motor is. When I was looking for base gaskets a while back Stoddard had oversize (>.25mm) gaskets for pre-Carrera's but only normal size ones for Carrera's ('84-89). Competition Engineering advertises that the have a wide selection of cylinder base shims.

-Chris
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Old 12-04-2003, 09:37 AM
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Sorry, it's a 2.7 7R case. I've posted so many problems here I sort of assume everyone in the P-Car universe knows my 'situation' .

I remeasured today using a ground steel plate on the surface of the two open cylinders, then reached up to the piston with a dial indicator. This gives me a very accurate reading. (After doing this I would never use a the back end of a vernier again.)

Anyhow, I get 0.25mm on cylinder #1, 0.5mm on cylinder #4, so I will install 1.0 + 0.25 on 1,2,3 and use 1.0 on cylinder 4,5,6. that will give me close to 1.25 mm on both sides.

Time to move on, thanks for the help.
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Last edited by Porsche_monkey; 12-04-2003 at 09:56 AM..
Old 12-04-2003, 09:53 AM
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LOL...Chris remembered right,....

I aim for .038 to .040 (max) for deck height. Certain race motors that get higher compression will run .035 but thats really close.

I do not permit a deck height greater than .040 (1mm) since that really makes these engines more prone to detonation; something that all hemispherical-headed motors have problems with. Its worth the extra effort, IMHO.

Stacking two base gaskets (shims) is fine.
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Old 12-04-2003, 12:03 PM
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I have two .25mm base gaskets under each cylinder. Didn't want to wait/search for the .50 gaskets. My heads were flycut.

I got some advice from a reputable Porsche 911 mechanic in the Phinney district of Seattle, who reports that when you use two base gaskets, the earth continues to revolve on its axis and orbit the Sun. Meteors and comets do not strike the earth in unusual numbers as a result of using two base gaskets. Most of you know, or can guess, who my mechanic/mentor is by now.
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Old 12-05-2003, 11:27 AM
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EBS Racing has .50mm shims in stock.
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Old 12-05-2003, 12:17 PM
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Be prepared to blame me for any cosmic distruptions next week.
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Old 12-05-2003, 12:35 PM
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