Deck height too large (?) Even without gasket

[Adrock]

Originally Posted by 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.

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.

[Trackrash]

You are correct. There is some discussion here. Deck Height

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

[targa72e]

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

[KTL]

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.




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

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

Upgrading 80 SC w/ 88 3.2 P&C's

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!

[mikedsilva]

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

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

[Adrock]

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.



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.

[mikedsilva]

Originally Posted by Adrock

Turns out that the deck height isn't as big as I first thought!

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

[Adrock]

Originally Posted by mikedsilva

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!

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