Elimination of all cylinder base shims - yes/no?

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Steve Hatfield
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Elimination of all cylinder base shims - yes/no?

#1 Post by Steve Hatfield »

I’m dealing with a set of S90 heads nicely prepared and machined to 30degree bevels which now cc at 66.

If I eliminate the only .010” copper I’ll still have a safe squish of .040” and a deck height of .95mm and I can achieve a respectable 8.727:1 CR which should work well with the high duration wide lobe cam and cast NPR 30 degree 86’s and cast iron cylinders.

So my question is: OK to assemble with no shim/gasket?

If I go with one ten thou shim my CR drops to 8.42:1. Not great.
Steve Hatfield
Fort Walton Beach, FL
'63 S90 Sunroof Coupe
'06 997S Coupe

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DonCichocki
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Re: Elimination of all cylinder base shims - yes/no?

#2 Post by DonCichocki »

Steve,
I didn't use any shims on my last rebuild, just used a small bead of Yamabond at the base of the cylinders. 9:1 compression, .040" deck height, and all has been well for over 5,000 miles.

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C J Murray
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Re: Elimination of all cylinder base shims - yes/no?

#3 Post by C J Murray »

I often go without base gaskets. Yamabond or similar.
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Steve Hatfield
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Re: Elimination of all cylinder base shims - yes/no?

#4 Post by Steve Hatfield »

Thanks guys
Appreciate your inputs
steve
Steve Hatfield
Fort Walton Beach, FL
'63 S90 Sunroof Coupe
'06 997S Coupe

Tim Berardelli
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Re: Elimination of all cylinder base shims - yes/no?

#5 Post by Tim Berardelli »

Steve,

Yep on no shims. How did you determine your CR? What was your technique?

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Re: Elimination of all cylinder base shims - yes/no?

#6 Post by Eric Wills »

Well, I'll try this for the second time......Computer or internet acting up.

Steve, Not that you need anymore confirmation, but sealant instead of copper gasket works fine. We use Threebond 1184. Supposedly the same as Yamabond/Hondabond. But CCing an S-90 head to 66 seems very large, even if modified. How is that done without cutting into the exhaust seats quite deep? Just asking, cause I'd like to know.....

EW
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Harlan Halsey
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Re: Elimination of all cylinder base shims - yes/no?

#7 Post by Harlan Halsey »

I think you have your answer, but I'd machine the cylinders enough to get 9.5:1 with a gasket. CR is where the power is. (Dema Elgin told me in answer to my question that 9.5:1 is about the maximum with todays street gas. And that is what I shoot for.)

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Re: Elimination of all cylinder base shims - yes/no?

#8 Post by Michael Foster »

I go with Harlan on this one, machine cylinder base to up your compression to 9.5 with proper shim. Otherwise someone will be calling you a previous turkey owner who left out shims.
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Steve Hatfield
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Re: Elimination of all cylinder base shims - yes/no?

#9 Post by Steve Hatfield »

I’ve been called worse ... but
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Fort Walton Beach, FL
'63 S90 Sunroof Coupe
'06 997S Coupe

Michael Foster
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Re: Elimination of all cylinder base shims - yes/no?

#10 Post by Michael Foster »

Hey Steve, I didn't call you a Turkey owner....just saying SOMEONE may call you that in future.

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Steve Hatfield
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Re: Elimination of all cylinder base shims - yes/no?

#11 Post by Steve Hatfield »

Capish
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Re: Elimination of all cylinder base shims - yes/no?

#12 Post by C J Murray »

Gee wiz, I'm a Turkey! :x
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Re: Elimination of all cylinder base shims - yes/no?

#13 Post by Mike Horton »

Steve, the S90 I just finished, using the 86mm AA cast iron 22*cylinder set, and a (-17cc dome) volume number, for the hypereutectic pistons, as published by AA, and once cut .010" heads, with the original 22* head angle, cc'd at 59.5, and with 3 ea., .25mm/.010" shims, calculated at 8.94:1, C/R... , a cross country cruiser, and in deference, to the "10% ethanol fuel" out on the roads, today,
Mike

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Steve Hatfield
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Re: Elimination of all cylinder base shims - yes/no?

#14 Post by Steve Hatfield »

Mike etal
The truth is I’m saddled with a pair of S90 heads that thirty years ago a previous owner thought he/she could both improve glow and eliminate the 22 deg sculpting. So they now cc 84 vice 60. With a deck height of 1.5 mm (new old stock NPRs), I don’t have much room to trim the barrels. If I did cut the fins which I’ve done in the past to match up a set, I’d only trim 8-9 ccs and still have 76 ccs to reduce with enlarged dome volume pistons. But then I may have flow issues.

So following Leonard’s (Shasta) advice I’m looking for new heads? Preferably SC heads.

This engine otherwise has been a sheer delight discovering near perfect bottom end and unused period NPR set.

Here’s a pic of the enlarged chambers on the Buret bench
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Steve Hatfield
Fort Walton Beach, FL
'63 S90 Sunroof Coupe
'06 997S Coupe

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Re: Elimination of all cylinder base shims - yes/no?

#15 Post by Michael Foster »

So you can blame the previous owner of the heads . Your not the Turkey, it's the previous owner who mistakenly cut the heads. Capish?

Michael
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