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  #1  
Old 08-17-2011, 11:38 AM
Subaru Alliance Subaru Alliance is offline
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Head gasket information

i am talking with cometic head gaskets and the offer the svx head gasket in three sizes .065, .066 and .075. they will make any thickness i require up to .140. does anyone have any idea or know the math behind how these sizes will affect the compression ratio, or will it be to small of a change to notice?

I believe the standard size is .066. thanks guys
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  #2  
Old 08-17-2011, 01:21 PM
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Re: Head gasket information

Cometic gaskets for the EG33 come in 3 bore sizes. 98mm, 100mm and 102mm. they are the BORE sizes. The stock engine takes the 98mm bore.


All three gaskets are the same thickness at .066".

Difference between the .066" and the .140" is .074" (or 1.88mm)




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Last edited by svxcess; 08-17-2011 at 05:02 PM.
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  #3  
Old 08-17-2011, 01:28 PM
Cam Cam is offline
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Re: Head gasket information

John, I believe cometic sells the headgaskets in 3 different bore sizes, and 3 different thickness measurements on their website. 66mm is the correct size for our stock HG, though.
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Old 08-17-2011, 01:43 PM
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Re: Head gasket information

Quote:
Originally Posted by Cam View Post
John, I believe cometic sells the headgaskets in 3 different bore sizes, and 3 different thickness measurements on their website. 66mm is the correct size for our stock HG, though.
Their website shows 3 bore sizes, but all the same thickness.

Part numbers are different, but the prices are all the same.

Stock is still 98mm x .066"

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1996 Polo Green LSi #216..138,100 miles...SOLD

JFICX8659TH100216.....Date of Manufacture: November 16, 1995.....
Fuji Heavy Industries..Ōta North Plant....Ōta City,. Gunma Prefecture, Japan

In-Service Date: January 2, 1997



"The Pristine Green Polo Machine”
First Polo Green on the Network
First Clear front turn signals, JDM Alcyone hood emblem, rear panel, and BOXER engine cover on the Network (US) (2000)
First 5000K HID factory fog lights (2007)
First SVX JDM BBS wheels on a USDM SVX (2013)

HID lighting (5000K) for headlight and H3 fog lights, PIAA SuperExtreme 120W high beams, rebuilt EG33 longblock, Cometic head gaskets, Phase II flexplate, AMR aluminum radiator with custom silicone hoses, 160A high-output alternator in aluminum-ceramic coated case, new design alternator wiring upgrade v.4, rare factory headlight protectors, refinished JDM BBS mesh aluminum wheels and custom, polished billet aluminum new hex center caps, LED grille mod, R1 Concepts high-carbon cryo slotted rotors, Akebono ceramic pads, Goodridge S/S braided brake lines, Smallcar Stage 1 shift kit, ThermalTech aluminum/ceramic-coated valve covers, Energy Suspension urethane front & rear swaybar bushings, Bontrager22 rear swaybar with QS Components Chromoly Teflon/Kevlar endlinks, "$15.00/5 minute" suspension mod. Hella Supertone horns, Custom stainless steel exhaust system with 2" headpipes, Magnaflow cats, AeroTurbine AR25 resonator /AWD "Bullet" muffler.


R.I.P. 2010 Subaru Outback Limited 2.5 CVT...338,000 miles. Totaled by a 1,300 lb. COW March 4, 2016

2014 Hyundai Avante Limited ...178,000 miles. Actually quieter and smoother than the Outback

2007 Mazda Miata MX-5 PRHT...102,000 miles.
Plenty of parts, service and windshields.


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My NEW locker..I...My Email..I..Wikipedia/SVX .

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  #5  
Old 08-17-2011, 02:23 PM
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Re: Head gasket information

Quote:
Originally Posted by svxcess View Post
Their website shows 3 bore sizes, but all the same thickness.

Part numbers are different, but the prices are all the same.

Stock is still 98mm x .066"
If you talk with them, you should be able to get a lot more options. The stock thickness should be used unless the machine shop needs to take off a lot of material when the mating surfaces are refinished to get them just right. The stock bore should be used unless you're having the cylinders bored out.
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Old 08-17-2011, 04:02 PM
Subaru Alliance Subaru Alliance is offline
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Re: Head gasket information

my bad guys i should have given some more specifics. I am talking to cometic through e-mail. I understand they come in three different bore sizes, I am gonna order a set of head gaskets in the stock bore size. They offer three different thickness' of head gasket in the stock bore size and that is .065, .066, and .075 however they make a gasket in any thickness up to .140

I am wondering if anyone knows the math, or some way to see how these different thickness' of head gasket will affect the compression ratio.

In Example
A stock engine with a standard thickness of .066 = 10:1 compression ratio.

A stock engine with a thickness of .075 = 9.9:1

A stock engine with a thickness of .140 = 9.5:1 etc. etc. I am just trying to see how the different thickness of the head gasket will affect the c.r. or will it be to small of a difference to even notice.

any help will be appreciated
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Old 08-17-2011, 04:23 PM
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icingdeath88 icingdeath88 is offline
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Re: Head gasket information

It is my understanding that on a boxer engine, changing the headgasket thickness more than just to compensate for the machine work will affect the timing. I am not 100% sure on any details, but I would look into that before making a decision.

That said, it could be estimated mathematically, by calculating the cylinder volume both at TDC and VDC assuming the stock headgasket thickness and compression ratio, then factoring in the additional volume from the headgasket. All you need is a couple of formulas from geometry class, and the bore/stroke.

Compression ratio 10
Bore, mm 96.9 (headgasket bore is 98mm for a little bit of extra space, I guess - for example if the cylinders are honed)
Stroke, mm 75

I can work it out later if you want.
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'94 Laguna Blue LSi ~159k.......JDM ultra short-geared 3.900 STi Version 7 6-speed w/ Cobb shortshifter, ECUtune 244,8.1mm/256,9.1mm i/e cams, group N motor mounts, '97 grille, JDM clear corners, Momo JDM Legacy GT steering wheel, apkarian's LED tails, silver STi BBS wheels, PWR radiator, redstuff pads f/r, drilled/slotted rotors, bontragerworks rsb #18, Koni/GC 450f/375r coilovers, Megan Racing adjustable lateral links, KMac c/c plates, Stebro exhaust, ECUtune 1v5, Optima battery in the trunk where it belongs. Turbo project

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Now a mod "over there" ............Photo album
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Old 08-17-2011, 05:11 PM
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icingdeath88 icingdeath88 is offline
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Re: Head gasket information

First we have to calculate the cylinder volume at TDC.

The CR is 10:1, the stroke is 75mm, the bore is 96.9. (If any of these values are wrong, please let us all know.)

If the CR is 10:1, then that means that the volume at BDC (bottom dead center) is 10 times the volume at TDC (top dead center).

The difference between the volumes (BDC - TDC), is given by the volume of a cylinder of height = stroke, and diameter = bore: V=pi r^2 h = pi x 48.45^2 x 75 =

10 x TDC = TDC + 553093.68 mm^3
...algebra...
TDC vol = 61454.853 mm^3
...more algebra...
and BDC vol = 6145485.33 mm^3

Any thickness added or subracted to the headgasket corresponds to volume added or subtracted to the TDC and BDC volumes. Lets say that we're considering a .140" headgasket and the stock is .066". (remember, when talking headgasket thickness, we are always talking about "compressed height", the height when it's all assembled and torqued to factory spec). The difference is .074", which is 1.8796 in mm.

This volume comes out to 13861.26 mm^3.

Add that to TDC and BDC, and we have 628409.80 mm^3 and 6159346.60 mm^3, respectively.

The compression ratio comes out to: BDC/TDC = 9.801:1.

Still pretty high, but you might be able to get away with 87 octane. Not exactly turbo friendly though.

There ya go. God, I'm such a nerd.
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'97 Ebony LSi ~137k #036.......Power mode mod, JDM clear corners, BBS wheels. AUX/pocket mod

Now a mod "over there" ............Photo album

Last edited by icingdeath88; 08-17-2011 at 05:14 PM.
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  #9  
Old 08-17-2011, 07:29 PM
Subaru Alliance Subaru Alliance is offline
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Re: Head gasket information

Nice work icingdeath88. ha ha I know enough about math to count money and that's pretty much it and by counting money I mean I can count to 10. None-the-less thanks for figuring that out. I was thinking if it would come out to around 9.6-9.7ish that might be good enough to run 87 octane, but I am not sure about the ignition timing, it may be to far advanced or to far retarded, or maybe it won't have an effect. thanks again.
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Last edited by Subaru Alliance; 08-17-2011 at 07:41 PM.
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Old 07-08-2014, 02:19 AM
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Re: Head gasket information

So stock is 98 bore and I should go with .066 thickness? I found this thread searching around and I think it answers some of my questions. I tried to read all of the math on those posts, but it was hard to read plus it is late. Any befit to trying anything other than stock, larger or smaller (both in bore and thickness)?
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