Can I use an oversized Head Gasket? 87mm on 86mm pistons?

[chicos240]

Im pretty sure my Cometic HG has given way, here is why.I have used it for more than one head and have torqued it aproximately 6-7 times. Over the past few months I have been happily running 15 psi on a rebuilt s14sr20det. This weekend I added a Circuit Sports FPR and a Circuit Sports Plenum Gasket.I am now noticing white smoke coming out of my exhaust, it is heavy white smoke when the engine is warming up. The smoke does get a little lighter as the car warms up, but it is still there.So im pretty sure its the Head Gasket is not creating a good seal and some coolant getting into the cylinder.I have a Tomei 87mm 1.5mm HG that I purchased along time ago, still in the package. Is there any thing wrong with using it on my stock head and stock block, ie 86mm. The only thing not stock on this block is the CP Pistons.

[DuckyD]

Is there any thing wrong with using it on my stock head and stock block, ie 86mm. The only thing not stock on this block is the CP Pistons.

You must use an 87mm headgasket for your 86mm bore. Off the top of my head, neither Tomei nor Apexi make a 86mm headgasket.

[chicos240]

You gave me an idea.Looks like 87 mm @ 1.5 mm Tomei HG on 86mm Pistons is exactly 8:5:1 ! Which is perfect!So I went and looked online...lookie what I found for S14 SR20DET. Available Sizes:TOMEI: * 11044R205G - 87.0 x 0.6mm * 1331870101 - 87.0 x 1.0mm * 1331870121 - 87.0 x 1.2mm * 1331870151 - 87.0 x 1.5mm <-------------- You are buying this one! * 1331870181 - 87.0 x 1.8mm * 11044R300G - 88.0 x 0.6mm * 11044R304G - 88.0 x 1.0mm * 11044R305G - 88.0 x 1.2mm * 11044R306G - 88.0 x 1.5mm * 11044R307G - 88.0 x 1.8mm



Compression ratio examples:

87.0mm Gasket .......... 86.0mm Piston .............. 86.5mm Piston 0.6mm ......................... 9.2:1 ......................... 8.9:11.0mm ......................... 8.9:1 ......................... 8.7:11.2mm ......................... 8.7:1 ......................... 8.5:11.5mm ......................... 8.5:1 ......................... 8.3:11.8mm ......................... 8.3:1 ......................... 8.1:1

from http://www.frsport.com/index.p...c115c

[DuckyD]

Those CR examples seem kinda high.Enthalpy has a good compression ratio calulator:http://rs-enthalpy.com/tuning/sr-compcalc.xls

Enthalpy puts a 8706 gasket (87mm bore 0.6mm thickness) with 86mm piston at 8.8 CR. FRSport claims 9.2

For a 8715 with the same piston, Enthalpy says 8.2:1 FRSport says 8.5:1.

I guess it would be better for FRSport to error on the side of safety (lower actual CR).

[chicos240]

d!ck for you oin that Entahlpy CR worksheet.How do you calculate for decking your head or your block.Most shops only take .010 mm off when decking.How do you account for that?Is it by changing the deck clearance?

Bore (mm) 86.00Stroke (mm) 86.00Displacement (cc) 499.56Cylinders (qty) 4.00Displacement (cc) 1998.23 Gasket Bore (mm) 87.00Gasket thickness (mm) 1.50Gasket Volume (cc) 8.92 Deck Clearance (mm) 0.05Deck Clearance Vol (cc) 0.29 Piston Volume (cc) -15.00 Head volume (cc) 45.00 BDC Volume (cc) 568.77TDC Volume (cc) 69.21 Compression Ratio 8.22

[DuckyD]

Edit: I really do not know.
Modified by DuckyD at 8:23 AM 2/4/2010

[chicos240]

what about the head volume?That would change as you shave metal off.

[OM3GA]

Its hard to calculate also because how much does the HG flatten out when the head is torqued? Or is the 1.5mm the thickness after the head is torqued?

There is a lot that goes into it.

I will definitely be getting a Tomei headgasket sometime down the way.

I was thinking about getting the 1.8mm one to drop the compression down a bit more, but how low is too low?

[DuckyD]

d!ck

I see what you did there...

Most shops only take .010 mm off when decking.How do you account for that?Is it by changing the deck clearance?

I am still uncertain how decking the head affects overall CRs.When I spoke to my machine shop they said they typically take five to ten thousandths of an inch off (.005" to .01"). That corresponds to .12mm to .25mm.I would just subtract that material from the headgasket thickness and call it a day, although I am sure there are better methods.

[chicos240]

I am in agreement. If you take metal off the head, or the block, you are doing two separate things.1. Bringin the piston closer to the valves, which could affect your piston stroke, if you take too much metal off the block.2. Reducing cylinder volume or head volume, which is Vc(clearance volume)These 2 things equally affect CR ...according to wakopedia http://en.wakopedia.org/wiki/Compression_ratio So, if you want to keep things equal you must add whatever your machine shop shaved off, to your head gasket thickness. This will ensure that you are as close to factory spec compression ratio as possible.How much your Gasket crushes is probably directly related to the quality of your Head Gasket. This brings up a question on block decking, how much shaving is too much. I am sure there is point where you can no longer shave off.

[DuckyD]

As far as how much the headgasket gets "flattened", I am fairly certained that the thickness advertised is the "flattened" thickness after it has been torqued.
Modified by DuckyD at 8:45 AM 2/5/2010