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Another Cometic MLS Gasket Thread / Dished Piston Measurement

papsimagoo

papsimagoo

Delicious Boost! Nom nom
Joined
Jan 3, 2007
Location
Los Angeles
Heads off, this is my first cometic replacement. Don't know what thickness originally was used. If they print any ID/PN on them let me know, I didn't see any.

Piston to deck measurements- ~ 0mm-.4mm below block deck height on cylinders 1 and 4 averaged out...

So I believe it goes .003" for every 1000 rpm, redline 7k so 7x.003=.021 +.003" for safety puts me at ordering around a .030" compressed....BUT i have waveguide/large valve relief pistons..want/wanted to make sure I am doing this right before trashing a motor. Do the pros factor reliefs/dished pistons into their CRs?

BTW going to use real hylomar.

Engine Specs:
2.5l B230F + T by RSI
RSI 86mm Billet Stroker Crank
RSI 158mm H-Beam Connecting Rods
RSI Waveguide 8v Turbo Pistons
Cometic MLS Head Gasket
RSI Stage 2+ Cylinder Head Prototype
RSI Race Valvetrain Kit
RSI Stage 3 Turbo Cam Prototype
RSI Square Tooth Adjustable Cam Gear
RSI Twin Scrool SS Turbo Header
RSI Poly Engine Mounts
Twin Tial MVS 38mm External Wastegates
Greddy Type S Blowoff Valve
Holset HX40/Garrett Hybrid Turbo
Isuzu NPR Intercooler
NIW Motoring Intake Manifold
Megasquirt 2 EMS
Mitsubishi Coils
Injector Dynamics 1600cc Fuel Injectors

<a data-flickr-embed="true" href="https://www.flickr.com/photos/vividvideo/43132155281/in/dateposted-public/" title="Head off1"><img src="https://farm2.staticflickr.com/1786/43132155281_a7dfa45dc5_z.jpg" width="640" height="480" alt="Head off1"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>
 
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Yes, you must factor piston dish volume.

The formula is:

(Cylinder volume + combustion chamber volume + head gasket volume + piston dish volume (whether positive or negative) + piston deck volume + ring land volume)/(combustion chamber volume + head gasket volume + piston dish volume (whether positive or negative) + piston deck volume + ring land volume)=compression ratio

Any machinist will tell you that both the block deck and head deck need to be cleaned up to use an mls gasket, but I have gotten away with just doing the head every time in N/A applications.
 
Been running a MLS gasket for 20K miles with a planed and polished head, block just wet sanded with 400 and 800 grit on a flat steel plate. Remember, the head being aluminium is what needs to expand with its 2.5x thermal expansion rate vs. steel/iron (or thereabouts ).

Below .036" there is negligible gain per the SAE.
 
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Went with .030" cometic, old looks like a .027”. Will report back. If anyone reads this and knows a hylomar source in PDX... I'd like to know.
 
From Cometic

WHY DOES COMETIC RECOMMEND MLS GASKETS TO BE INSTALLED DRY?
Cometic Multi-Layer Steel (MLS) head gaskets go on dry because they are coated with a sealant. Each MLS head gasket is coated with a .001" thick Viton rubber that is bonded to the outer stainless steel layers. Adding an additional sealer can hinder the performance of an MLS head gasket.
 
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