head gasket re install question

[Titanium Man]

good evening,

If a fresh motor is put together with a cometic gasket, everything torque plated, and down the line the gasket blows...When re installing, do you install another cosmetic or a composite gasket? since the head/block surface will not longer have the same Ra when first assembled

Thank you

 

[sidewaystsi]

Check the decks with striaghtedge and feeler gauges,that should give you an idea.Just to pry, what was the reason the gasket let go?

 

[Titanium Man]

it has not. I am about to ascertain a fresh motor and I am wondering what would be the right way just in case. Thank you

 

[turboglenn]

"Cometic" is a brand (not cosetic,but i'm not picking just clarifying) and Cometic makes MLS (multi layer steel) and i think they make composit gaskets too...

As far as the roots of your question i'll explain it to you as i've heard it explained to me.

If the block was machined to the proper RA to start with and didn't warp it should be fine with a clean up (i'd really like to hear bogusSVO or some one who's actually been in this spot first hand though and what they have to say)

The head, being aluminum should always be sent to the machine shop before re-assembling with a new MLS gasket (or even complosite) it takes a lot of detonation, pressure or for something to go wrong pretty majorly to blow an MLS head gasket from what i understand, so if one lets go there's usually more to look for than surface finish (in fact i hear that usually pistons get holes in them, and rods or other parts fly out the block before a properly seated and torqued MLS fails to hold)


That's why some people, (myself included) run the composite gaskets as an intentional "weak link" to eep repair prices low when starting to push the edge of what's gonna hold (my composite parts store Fel-Pro is holding in excess of 500hp with a set of ARP L19 head studs torqued to 100 lb/ft which is ARP spec for the 6 bolt on these studs)

Another option i've heard about but haven't seen and am really interested in is what i've been told is a multi layer COPPER gasket..supposedly has the extra holding power of an MLS but being that copper is more maleable and can conform to imperfections in the block and head doesn't require such a fine finish and can be put on like you would a composite with only slightly less holding power when pushing the ragged edge of cylinder pressures and detonation

Personally for my goals i want to stick with composite, it's too mpainfull for me (and more than i have for money) to have a head and block pulled when a HG blows, so ultimately i would like soething similar to what Paul Volk is running and that's to have the block machined for O-rings, run a composite gasket and keep using the L19 studs which have been perfect so far.

I should also mention that i've popped two HG's this past summer from stretching older ARp 2000 studs and the head lifting on me under pressures that were indeed quite high (high timing with 500hp of airflow) I didn't take the head in the first time, and the second I had checked both the block and head with a machinist flat edge (my machinist stopped by the house as he passes me on the way home and to work) said the finish was a little rough (too much to go MLS for sure) but that composite gaskets will hold still and so i've been running it that way and pushing the tune even further since wihtout a single hiccup

Anyone with more info on this copper gasket please pass it along.. i would like to decide if this is what's right for me, or at least hear more about it than i have so far

 

[BogusSVO]

Glenn... you are right about building in a "weak link" better to have a $40-50 HG fail, than to burn the tops of the $400 pistons, or sling a rod on the outside of the block.


Surface finish and warp are 2 of the things to check, the third is fire rings, this is where the bead on the HG, digs/wears a slight groove into the mating surface, more common in the alum head than the cast iron block, but still happens.

The grooved area will have less clamp force and allow the HG to fail again.

A pic of a partialy milled block, notice the fire ring is still not fully removed.

Checking with a straight edge and feeler gauge will not show this imperfection, beacuse the feeler gauge will bridge this.

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[viperlp01]

"RA" is a term often used and I don't think people actually understand the purpose or meaning. RA is the roughness average of the surface.
[ame=http://en.wikipedia.org/wiki/Surface_roughness]Surface roughness - Wikipedia, the free encyclopedia[/ame]

What this means is as the cast iron block heats up it expands at a different rate than the aluminum head. What happens is the roughness of the surface moves along head gasket. This moving, if a rough surface is used, causes scraping on the head gasket. The finish is often shinny when first machined but after a head gasket is laid on the surface, torqued down, and has some miles put on it, it tends to have an imprint of the head gasket on the mating surface. The "RA" hasn't changed, it only has a color imprinted onto the surface.

In all reality with how many miles are put on our cars as they normally aren't our DD's or if they are see way less than normal miles the RA on a MLS is not as a big of a deal as everyone thinks it is. All it prevents is gasket damage over hundreds of heat cycles. Even then you pull head gaskets off cars that have 100k miles on them they normally look beat to shit anyways. We have made over 1600 hp with less than optimal "RA" on a blown alcohol car and never had sealing issues(iron block, aluminum heads). Don't get me wrong if "RA" wasn't important I would't spend 8 hours indexing the cutter on my decking machine to all within .0001 in the X/Y direction. Just some food for thought.

 

[98zx23369]

Alot of very good info here. Thank you