Head gaskets revisited

[Robyn]

After the debacle with the Block and other things On the DaHooooley rebuild I have been thinking about the Cometic head gaskets again.

The Cometic gaskets for those not familiar are a "Multi layer stainless" gasket.
They are made specific for RH and LH sides.

The passage at the front that allows coolant to lay against the block on a standard gasket contributes to electrolysis around the Number 1 and 2 cylinder fire rings.

The Cometic gaskets do not have the front passage (Not needed) and thus do not allow the coolant to lay against the block.

The Cometic gasket also is a solid metal gasket (Many layers) and has coined up areas that seal around the various ports as well as the cylinder bores.

The one thing I noticed right off what that the bolt torque on the head with the Cometic reached and held torque quickly.

The final 1/4 turn was far firmer than with the Felpro comp gasket.

I was told by the techs at Cometic to spray the gasket lightly with copper coat and then install.

The block and head surface needs to have a very smooth finish to use these gaskets though.

Do not use on a block that has errosion.
The machine finish must be very smooth.

Now the folks at Cometic will make these gaskets to spec for you.

The stock thickness for the 6.5 is .045" (All specs from Cometic are for a compressed gasket thickness)

A .055 gasket would be used for a .010" decked block
My 929 block in the DaHoooley needed one gasket at .070" to make up for the fact that the block had seen the one deck done previous to my owning it and NOT CHECKING THE DECKS BEFORE BUILDING :eek:

I am very convinced at this juncture that the Cometic gasket may very well be a far better choice for these engines.

No fabric to break down with heat.
No narrow fire ring to go away.
No coolant contact with the block at the front like stock.
More even torque maintained in the head bolts.

These are not cheap though. About $150 per side.

If good is your desire for long term hard use, this may be the best choice when doing a rebuild.

These gaskets can be ordered to your specs to allow for extra decking.

Stock is .045" thick and the piston to head clearance with head bolted tight is .035" (This is spec from GM with stock gaskets)

*******KEEEP IN MIND to order the thickness you want when compressed *******
Stock Felpro compressed is about .041" (This is the best I can get from measuring a new gasket that was removed unfired)

So if you have a block that has been decked once and now needs it again.

You can simply measure the protrusion of the piston above the deck with the piston at TDC and then calculate the needed gasket thickness to allow for the needed .035" clearance of piston to head.

Sadly these gaskets like all other head gaskets are a one time use.

Cometic also offers reusable head bolts that torque in the normal manner and not the TTY.

You can use TTY bolts.

I recommend that when using the TTY bolts that you go through the torque sequence twice on the first two steps and then do the 1/4 turn thing.

Once all the bolts are done, go back through with the torque wrench and start with the last bolt done with the 1/4 turn, sneek the torque up until the bolt just starts to move then observe the reading.

Now go back through the entire pattern from the start and sneek the torque up and if the wrench moves take the torque up to the final reading you obtained with the first bolt checked.


I did this and found a couple of the first bolts tightened that last 1/4 turn were not as tight as the rest.

This will get all the bolts to the same torque.

Having a good even torque is essential for a long life engine.
Having some bolts somewhat different can cause stress levels to differ in the heads and the block. NOT GOOD :eek:

These sorts of stress differences can and do effect the whole block assembly.

Just some observations that I have had time to reflect on since building the DaHoooley.

Hope someone finds this useful.

Maybe worthy of a sticky ?????? ;)

[tommac95]

Sounds like a 'good news' solution for a block with decks that have allready been surfaced (like mine). Mine had a fair bit of corrosion along the lower side of Most jug holes.

I think 'electrolysis' is a decent analysis. I Hope that the new , low-sulfur fuel will leave less corrosive residue.

While eliminating the "passage" {??} sounds like a sort of cure ... it bears mentioning that stainless steel , in contact with steel/castiron may well Promote corrosion of the 'less noble' base metal , More than if a gasket of a same material as the block were installed.

That is to say , that the most basic rule regarding 'galvanic' corrosion is to Avoid the contact of 'dissimilar' metallic alloys in solution.
See : www.engineersedge.com/galvanic_capatability.htm
en.wikipedia.org/wiki/Galvanic_corrosion
www.corrosion-doctors.org/Definitions/galvanic-series.htm
www.ceoe.udel.edu/seagrant/publications/corrosion.html

[Another thing to note , regarding StainlessSteel , is that the various alloys of SS have vastly varying corrosion resistance under different environments-- thus a Steel , like alloy UNS316 ,which generally retains a beautiful luster in marine spray environments , may suffer significant pitting when submerged in brine] . SS316 usually costs 2-4X as much as the hardware-store SS302/SS304 (etc). Better alloys for corrosion resistance immersed in brime , cost 3-5X as much as SS316 !! {etc., etc.}


The "stiffer" response while torquing may reflect the use of a thicker ring in the Cometic gaskets than the OEM/Felpro. I was surprised at how narrow the Felpro gasket was , compared to those in my old 1980 VW rabbit diesels (which Never sealed well)....
I wondered , at teardown , if the (deck) pitting corrosion was not partially caused by extreme pressure (stress-corrosion), too.
- T m

[rogers]

While eliminating the "passage" {??} sounds like a sort of cure ... it bears mentioning that stainless steel , in contact with steel/castiron may well Promote corrosion of the 'less noble' base metal , More than if a gasket of a same material as the block were installed.

Most of the cometic gaskets I have seen (admittedly only a few) are covered in a rubber/viton layer. I believe the 6.5 ones are too. Shouldn't this prevent said corrosion from dissimilar metals?

By the by, RPM Machine sells the standard cometic gaskets on their site for $98. In case anyone wants some.

[Robyn]

The Cometic Gasket that I got did seem to have a coating but it seemed more like a plating or ???? extremely thin no matter what it is.

The fire ring on the Cometic is not a separate item as is the case with comp gaskets like Felpro.

The ring is formed into the gasket during production. (Stamping)

The corrosion that forms around the number 1 and 2 cylinder fire rings is due mainly to the action of the coolant coming into contact with the iron block and the stainless ring. (Dissimilar metals = elctrolysis)
I think that the fire rings actually move a little too and help cause some "fretting"
The one piece Cometic may not see this action at all.

This area is the usual location of deck errosion on the 6.5/6.2 engines.
Also the reason given as to why Cometic makes their gaskets as RH and LH specific.

There is not a front coolant port in the gasket (Not needed)

The standard Cometic gasket at $98 is not a bad deal.
The price heads North when you start ordering the custom thickness.

Additional thickness adds $$$$ as the gasket gets thicker.

Still beats the heck out of having to scrap a perfectly good block just because the decks need a second KISS.

.010 the second time around will definately not compromise the block.

The different stainless materials all do different stuff and react different in any given environment.

5086H16 is some great stuff.

Marine grade aluminum that is very tough and resists corrosion under nasty salt conditions.

The tubs on my transfer truck are made of this stuff.

Pollishes to a very high luster and holds up to years of being in the elements.
Will tollerate a lot of flexing and tough use.

Dump truck boxes/tubs are a perfect example of HARD USE.

I lined the lower section of my tubes with a poly liner to keep the material off the aluminum as even as good as the 5086 is it wont tollerate abrasion like Hi tensil steel.

The poly is cheap to replace and also is slippery.

Robyn

[tommac95]

>>Most of the cometic gaskets I have seen (admittedly only a few) are covered in a rubber/viton layer. I believe the 6.5 ones are too. Shouldn't this prevent said corrosion from dissimilar metals?

I would think that any insulating coating on the gasket would prevent corrosion ,
at least , at the waterjacket boundaries, where the pressure/temperature are moderate.
At the cylinder/head combustion seal , i fear the pressure/temp would destroy anything softer than hard metal/ceramic.

Corrosion engineers consider protective coatings (proprietary commercial paints) to be protective from corrosion because they are dielectric (insulating) coatings. Electrolysis/galvanic corrosion requires the dissimilar metals to (I) be in good electric contact , and (II) be submerged in a conductive solution . The rubber/paint coating insulates , cutting off the connection.

>>5086H16 is some great stuff.

It sure is . I think 5083 is another fine aluminum alloy (for corrosion resistance). These are employed in the building of aluminum-hulled vessels for marine service. The superstructure is often left unpainted ; the underwater hull is painted primarily to prevent 'fouling' by algae, barnacles , etc.

It turns out that the 6061 alloy -- found in most extruded aluminum tubes , etc , is also excellent for resisting corrosion.

Tubs , toolboxes even truck bodies (not to mention aircraft) are good uses for aluminum.
Particularly if the environment is moist , it helps that the aluminum [truck body , toolbox , tub] be insulated from contact with the steel frame ....

[More Power]

The multi-layer SS gaskets, like those used on the Duramax have held up very well. Like the 6.5, the Duramax also uses TTY head bolts.

The only way to know whether the Cometic gaskets are better than OEM or Fel-Pro is to run them, long-term, then form an opinion. Are Cometics worth the 3-4x the cost? The last time I bought Fel-Pro gaskets/bolts, I paid less than $100 for both sides, and they lasted for 250,000+ miles in a built 6.5.

Personally, I believe the erosion we see on 6.5 head decks, especially on/near the fire ring, is due to steam erosion. Molecules of water (coolant) enter pores of the cast iron, which exposively turns to steam during combustion. Slowly over time, this can produce pitting or erosion.

Jim

[Robyn]

This is an interesting concept in that its caused by steam.

The outcome is the same though with the RH and LH gaskets having the front water port left out.
This protects the area from contact by the coolant.

I am also suspect that another reason for gasket failure is the breakdown of the intermediate material of the Felpro gasket.

Many engine that I have pulled apart have shown the head gaskets to be in a very sad state in the area around the fire ring.

The material is actually crumbling and disintegrating all around the fire ring.

Once the support for the ring goes away then the gaskets integrity is gone.

Having the soft material sandwiched between the two layers of stainless leaves the structural integrity of the gasket at risk as the miles and time add up.

The latest incarnation of the felpro gaskets seems to be fairly good at holding together.

Having the complete stainless gasket though I think with the mechanical seal portions stamped into them is a very good design.

Strange though that GM did not use the Felpro gasket in the late 6.5's

My DaHooooley has a Cometic on one side and the Felpro .010 OS on the other.

I dont anticipate any issues with either of the gaskets with the usage that I give the rig.

Heat, as in too much, can for sure aggravate the degradation of the soft material type gaskets.

I have seen some OEM gaskets that came off with the area around the fire rings literally having turned to what looked like Carbon and that crumbled as the gasket was removed.

This would indicate that failure was not all that far off.

I had the not so good fortune to have purchased a 6.5 Burb and upon seeing it for the first time it ran great great and while we discussed thing a head gasket failed, the engine started to miss and coolant puked out the overflow.

I had flown into SanDiego to pick this rig up and the failure occured shortly before I handed the seller the check. :eek:

He was uptset as was I.

Definately not his fault but had this failure happened just a few minutes later I would have been stuck in SOCAL with a dead rig and a real dilema.

Leave it and fly home :confused:

I got my deposit back and he had the mess to deal with.

I spent the day on a Jet airplane. :( Round trip flight from PDX to SanDiego

AAAAAARRRRG

Could have failed somewhere between here and there and really been a mess.


Missy