Hylomar Vs Copper Coat on a MLS Head Gasket

I'm looking for people that have tried both, not people who say "I always use copper coat and had no problems".

What is the difference? Pros? Cons?

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not saying our oinions don't matter... but a quick phone call to a few gasket makers and they might give you more accurate info... instead of, "I always use copper coat and had no problems"

I used the Hylomar on my 428SCJ exhaust manifolds....

I find the gasket makers want to cover their rear ends and recommend no coatings. They don't want the blame for any gasket failure.

Normally I would not use any coating on a head gasket but when the head gasket is an all metal job and not the composite style that was common for so may years I have used the copper spray on it. The engine I did this on was a Madza BP 1.8L engine intended for Lemons racing. The engine has seen 5-6 races without any head gasket issues.

The Hylomar's advantages seem like a perfect fit for the MLS gasket. It never really hardens, holds up to 250 degrees, and seals superbly. My concern, since I've never used it, is will it cause a build upon the head gasket that can't be compressed under torque? Will it make my head leak?

I've used plenty of Hylomar in my days drag racing. we'd use it on holley carbs but never a head gasket. now a days everyone uses chapstick on carbs.

I used to use copper spray and then switched to hylomar, but not on mls head gaskets. Looking back it seems like a band aid for poor head and block prep or poor quality gasket material. Now I put on mls gaskets dry and I am very careful to meticulously prepare the block and heads.

The black MLS gaskets have sealer built into them.

The black coating is a sealer.

An engine builder I respect a lot says he uses Permatex High Tack on MLS gaskets. Personally, I've never used anything and haven't had a problem.

I've used Hylomar extensively on gaskets that are oil or fuel barriers. You used to be able to get the original stuff at Jaguar dealers, versus the Permatex, which is different. It strikes me as misuse to put it on a head gasket, though I have no scientific support for that view. I look at Hylomar as an alternative to RTV silicone and you wouldn't put that on a head gasket, would you?

I've also used Copper Kote successfully on old-school head gaskets and I don't see why it wouldn't work on MLS.

I used Permatx Hylomar on the case seam on a Harley I did.

Never again! (Back to YamahaBond, I shoulda known better)

On a head gasket? NO WAY! MLS really only needs (if anything) a micro sealer for the machine finish. The mechanical part of the seal is the embossed ridges that get squished and retain some give for thermal expansion.

An un-coated MLS I would put a very very light coat of thin aluminum paint and torque before it dries. (I have never seen an un-coated MLS)

f6sk wrote: I find the gasket makers want to cover their rear ends and recommend no coatings. They don't want the blame for any gasket failure.

Yes, but you're going to want to spray something on the gasket when you re-use it after it's been heat cycled and half of the original coating stuck to the head/block.

The gasket manufacturers will also tell you to never reuse a gasket, too. Heck, a lot of OEMs will tell you to use NEW bolts because they come with a little dab of blue Loctite and they don't trust you to sufficiently clean the old bolts and put new Loctite on.

(I install new MLS gaskets bare. Copper-Cote on used ones)

To the OP, I'm unfamiliar with Hylomar except for a thick blue toothpaste-like crud that I used to use for assembling Mazda 12As. I assume that this is not the stuff you're talking about.

Basil Exposition wrote: I look at Hylomar as an alternative to RTV silicone and you wouldn't put that on a head gasket, would you?

One of the Buick engine builders does this. Apparently they sell a little paint-roller kit so you can roll a thin layer of RTV on both sides of the head gasket before you slap it on.

Buick MLS head gaskets are weird. They like to wick oil through the gasket itself out of the valley. You have to either caulk the head to the block inside the valley (which requires new, surgically-clean parts for it to take) or just turn up the boost enough that you split the block before the oil leakage gets to be too annoying. And you can build those kind of cylinder pressures with an O-ringed block and a regular ol' graphite gasket that won't leak in the first place, no need to get fancy with MLS.

MLS gaskets are mostly for aluminum head motors where the block and heads have dis-similar expansion rates.

It's not just for dissimilar-metals engines. They are used on aluminum/aluminum engines too.

I think just about the only they aren't used is on older engine designs where the manufacturing tooling (which was bought and paid for years/decades earlier) doesn't hold the surface finish/flatness tolerances required for MLS. It blew my mind when I found out that they would "machine" the decks by ramming the blocks through a big broach. No wonder the surface finishes always looked horrible and there'd be a significant amount of deck height variance from one end to the other.

Knurled wrote: It's not just for dissimilar-metals engines. They are used on aluminum/aluminum engines too. I think just about the only they aren't used is on older engine designs where the manufacturing tooling (which was bought and paid for years/decades earlier) doesn't hold the surface finish/flatness tolerances required for MLS. It blew my mind when I found out that they would "machine" the decks by ramming the blocks through a big broach. No wonder the surface finishes always looked horrible and there'd be a significant amount of deck height variance from one end to the other.

A good surface finish helps....

They make more than one kind of hylomar. A spray, and a tube. I think the spray would be best for this application. I am re-using a MLS gasket (I know tisk tisk)- And after cleanup the black coating has come off. I want to make sure I don't have any issues and that is the reason for the question.

You do not want to add a coating onto an MLS gasket, you also do not want to reuse them. Its not a bit conspiracy to get you to buy more parts from the parts department. Gaskets that take a permanent set, and torque to yield bolts need to be replaced between uses. You can get away with reusing less critical metal gaskets like intake and exhaust manifold gaskets, but you dont want to do it on head gaskets or water pumps.This is grassroots motorsports, not cheap E36 M3ty hack garage.

Torque to yield bolts should be replaced, yes. But if you measure them against a new bolt, and they're the same length, then they have not yielded and can be reused.

95% of the time, they tell you to use new bolts because the coating/sealant gets wrecked. VW head bolts don't yield but the moly coating on the threads sure gets used up. The first time they torque like butter, the second time they make HORRIBLE popping/cracking noises. But since the tightening spec is an angle instead of a torque, the friction doesn't matter since the added friction comes AFTER the initial "baseline" torquing. It takes one heck of a lot more torque to get that 180 degrees but that is because the coating is gone. The tension on the bolt ends up being the same.

That is why they tell you to torque by an angle, after all... it's far more repeatable a process than torque. Ideally you'd prestress a stud and thread a nut down fingertight, but that is beyond the scope of what is economically feasible for lowly automotive engines

I use a Cometic MLS head gasket on my "older design" MG Midget, as do a number of vintage racers I know. We also reuse them, to no ill effect. Supposedly they are made from a spring steel that doesn't take a set or crush, unlike a copper gasket or the composite ones with crush rings. I suppose if I were building a motor that was supposed to last 50k to 100k miles and several years I'd put a new one on, but considering how often the head comes off on this one, I'm reusing it.

But maybe these are a different design or manufacture than the ones y'all are using on "newer design" engines, IDK.

I follow the headgasket manufacturer instructions...no issues so far...knocks on wood...