Fel-Pro vs Cometic vs Mahle MLS Head Gaskets — The Real Differences

TL;DR: All three brands sell multi-layer steel head gaskets, and on a stock-bore engine at stock power all three will seal. The difference that actually decides the purchase is whether the brand lets you choose a thickness. Cometic is the only one of the three that does — every Cometic part number on our shelf is specified as a bore and a thickness (4.100" × .040", 4.100" × .051"), while Fel-Pro and Mahle sell one thickness per application, whatever the factory used. That single fact splits the three brands cleanly: Fel-Pro and Mahle are for putting an engine back the way it was, Cometic is for engines where the deck has been cut and the quench has to be set on purpose. The second difference nobody mentions on the product page is deck surface finish — a race MLS gasket will not seal on a machine finish that a composite gasket would have tolerated.

The three catalogs, side by side

Every row below is a real listing on our own shelf. This is the comparison that matters, because it shows what each brand chooses to sell rather than what its marketing says it can do.

Two things jump out of that grid.

Fel-Pro is the only one of the three that still sells composite. Its SBC 400 gasket with steam holes, its Vortec L31 gasket and its 4.3 V6 gasket are all composite — because those engines were designed around a composite gasket and a rougher deck finish, and putting a race MLS on a forty-year-old cast-iron deck that has never been cut is a good way to buy a second head gasket. Breadth is Fel-Pro's actual product. It covers the odd engines.

Cometic's entire GM catalog is one product sold in dimensions. There is no good/better/best ladder. There is a bore number and a thickness number, and you are expected to know both. That is not a premium tier of the same thing Fel-Pro sells — it is a different purchase, aimed at somebody who has a deck-height measurement in hand.

Browse the full range in head gaskets, or the complete top-end kits in gasket sets.

What multi-layer steel actually is, and what it demands in return

An MLS gasket is typically three to five layers of spring-steel shim, the outer layers embossed into raised beads around each cylinder and each coolant port, with a polymer coating — usually a fluoroelastomer such as Viton — bonded to the sealing faces. Layer counts and coating chemistry vary by brand and part number. The bead is a spring. Torque the head down and the bead flattens elastically; every combustion event flexes it a few ten-thousandths and it springs back. That is why MLS survives boost, detonation and thousands of thermal cycles where a composite gasket relaxes and eventually loses clamp.

The price of that spring is a flat, fine surface for it to spring against. A composite gasket has a thick, conformable body that fills machining marks — it will seal a deck in the 60–110 Ra range without complaint. An MLS bead has almost no compliance, so the deck and the head both have to be finished fine enough that the polymer coating can bridge the remaining peaks. Cometic publishes a specific Ra requirement for its MLS gaskets — considerably finer than a composite needs — on the installation sheet for each part number; give that number to your machine shop.

This is the most common reason a brand-new MLS gasket weeps coolant on a fresh build: the block was decked with a coarse insert and nobody measured the finish. It is not a brand problem. It is a machine-shop spec problem, and it applies to a Fel-Pro MLS exactly as much as to a Cometic — Cometic's just gets blamed for it more often, because it is the one people buy for the hardest builds.

Two practical consequences:

• If your deck and heads are being machined anyway, tell the shop the target finish before they cut, not after. A re-cut costs more than getting it right once.
• If the engine is going back together on original, un-machined surfaces, a gasket designed around OE finish assumptions — Fel-Pro or Mahle in the OE application — is the lower-risk choice, and "lower risk" here means it is genuinely the better part, not the cheaper compromise.

The thickness question — the only place these brands genuinely diverge

Head gasket thickness sets quench (also called squish): the distance between the flat of the piston at TDC and the flat of the combustion chamber. Quench is what shoves the end gases out of the cool corner of the chamber and back toward the plug, and a tight, well-controlled quench is one of the cheapest sources of detonation resistance in a naturally aspirated pump-gas engine. Thickness also moves compression ratio — a few thousandths is worth a meaningful fraction of a point on a small-bore engine.

You do not get to choose that number with Fel-Pro or Mahle. You get the factory thickness, which is correct for a factory deck height, which is correct right up until somebody decks the block or the heads.

Here is what the choice looks like on our shelf where it exists:

Look at the LT1/LT4 pair. Same engine family, same bore, two thicknesses — and the thicker one is the supercharged application. That is the whole argument for Cometic in one row: on a boosted build, gasket thickness is a compression-ratio adjustment you make deliberately, and Fel-Pro and Mahle do not sell you that adjustment.

Look also at the SBC entries: 4.040", 4.060", 4.155". Cometic sells by finished bore, not by nominal displacement, because a race gasket is sized to the cylinder you actually have after boring. If you do not know your finished bore to three decimals, you are not ready to order a Cometic gasket — you are ready to call your machinist. Shop by engine family in SBC Gen I, LS Gen III, LS Gen IV or LT Gen V, and confirm year and engine on the fitment page.

A bore warning worth reading twice

MLS gasket bore should be slightly larger than the cylinder — by a small amount the gasket manufacturer specifies, so the fire ring sits on the deck and not out over the cylinder. "Slightly" is doing real work in that sentence. A 4.000" gasket on a 3.780"-bore 5.3 is more than two tenths of an inch oversize: the sealing land is enormous, the fire ring no longer sits where the head's chamber edge supports it, and the gasket has nothing to clamp against where it matters. That is not a close-enough situation.

This is a live trap in our own catalog, and we would rather say so than let somebody order into it: one of our Cometic rows is titled for "LS 5.3L / 6.0L iron block" and carries a single 4.000" bore spec. That gasket is right for a 6.0 (4.000" nominal) and wrong for a stock-bore 5.3 (3.780"). Read the bore number, not the displacement in the title. We are correcting that row.

Where Mahle actually fits

Mahle is the brand people struggle to place, because its marketing does not shout. The useful way to think about it: Mahle is an OE supplier that also sells to the aftermarket, so a Mahle Original gasket is frequently the same design philosophy as the part the engine left the factory with, in OE-equivalent construction, with no thickness menu — because the factory did not have a thickness menu either.

That makes it a straight substitute for Fel-Pro in the OE-replacement job, and the honest answer to "Mahle or Fel-Pro for my stock rebuild?" is whichever one covers your engine and is in stock. On our shelf Fel-Pro covers more engines — 13 GM rows to Mahle's 6, including the SBC Gen II Opti-Spark LT1/LT4 and the SBC 400 that Mahle does not list at all. That is a coverage answer, not a quality one.

One naming trap worth knowing when you cross-shop: MAHLE sells under more than one label, and our own catalog shows it — the GM rows carry the vendor name "Mahle Original" while the Ford Modular and Coyote rows carry "MAHLE". Same manufacturer, two spellings, and on a faceted storefront that is two different filter entries. If you filter by brand and come up short, check both. (We are consolidating ours.)

The decision, in one table

The part nobody budgets for and everybody should

None of this matters if the clamp load is wrong. Torque-to-yield head bolts stretch permanently by design and are not reusable — reusing them is one of the most common causes of a repeat head-gasket failure, and it will kill an expensive MLS gasket exactly as fast as a cheap composite one. If the build is boosted, high-compression, or you simply want the option of pulling a head again without buying hardware twice, studs are the answer rather than bolts.

We cover the material choice — ARP2000, L19 and Custom Age 625+ are three different metals, not three price tiers — in the ARP stud material guide. Browse head studs and bolts alongside the gasket, and if you are working on a Ford, the same three brands compete there too: see Ford Windsor small-block and Coyote.

Frequently asked questions

Is a Cometic head gasket better than a Fel-Pro?

Not in the general case. On a stock-bore engine with untouched deck surfaces, a Fel-Pro MLS in the correct application is the better-matched part, because it is designed around the factory deck height and the factory surface finish. Cometic is better specifically when you need to choose a thickness or a non-standard bore — which is a different job, not a higher grade of the same job.

Can I use an MLS head gasket on an un-machined old iron block?

Often not successfully. MLS beads have very little compliance and need a fine, flat surface. An old deck that has never been cut is frequently rougher than the finish an MLS gasket wants, and it may not be flat either. On those engines a composite gasket — Fel-Pro PermaTorque or the Vortec composite, depending on the engine — is the correct part, not a downgrade.

How do I pick between a .040" and a .051" Cometic?

From measurements, not from horsepower. Measure your actual piston-to-deck height at TDC, add the gasket's compressed thickness, and you have your quench. Then check the compression ratio that results. A thicker gasket opens quench and drops compression — which is what you want on a supercharged engine and what you do not want on a naturally aspirated pump-gas build that is already loose in the hole.

Does Mahle make the same gasket the factory used?

Sometimes literally, sometimes not. MAHLE is an OE supplier across many platforms, and a Mahle Original gasket is built to OE-equivalent spec for the application. Whether a given part number is the identical OE part depends on the engine and the model year, and we do not claim it row-by-row without confirmation — ask us about the specific part number.

Do I need new head bolts with a new head gasket?

If the engine uses torque-to-yield bolts — as the LS, LT, Coyote and most modern engines do — yes, always. TTY bolts are stretched past their elastic limit on the first install and cannot deliver correct clamp load a second time. If you are already buying hardware, compare the cost of a second set of TTY bolts against a set of studs you can reuse indefinitely.

Is a thicker gasket a fix for a marginal head-to-deck seal?

No. Thickness is a quench and compression adjustment, not a sealing repair. If a gasket is leaking, the cause is one of: an out-of-spec surface finish, a deck that is not flat, insufficient or uneven clamp load, reused TTY bolts, or a cracked casting. A thicker gasket changes none of those and will fail the same way.

Are these three brands interchangeable inside one gasket set?

Physically, yes — many builds run a Cometic head gasket with a full set from another brand for everything else, and that is normal practice. What you cannot do is mix thicknesses between banks. Both cylinder heads get the same gasket part number, always.

Related reading

• LS head gasket buyer's guide — bore and generation first
• ARP2000 vs L19 vs Custom Age 625+ head studs
• LS3 head flow showdown