MLS vs Composite Head Gaskets

Three constructions, not two

Fel-Pro head gaskets in this catalog are not all the same stack of steel. The part type line and the attributes on the part page tell you which construction you are buying. The three you will actually see are multi-layer steel (sold as PermaTorqueMLS), a composite gasket with a fiber or graphite facing on a steel core, and a performance gasket with a pre-flattened steel wire ring around the bore. Picking among them by price, or by a photo of the box, is how a fresh head job leaks.

Use the construction the catalog lists for that year, engine, and bore. A gasket that seals a different engine in the same car family can be the wrong thickness or the wrong bore, and thickness changes compression and valve-to-piston clearance.

Multi-layer steel (PermaTorqueMLS)

An MLS gasket is several layers of spring steel, often three or five, with raised beads around the combustion openings and the oil and coolant holes. A thin elastomer coating fills small tool marks. The beads, not a thick facing, make the seal. Fel-Pro’s version of this design is PermaTorqueMLS. It is the usual gasket on late-model engines that came with MLS from the factory, on aluminum heads, and on engines with higher combustion pressure, including many turbo engines.

MLS is less forgiving than a composite gasket. The deck has to be flat, and the surface finish has to be fine enough for the coating and the beads to seat. Fel-Pro prints the finish requirement on the instruction sheet in that box. A milling job that was acceptable for an older composite gasket is often too rough for MLS, and the engine will push combustion gas or coolant through the joint even when a straightedge says the head is flat. Do not reuse an MLS gasket. The beads take a set the first time the head is torqued.

Composite gaskets

A composite head gasket uses a graphite or fiber facing attached to a perforated steel core, sometimes with a printed bead or a fire ring. The facing conforms to small pits and a coarser finish. That is why it is still the right part on many older cast-iron engines that were designed for it, and why it can seal a lightly pitted deck that an MLS gasket will not.

Conformability is not a reason to skip measuring the head. A composite gasket will not pull a warped aluminum head back into shape, and it will not make up for a deck that is eroded around a coolant hole. If the catalog lists a composite gasket for the engine, use that. Substituting an MLS gasket of a different compressed thickness because it is “more modern” changes the quench and can leave valves and pistons with less clearance than the engine was built with.

Steel-wire performance gaskets

Some Fel-Pro performance head gaskets seal the bore with a pre-flattened steel wire ring rather than an MLS bead stack. The attributes on those part pages call out the wire ring and the bore size. They are for engines the catalog lists them for, usually higher cylinder pressure than the stock composite or MLS part. They are not a generic upgrade for a daily-driver MLS engine, and they still need a flat deck and new torque-to-yield bolts when the engine uses torque-to-yield bolts.

Bore size on the part page has to clear the cylinder. A wire ring that overhangs the bore, or a gasket bore smaller than the cylinder you just bored, will not seal and can get hit by the piston. Read the bore attribute against the actual cylinder, not against the stock spec, if the block has been bored.

Finish, flatness, and thickness

Flatness and surface finish are different measurements. A straightedge and feeler gauge tell you whether the head or the block is warped. A profilometer, or the machine shop’s record of the milling head they used, tells you the roughness. MLS needs both: flat, and smooth enough for the sheet in the box. Composite gaskets tolerate a rougher finish and still need the head inside the warp spec for that engine.

If the head is milled, compressed thickness matters. A thicker gasket lowers compression and restores a little clearance; a thinner one does the opposite. The thickness on the Fel-Pro part page is the number to compare with what the machine shop removed. There is no universal “MLS is thinner” rule across part numbers.

• Stock engine, catalog lists PermaTorqueMLS: use the MLS part and follow its finish note.
• Stock older iron engine, catalog lists composite: stay with composite unless the deck is being prepared for MLS on purpose.
• Bored or high-cylinder-pressure build: use the performance part whose bore and thickness match the build, not the stock replacement.
• Head was welded, pitted, or overheated: measure it. No gasket construction fixes a deck that is out of spec.