MLS gaskets are awesome—until they aren’t. If you’re running MLS head gasket surface prep like it’s a basic composite gasket job, you’re basically scheduling your next tear-down. MLS needs flat, smooth, clean, and properly clamped surfaces. Miss one detail and you’ll get seepage, combustion leaks, or that slow coolant mystery that turns into a full-blown headache.
Below is the “do it once, do it right” checklist.
The MLS Head Gasket Prep Checklist (Do These or Don’t Bother)
1) Know what MLS actually wants (it’s not forgiving)
MLS uses multiple embossed steel layers and coatings to seal under high cylinder pressure. That sealing strategy depends heavily on surface quality and clamp load—more than composite gaskets do. Victor Reinz specifically calls out strict limits for surface unevenness/waviness/roughness for MLS installs.
2) Measure flatness—don’t “eyeball” it
Before you even think about surface finish, confirm the head and deck are within spec for flatness. If the head or block is warped, MLS won’t “crush and conform” like softer gaskets.
Tip: Use a precision straightedge + feeler gauges, and check multiple directions (lengthwise, crosswise, diagonals).
3) Hit the correct surface finish (Ra/Rz) for YOUR gasket brand
This is where most repeat failures are born.
- Fel-Pro publishes Ra guidance and explains why the number matters.
- Cometic states a 50 Ra or finer recommendation for MLS sealing surfaces.
- Heavy Duty Parts Company also breaks down Ra/Rz and why MLS gaskets can seep if the finish isn’t right.
Bottom line: Don’t guess. Match the surface finish target to the gasket manufacturer’s spec.
4) Use the right machining process (and tell the machine shop it’s MLS)
Not all surfacing methods produce the same “texture profile.” MLS generally wants a smoother, more consistent finish than old-school stones or rough milling.
When you drop off the head/block, say this clearly:
“This is for an MLS gasket—what Ra can you guarantee, and how are you measuring it?”
5) Clean like you’re prepping for surgery
MLS doesn’t tolerate junk on the deck:
- Old gasket material
- Coolant crust
- Oil film
- Scotch-Brite “fuzz” and embedded abrasives
Clean until a white towel with brake cleaner wipes off clean. Then clean it again.
6) Don’t stack “miracle sealers” on MLS
Unless your gasket manufacturer specifically calls for a spray or coating, don’t add it. Many MLS gaskets are designed to seal with their built-in coating and embossing. Adding extra chemicals can actually reduce friction behavior and clamp consistency.
7) Fasteners matter more than people admit
Reusing torque-to-yield head bolts, mixing hardware, or using mystery bolts is a recipe for uneven clamp load.
If you’re doing a proper rebuild, use the correct bolts/studs and follow the correct procedure. Heavy Duty Parts Company has a solid overview on why the right head gasket set and bolts matter.
8) Torque procedure: slow, staged, and by-the-book
MLS hates uneven clamping. Follow:
- The correct torque sequence
- Correct steps/stages
- Correct lube (especially with studs)
- Any required angle-torque steps
And if the gasket/fastener maker calls for a retorque or special process, follow that—not what your buddy “always does.”
9) If you’re unsure—use the gasket supplier’s guidance first
AA1Car gives a straight-shooting rule: use the gasket supplier’s surface recommendations (especially for MLS) or the OEM spec if you’re installing OEM.
That’s the “stop arguing on forums” answer.
Conclusion
MLS gaskets don’t fail because they’re “bad.” They fail because the prep is sloppy, the surface finish is wrong, or clamp load is inconsistent. Nail the MLS head gasket surface prep checklist above, and you’ll stop donating your weekends to repeat head gasket jobs.
If you’re pairing a quality cylinder head with the right gasket hardware, check out HeavyDutyParts Cylinder Head options here:
- Browse cylinder heads (includes head + gasket/bolt set options on select listings)
- Read: Why Your Cylinder Head Might Not Be the Problem (But the Head Gasket Is)
Helpful Reads:
- Cometic: MLS head gasket materials overview
- JE Pistons: Tips to properly install an MLS head gasket