Head gasket failures aren’t always about torque specs or “cheap gaskets.” A lot of repeat leaks come down to one boring, easy-to-miss detail: RA/RZ surface finish for head gasket sealing. If the head or block deck isn’t the right kind of smooth, even a premium MLS gasket can seep coolant, push compression, or stain the fire ring—then you’re tearing it back apart.
Below is the practical, shop-floor breakdown of what Ra and Rz mean, why gasket type changes the target, and how good machine shops prevent sealing problems before the first bolt gets torqued.
10 Things RA/RZ Numbers Tell You (and How They Affect Sealing)
1) Ra is the “average roughness”… and averages can lie
Ra (roughness average) is the arithmetic mean of tiny surface deviations from the mean line. It’s a great general metric—but because it’s an average, it can hide the occasional deep scratch that becomes a leak path.
2) Rz is the “peak-to-valley reality check”
Rz looks at the height difference between peaks and valleys (a peak-to-valley style metric). It’s more sensitive to surface extremes that matter for sealing, especially on thin, spring-steel MLS layers.
3) MLS gaskets demand smoother decks than composite gaskets
MLS gaskets generally want a flatter and smoother surface than older composite-style gaskets because the layers don’t “compress and conform” the same way.
4) Real-world targets: the numbers you’ll see most often
Different manufacturers publish different targets, but these are common ballpark references:
- Fel-Pro points to roughly 50–60 Ra for aluminum and 60–100 Ra for cast iron in their surface finish guidance.
- Cometic recommends 50 Ra or finer for MLS sealing.
- Some guidance also references Rz limits (you’ll see ranges depending on gasket style/coatings).
Bottom line: don’t guess—match the finish to the gasket type + manufacturer requirement.
5) “Too rough” doesn’t just leak—it creates micro leak highways
If the deck is too rough, the valleys can connect into channels. Combustion pressure and coolant don’t need a big gap—just a continuous path.
6) “Too smooth” can also be a problem (yes, really)
Certain gasket coatings and materials seal best with a controlled texture. Ultra-polished surfaces can reduce “bite” and limit how coatings flow and grip—especially if clamp load and flatness aren’t perfect.
7) Waviness and flatness matter as much as Ra/Rz
A surface can hit the “right Ra” and still fail if it has waviness, low spots, or distortion. Ra/Rz describe texture—not overall shape. That’s why good shops check flatness and not just finish.
8) The machining method changes the texture pattern
CBN, PCD, broach, flycut, belt finish—each can leave a different surface “lay” and micro-tear pattern. Two decks can measure similar Ra but seal differently because the texture is different. (This is where experienced machinists earn their money.)
9) Measuring correctly is half the battle
Surface roughness numbers depend on sampling length, filter cutoffs, and instrument setup. If one shop measures differently than another, you can get “correct” numbers that don’t actually match the spec intent.
10) The block deck is guilty until proven innocent
A fresh head surface won’t save you if the block is rough, pitted, or has corrosion around coolant ports. MLS sealing is a system: head + gasket + block + fasteners + torque method.
Quick Checklist (What Good Shops Do)
- Match gasket type to finish requirement (MLS vs composite; coated vs uncoated).
- Verify both surfaces (head and block deck).
- Control cleaning (no aggressive abrasives that change the finish and create low spots).
- Use the right fasteners + torque procedure (clamp load consistency matters more on MLS).
Need a Cylinder Head That Seals the First Time?
If you’re chasing a mystery head gasket leak, don’t just throw another gasket at it—start with the surfaces and the parts that control clamp load.
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Browse our Cylinder Heads catalog (new and reman options).
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Looking for a full kit? Check options tagged Head Gasket Set with Bolts.
Conclusion
RA/RZ surface finish for head gasket sealing is one of those details that feels “too technical”… right up until it costs you a weekend, a set of torque-to-yield bolts, and another round of coolant in the oil.
If you’re building or repairing an engine that must seal—especially with MLS—treat surface finish like a required spec, not a suggestion. Then buy parts that are built and supported by people who actually understand machining.
Shop our cylinder heads, grab the right gasket/bolt combo, and if you’re unsure—contact us and get a machinist on the phone.