ROUGHNESS TESTING: WHAT IS IT AND WHY IS IT IMPORTANT?
When resurfacing a cylinder head, the surface finish can mean the difference between a long-lasting engine repair and a repeat head gasket failure.
In engine reconditioning, precision is everything. When it comes to cylinder heads and engine blocks, one of the most critical yet often overlooked measurements is surface roughness. Roughness testing ensures that a machined surface is suitable for proper head gasket sealing and long-term engine reliability.
But what exactly is roughness testing, and why does it matter so much?
What Is Roughness Testing?
Surface roughness testing measures the microscopic peaks and valleys left behind after machining a surface. Even a freshly machined cylinder head deck that appears smooth to the naked eye contains tiny ridges and grooves.
The most common measurement used is Ra (Roughness Average). Ra represents the average height variation of the surface profile from the centre line.
It is typically measured in:
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Micrometres (µm)
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Microinches (µin)
When resurfacing a cylinder head or engine block, matching the surface finish (Ra) to the head gasket type is critical for proper sealing and long-term reliability. At Southside Cylinder Heads, we always machine surfaces to suit the head gasket being used for each application.
Common Head Gasket Types and Preferred Ra
| Head Gasket Type | Typical Preferred Ra |
|---|---|
| Composite | 60–80 µin |
| MLS (Multi-Layer Steel) | 15–30 µin |
Composite Head Gaskets
Common in older engines, these gaskets are made from graphite or fibre material bonded to a steel core.
These gaskets are more forgiving because the softer material can conform to slightly rougher surfaces.
MLS (Multi-Layer Steel) Head Gaskets
Used in most modern petrol and diesel engines, especially turbocharged applications.
MLS gaskets require a much smoother finish. If the surface is too rough, combustion gases can leak past the sealing layers.
Other Factors to Consider
While these are the most common circumstances we see, other variables can influence the ideal surface finish, including:
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Boost levels
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Head bolt or stud clamping load
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Gasket material (such as copper or brass)
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Gasket coatings and sealing layers
Selecting the correct Ra ensures the gasket can seal properly under operating conditions.
Why It Matters
Correct surface finish is critical for modern engine reliability. At Southside Cylinder Heads, we measure and machine every cylinder head to suit the correct gasket specification, ensuring the best possible sealing and long-term durability.
If you’re unsure what surface finish your engine requires for your next build, give us a call on 07 3343 5899 or get in touch via our contact page.
Southside Cylinder Heads — high quality engine repairs backed by decades of experience and unbeatable industry knowledge
