A carbon seal face lapped to less than one light band at the factory can read three light bands after a few weeks in service — and still seal perfectly. That single fact trips up more maintenance technicians than any other aspect of lapping.
The usual approach is to jump straight into process steps. But the harder question comes first: does this face actually need lapping, or is it headed for the scrap bin? Getting that call wrong costs real money — either burning hours on a face that needs replacing, or replacing one that a 20-minute lap job would have saved.
What Is Seal Face Lapping
A seal face pressed against a cast-iron plate with fine abrasive between them loses its high spots one micron at a time — that is lapping. The controlled motion gradually restores flatness until the surface meets tolerance.
Before you start, make sure you understand what lapping actually fixes. Two parameters define a seal face’s condition: flatness and surface finish. Flatness describes whether the face is level — free of waviness and warping. Surface finish describes roughness at the microscopic level, measured in Ra (micrometers). Lapping primarily corrects flatness. A face can have good surface finish but poor flatness, and it will leak.
Flatness is measured in helium light bands (LB), where one light band equals 0.3 microns. Place an optical flat on the seal face under monochromatic light — interference bands appear, and each one represents one half-wavelength of deviation from flat. Fewer bands, flatter face.
Manufacturing lapping and maintenance lapping have different goals. Factory lapping brings new faces to specification — often below one light band. Maintenance lapping restores a worn face to serviceable condition, which accepts slightly wider tolerances depending on the application. The most common mistake I see is technicians holding maintenance work to factory-new specs, burning time that does not improve seal performance.
How to Tell If Your Seal Faces Need Lapping
Check the face with an optical flat first. If you do not have one, you cannot make this call — guessing based on visual appearance alone leads to bad decisions.
Flatness tolerances by material:
| Face Material | Maximum Light Bands | Notes |
|---|---|---|
| Tungsten carbide, silicon carbide, ceramic | 1-2 LB | Hard faces hold flatness well |
| Carbon, glass-filled PTFE | 2-3 LB | Softer materials allow slightly more deviation |
| Any material above 40 bar (580 psi) | 1 LB | High-pressure service tightens the limit for all materials |
| Shafts over 100 mm diameter | Up to 5 LB | Larger faces tolerate more due to contact area |
If your reading exceeds these limits, the face needs either lapping or replacement. Which one depends on the type of damage.
Lappable damage: Minor scoring, light wear tracks, and surface discoloration from normal operation. These are surface-level issues that lapping removes by taking off a few microns of material. Heat checking — the fine radial cracks from thermal shock — can be lapped out on hard faces like silicon carbide and tungsten carbide if the cracks are shallow.
Not lappable: Deep radial cracks, chipped edges, visible warpage, or grooving you can catch with a fingernail. If you skip this step and lap a cracked face, you will be back in a week pulling the same seal.
One thing that catches newer technicians: carbon and graphite faces relax after manufacturing. A face lapped to less than one light band at the factory will read as high as three light bands after installation. This does not mean the face is damaged. Carbon re-flattens against the harder mating ring during initial run-in. Check the mating hard face — if it reads flat, the carbon face is likely fine.
Heat checking on hard faces is the one damage type that creates the most confusion. Shallow heat checks on silicon carbide can be lapped out. Deep heat checks cannot — and the depth is not always obvious without magnification.
When to Replace Instead of Lap
Soft face materials — graphite, antimony, PTFE — should be remade from raw material rather than lapped. These materials are inexpensive relative to the labor of lapping and do not hold a lapped surface as reliably as hard face materials. If your seal uses a carbon or graphite primary ring, replacing it is almost always faster and cheaper than attempting a precision lap.
Hard face damage beyond lapping limits includes sub-surface cracks, chips larger than 1 mm at the sealing edge, and thermal distortion from severe dry running. A tungsten carbide face with edge chipping will not seal even if the undamaged area reads flat — the chip creates a leak path lapping cannot close.
For critical or hazardous service, the bar is higher. Repaired seals in ATEX or IECEx-certified environments must perform identically to the original documentation, including seal face temperature rise data. If you cannot verify that a lapped face meets the original seal’s performance envelope, replacement is the only compliant option.
Many styles of seals can have their faces re-lapped and elastomers renewed to return the seal to like-new condition. When replacing a larger seal, keep the old one — a properly repaired seal becomes your backup spare, cutting emergency lead times from weeks to hours.
Next Steps
Pull your optical flat out before the next scheduled shutdown and baseline your installed seal faces. A set of readings now — while the seal is running well — gives you a reference point when something eventually goes wrong. For hard faces reading outside tolerance with only surface-level wear, lap them. For soft faces, cracked hard faces, or anything in certified hazardous service — order new ones.
