Summary
The correct thickness of a cylinder head gasket is not chosen by simply measuring the compressed old gasket. On many diesel engines, it depends on the maximum piston protrusion above the block. The gasket manufacturer then translates this thickness class by a number of notches or holes visible on a tab.
Where to find thickness marks?
Before disassembly, look for an area on the edge of the gasket that protrudes from the cylinder head. It may have one, two, or three holes, notches, or an engraved reference. These marks identify a class specific to the engine family; three holes do not correspond to a universal thickness.
Photograph the mark and note the engine code and the gasket reference. A manufacturer or supplier catalog can then be used to match the marking to a nominal thickness.
Why measure piston protrusion?
After machining, piston replacement, or doubt about a previous assembly, the old mark is no longer sufficient. The method described by gasket manufacturers consists of cleaning the block face, placing a properly calibrated dial indicator, and measuring each piston at top dead center, at the prescribed locations.
The maximum value measured on all cylinders is retained. It is compared to the manufacturer's table to select the gasket class. Precision is essential: an error changes the clearance between the piston, cylinder head, and valves, as well as the compression ratio.
Can the old gasket be measured with calipers?
This measurement can help to roughly identify a part, but it is not a reliable basis: the gasket has been crushed, heated, and sometimes deformed. Deposits also skew the reading. First use the marks and engine data, then the protrusion measurement if the procedure requires it.
Does a machined cylinder head require a thicker gasket?
Not automatically. Some cylinder heads have a minimum dimension and should not be machined beyond it; other procedures provide a specific solution. Arbitrarily fitting a thicker gasket does not correct a cracked cylinder head or a deformed block. Flatness, surface condition, and residual height must be checked by a professional.
Essential checks before reassembly
Identify the engine by its code, use new screws if they are elastic, and respect the tightening order, angles, and torques. To place this repair in the general maintenance of a vehicle, our selection of interior car parts remains distinct from engine components and should never be used to deduce mechanical compatibility.