Auto Service World
Feature   February 1, 2000   by CARS Magazine

Most critical gasket service

Cylinder Head Sealing.

No matter how good the gasket, its ability to perform as advertised will be either enhanced or compromised by the preparation of the cylinder head.

The cylinder head gasket is by far the most critical gasket on an engine.

Preparation of the mating surfaces is absolutely critical in order to achieve a positive, long-lasting seal with cylinder head gaskets.

Before installing any gaskets, the mating surfaces must be thoroughly cleaned, flat and have the proper surface finish.

Before any new gasket is installed the mating surfaces must have the old gasket completely removed along with any carbon deposits, rust, corrosion or foreign material.

Once mating surfaces are cleaned, the fasteners or bolts must be checked.

On newer engines, torque-to-yield bolts have been used. Most of these cannot be used again.

When bolts are used again, thoroughly clean the bolt threads making sure to remove all foreign material.

Carefully check the entire bolt for damage such as stretched threads or evidence of galling. Where there is any doubt, replace it with a new bolt.

Every bolt hole must be inspected and cleaned. Most of the modern light-weight engines in use today have cylinder head bolt holes that enter coolant passages.

Check every hole to see if it enters a coolant passage. This is easily done using a long piece of brazing rod or stiff wire.

Carefully note every one that does and re-tap these holes first, making sure to clean and remove all of the old thread sealing compound from the threads.

Re-tap the rest of the bolt holes and make sure the loose and foreign material is removed from the bottom of the hole so it does not interfere with the installation of the bolt.

Finally, chamfer every bolt hole to remove any raised or distorted threads.

For the second step of the preparation process, use a straight edge and feeler gauge to check the mating surfaces for flatness.

Each surface must be checked four ways; For length, width, corner-to-corner and opposite corner-to-corner.

On a typical three-cylinder and V6 head, the surface must not be more than .003″ out of flatness.

A four-cylinder and V8 head cannot be more than .004″ out of flatness and a straight six-cylinder must not exceed .006″. A good rule of thumb to remember is .001″ for every cylinder bore. If the allowable tolerance is exceeded, the mating surface must be machined and brought back to proper specification.

If any of these are encountered they must be repaired and corrected before final assembly is attempted.

Afterwards, the surface finish must be checked to insure it is within specification.

Most modern engines today have an 80 RMS to 120 RMS finish which is the measurement of the roughness of the surface. RMS is an abbreviation for Root Mean Square which is the method of measuring the surface.

A mating surface that has been machined with too rough of a surface finish may cause premature failure of the gasket.

Before installing gaskets, make sure the proper surface finish has been applied by the machine shop doing the work. SSGM