Technical Tips

Surface Finish
Head and block surfaces can cause sealing problems if they are too smooth or too rough. Too smooth a surface allows excessive lateral motion of the head gasket, which can deform the combustion seal. Too rough a surface creates small leakage paths. We recommend checking the surface finish with Fel-Pro's Surface Comparator, Part No, SC-I. Prepare the head and block surface in the 90-1 1 O RMS. (80100 RA) range. That is just enough to feel with your fingernail.

Surface Flatness
To maintain constant contact between the gasket and the head and block, deck surfaces must start out flat and remain flat after the fasteners are torqued. The maximum initial "out of flat" as measured with a straight edge and feeler gauge should not exceed .0025" in any direction. To find out if the surfaces are staying flat during engine operation, it's necessary to "map" the used gaskets. Wipe them off and trace the outline on a piece of paper. Measure the thickness of the gasket body within 1/4" of every bolt hole and at a point midway between bolt holes. In addition, try to find an area of the gasket body that has not been compressed and measure the thickness there as well. The compressed areas should be 003-.004' thinner than the uncompressed areas, and all compressed thickness' should be within .001-002" of each other. If the compression is inadequate or uneven, the clamping force can be changed by raising or lowering the torque on the fasteners in those areas 5 ft lbs. at a time. This will fine tune the load on the gasket and result in less flange bending. Examine the gaskets again at the next tear down. If they are evenly compressed: the head and block are staying flat while the engine is running. This reduces cylinder bore and valve seat distortion which makes more power.

Clamp Load !Torque Procedures

Insufficient clamp load is the cause of many head gasket problems if the gasket is not tightly clamped between the head and the block combustion gases and engine fluids can leak past the gasket.

The key to sufficient clamp load, is to have adequately stretched head bolts or head studs. Proper clamp load will not be reached if head bolt assemblies (or stud/nut assemblies) have too much friction. This is because torque wrench readings will reach specification before adequate bolt stretch has been achieved. Clamp load normally declines moderately over time as the head gasket " relaxes" and bolt stretch is reduced. We measure this as "torque loss" of the bolts. Clamp load can decline excessively due to extreme engine temperature, excessive head motion or detonation. For proper installation:

Use hardened washers under bolt heads (or nuts) to  prevent galling of the cylinder head and to reduce friction.

Tighten head bolts (or nuts) with a smooth motion.  Sudden or jerky movement of the torque wrench gives false readings, resulting in clamp loads as much as 20%  below normal.

Follow the recommended torque pattern and tighten bolts or nuts in at least three steps, up to the specified  torque.

Fastener torque specifications shown often are for use with moly, anti-seize, or Gray Bolt Prep (GRA2)  Motor oil or EP grease is not recommended. Bolts which enter the engine's water jackets should be sealed with Teflon sealer or Gray Bolt Prep. If more clamping force is  required to correct a sealing problem, the torque can be  raised 5 to 10 ft. lbs. at a time until the problem is corrected. Just remember that excessive torque can  strip the threads, break the fastener or permanently warp the flanges. Approach the problem carefully and slowly.  Don't try to use excessive torque to make up for warp-age or defective surfaces -- it won't work!

With higher installation torque, it's more important to chamfer the bolt holes in the head and the block. This will prevent the threads from pulling up and distorting the block deck surface which can keep the gasket from being compressed properly. If you have doubts about how much chamfer is needed, examine the gasket around the bolt holes. If there are signs of gasket crushing within .050-.100" of the bolt hole, increase the amount of chamfer slightly.

In severe service, such as racing, retorquing is recommended to restore maximum clamp load. After a complete engine warm up and cool down, retorque the bolts or stud/nuts to specification.
In circumstances where retorquing after running the engine is not possible, the next best method is retorquing before the engine is taken off the engine stand. After torquing the fasteners, wait at least 10 minutes, back off each fastener, one at a time, 1/8-1/4 turn and pull them back up to the torque specification. This will allow for gasket relaxation, thread engagement variations and ensure consistent clamp load.

Retorquing bolts that use Gray Bolt Prep will require approximately five ft./lbs. additional torque to overcome the locking action of the compound. Retorquing will not adversely affect the sealing capability of the material.

Appearance of Used Head Gaskets
Learning to "read" a used head gasket can help you solve some problems and prevent others before they happen. A "plug light" with a magnifier is an ideal tool for close examination of the gasket.
Examine the combustion armor for carbon tracks which indicate either combustion leakage or that the armor was  hanging into the combustion chamber or cylinder chamfer.

Examine armors for discoloration of the stainless steel. This is a sign of excessive casting temperatures.

Look for signs of seepage around coolant holes. A gasket which was sealing properly will have distinct impressions of the castings around coolant holes. If impressions are not distinct or if the gasket coating has been  washed away, there was probably a coolant leak due to inadequate clamp load or excessive head bending.

Measure the used gasket with a micrometer to see if it  had been properly compressed as described in surface flatness section.

Most head gasket combustion seal failures occur because of excessive heat in the gasket sealing areas. To correct the situation, more coolant flow must be provided to these areas. This can be accomplished through the use of a more efficient water pump, higher pump speed, restricting the coolant flow out of the engine (increases jacket pressure) or revising the coolant holes in the castings and/or the gaskets. Remember to make only one change at a time so the effect of a particular change can be determined
Many OEM and aftermarket water pumps do not flow sufficient coolant evenly to both sides of the engine. We highly recommend the newer design high performance pumps introduced by manufacturers such as Edelbrock and Howard Stewart.

To maximize the life of head gaskets, the use of electric water pumps is not recommended. They often do not flow sufficient coolant to prevent hot spots in the engine, even if the "gauge" temperature is normal.

For an even more extensive look at gasket usage and tips on doing it like the "Pro's", Use this link to go to Fel-Pro's web site to read more.
Note: This link opens in a new window. (Fel-pro's site is one of the best we've seen for information and tips. Let's hope we see more companies doing what Fel-Pro have done.)


Directory  |  Diesel Engine  |  Tools  |
Home Page   |   Returned Mail   |   Odds n Ends    |  Boat Builders  |  Engine Page  |  Electric Page  |  News Page  |  Safety page  |  Trailering  |  More About Us   |   Related Sites   |   Special offers   |   Event Calendar   |   Opportunities