DIESEL HEAD GASKET LEAKAGE SUMMARY INFORMATION

There are various reasons why a cylinder head may not seal, that should be detected before a head gasket is replaced. Some may not be readily apparent to the technician because the theory of sealing is not fully understood.

First get an understanding of what is going on in the engine and what the gasket must accomplish. The pressure within the diesel engine cylinder is much higher than a gasoline engine, 1000 vs. 600 psi. This presents a unique challenge to seal the increase in pressure with the clamping load from the four (4) bolts around each cylinder.

The sealing concept is to use most of the clamping load, about 75%, to seal the compression. This is accomplished by placing a round wire ring inside of a thin metal shield that surrounds the cylinder bore. When the bolts are tightened, we literally have line contact around the bore between the cylinder head and the block. Because it is line contact, the pressure exerted by the ring to the head and block is extremely high. The clamping load is used to compress the metal ring. The body of the gasket is a few thousands of an inch thinner than the ring after it is crushed. Therefore, none of the clamping load is used to crush the body. The colored rings around the various holes in the gasket are a cured RTV sealer. The sealer is about .005 inches in thickness, on each side. It is thick enough so that it gets crushed between the head and block. The sealer keeps the combustion gases from going into the coolant and obviously keeps the coolant from leaking out through the gasket.

The gasket has another feature that needs explanation. The prechamber represents a special challenge because the wire ring must cross over the prechamber which, most likely, is not flush with the head. If the prechamber is recessed, the clamping load in that area will not be as great. If it is exposed, the clamping load beside the prechamber will not be as great. In either case, the leakage at the prechamber area is expected to be greater than at any other area on the gasket. The oblong hole in the shield and groove in the gasket are designed to allow those gases that leak in that area to be vented to the outside.

Dealer Technical Bulletin Bulletin 81-T-40 contained a wealth of information regarding head gasket sealing and precaution. Those items are still very important so review that bulletin. Some things that are not covered in the bulletin will be discussed now.

There has been undue concern regarding the flatness of the block and cylinder heads. But now it should be better understood that the sealing surface is the wire ring in the gasket where it contacts the block and head. Any damage to these surfaces will result in gasket leaks. Use of the motorized wire brush or grinder could remove a few thousands of metal. The head may then clamp the body of the gasket rather than the sealing ring. One engine block returned for examination was .018 inches lower at the bore than it was one fourth inch away. The technician had used a surface grinder to clean the block. This engine had repeat head gasket failure, at about 5,000 mile intervals.

Most technicians are aware of an indentation in the block and head surface where the sealing ring contacts both parts. While this appears to be quite deep, actual measurements have shown that the groove is only one or two thousands deep and does not affect sealing. There are new head gaskets available from WDDGM that are used with .030 inch oversize pistons. Use of these head gaskets will move the sealing bead outboard of the existing groove. These gaskets will be used in the various kits.

Another condition is one that is evident by looking at the gasket once it is located on the dowel pins on the block. The sealing bead is only slightly larger in diameter that the bore. The bead may extend into the chamfer at the top of the cylinder which results in an uneven crush of the wire and after a few miles will result in a leak.

To check for this, lay the old gasket on the block. Look at each cylinder, the gasket should be concentric with the bore. It may help to pull the metal ring out of the gasket so the block is more readily visible.

If the gasket is not concentric with the bore look at the shield on the side next to the block. There will be a crease line in the shield. If the line shows up on the shield, you definitely have dowel pin shift, if you are unsure, play it safe anyway.

There are two options to correct this condition. The new gasket which is used with .030 piston will result in the sealing ring moving outward .010 inches which may be enough to correct the condition. Another method is to enlarge the dowel pin holes in the head gasket until the gasket can be located with the ring concentric with the bores. The gasket, however, will not stay in the correct position when the cylinder head is installed. To keep it in place, put a small daub on contact cement on each end of the gasket between the bolt holes. Place the gasket on the block, then remove it to allow the contact cement to become tacky. Then place the gasket in the proper location on the block. Care must still be exercised when installing the cylinder head.

Another condition that could exist is to try to put a cylinder head on the block without the aid of dowel pins. Service blocks do not contain dowel pins. The dowel pins are very difficult to remove. Service blocks should not be installed without dowel pins. It is essential that the dowel pins be transferred for if they are not, the head gasket will leak. Should dowel pins be needed, they can be ordered using P/N 585927.

The importance has previously been explained of making sure that the bolt holes in the cylinder block are drilled and tapped deep enough. Some technicians have been advised to run a tap into all holes. The thought being that this will resolve the problem. Technicians then are only cleaning up threads rather than tapping the hole deeper. There is no need to put a tap in the hole if the hole does not need to be tapped deeper. The head should be placed on the block without a head gasket. Then run a .005 feeler gage around the edge of the head. There should be no clearance, this indicates that dowel pins are not holding the head off the block. Then by hand, screw each of the bolts in. The bolts should screw in far enough to contact the head. This will indicate that the holes are drilled deep enough.

The bolt threads should be wire brushed to clean them and the oiled. Oiling should be on the threads and under the heads of the bolts. Oil on the threads and under the head are critical so that the friction on the bolt is reduced during installation. Do not put the oil in the bolt hole, and excessive amount of oil could cause a hydraulic lock and prevent the bolt from tightening up.

We have a recent report of cylinder heads being replaced by bending up the injection lines and removing the cylinder head without taking the intake manifold off. Other home remedies such as painting gaskets and using sealants have been used. Sealants will sometimes attack the RTV sealer which results in a leak. We have tested gaskets supplied by various vendors and have found none to be as good as the gasket supplied for service. Some dealers have also tried some aftermarket gaskets and have confirmed that they do not resolve the problem.

We continue to have a number of people concerned about the hair line cracks in the valve seat area of the cylinder head. These cracks are very shallow and do no harm. They do not result in valve leakage or in water leaks. Heads should not be replaced because of these cracks even if they extend into the valve seat area.

Another concern has been the fact that some holes in the block or head are covered totally or partially by the gasket. It is far easier to control the size of a hole in the gasket than in a casting. Enlarging these holes may upset the water flow and result in uneven cooling.

These are certainly a number of items that must be considered when replacing a head gasket. The following checklist may help to make sure that you don't forget anything.

V8 DIESEL HEAD GASKET INSTALLATION CHECKLIST

Latest head bolts have "-" on top of bolt along with vendor code.

Wire brush head bolts to clean threads.

Oil threads of bolts and underneath head.

Dowel pins hold head off block.

Dowel pins off location.

No dowel pins.

Cylinder heads warped more than .010 inches.

Prechamber +/- .004 inches from head.

No damage in sealing ring area.

No stamps in seal area around water passage.

Water passage seal surrounds all water passages.

No chips in bolt holes.

Bolt holes in cylinder block drilled and tapped deep enough.

Follow torque sequency and installation torque procedure.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.