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.
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 ho’es 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 wire ring must cross over the pre-chamber which should be flush with the head. If the pre-chamber is recessed the clamping load in that area will not be as great. If it is exposed, the clamping load beside the pre-chamber will not be as great.
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.
While the cylinder heads are off the engine, they should be carefully inspected for a number of possible conditions, one of which is warpage. If any cylinder head is warped more than .006″ longitudinally, .003″ transversely, it should be replaced; resurfacing is not recommended.
Minor surface cracks in the valve port area of the cylinder head, especially between the intake and exhaust valve ports, are not a normal condition. These surface cracks may affect the function of the cylinder heads and they may require replacement for this condition. The use of magnaflux or dye check is recommended as cracks in the cylinder head that affect performance are not always readily visible to the naked eye, therefore magnafluxing is necessary.
There is 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 gaskets available 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 than 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.
Make sure that the bolt holes in the cylinder block are drilled and tapped deep enough. 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 then coated with a sealant lubricant (1052080). This should be on the threads and under the heads of the bolts. This is critical so that the friction on the bolt is reduced during installation. Do not put the oil in the bolt hole, an excessive amount of oil could cause a hydraulic lock and prevent the bolt from tightening up. Do not paint the head gasket with a sealant. Sealants will sometimes attack the RTV sealer which results in a leak.
