HEAD GASKET REPLACEMENT

If you are removing any head, open the block drains to drain the coolant below the surface of the head or coolant will leak into the crankcase when the head is removed. On engines with V configuration it can have one drain for each side. When removing the head follow the reverse of the tighten sequence and do it in stages to prevent warping of the head. In most cases the exhaust manifold can be removed attached to the cylinder head.

Mercedes Benz M104 engines from the 1990's are very commonly leaking oil from the back right hand corner. The failure was partly due to dissimilar metal expansion and the original head gasket design. An aluminum head expands faster than a cast iron block. This combined with the placement of the oil passage very close to the edge on the block. The head gaskets now have an updated design but still repeat failure is all too common. Careful surface preparation is crucial to having a good seal, often repeat failure is the result of corner cutting and poor preparation of the surface. The using of abrasive disks that are too aggressive and cut into the aluminum head surface. Mercedes-Benz does not recommend the use of abrasive disks they prefer gentle scraping or chemical removers, in the trade abrasive disks are used everywhere. If you are worried about bits of abrasive getting into your oil passages and later embedding in a bearing then either plug the open holes or be cautious of material falling into them. A vacuum can help remove dislodged material while scraping with the other. In practice using sharp scraping blades works well for poorly adhered material when done at shallow angles gently, don't gouge the surface.  If not done carefully you can cause damage that can require cylinder head resurfacing.  I prefer a combination of gentle scraping and scotch brite pads all done by hand. Finish the surface prep with 3M bristle disks available in three grades, white 120 grit, yellow 80 and green 50 grit.  Use the lightest grit possible on aluminum, and only very light pressure and keep moving.  You can plug any open oil feed passages but if the surface is already relatively clean this can be avoided. Be sure you do don’t forget to unplug them later or you will have bigger problems than leaving them open could ever cause.  Bristle disks are somewhat expensive unlike roloc disks, and they have an abrasive embedded in plastic fingers and produce very little fine debris. If you use an air die grinder most of the material is spun off radially and you can polish the surface and remove any material from small pockets. Use low speeds, light pressure, and shallow angles to prevent gouging. Another problem with inexpensive abrasive pads is that some of the fines that remain can pass through the oil filter and may continue to circulate in the oil increasing wear. With the abrasive embedded in plastic the particle is larger and can get lodged in an oil filter. Using disks for final surface prep can be a good idea, although the problems arise when they are used to remove bulk material.

Once the two surfaces are prepared be sure to check the surface for any deviation from flatness. Place a machinist bar evenly over its length to 0.0001" check longitudinally, transversely, and diagonally in two directions noting any deviation. It’s a good idea to check this before cleaning because it may need to go to the machine shop for resurfacing anyway. It must be clean enough for the straightedge to lie flat. If you can see light from a led flashlight under the bar check with a feeler gauge and quantify the measurement and make sure it is within specification. If not, it has to be stripped and sent to a machine shop for resurfacing. You can leave the valves in. If the surface is corroded it may also need to be resurfaced as this can inhibit good sealing, depending on where the damage is. A head should only be resurfaced when needed as it is not necessarily a good thing. When ready for assembly make sure both surfaces are totally clean. If oil is dripping out of the head wait until that has stopped before proceeding. Clean it just before assembly, as both surfaces need to be clean and oil free. It’s a very good idea to chase the head gasket bolt bores in the block to insure the bolts turn freely. Make sure you blow out the holes, as any liquid trapped in the bores will cause the bolts to hydro lock. A hydro locked bolt will stop turning and start twisting before the gasket is tightened completely because the fluid trapped in the bore is incompressible, causing the resulting assembly to fail very quickly. Most head bolts today are stretch bolts, which have a torque specification and an angle of rotation specification. This is typically done in three stages. The bolts stretch each time they are installed, so they have an overall maximum length specification above which they are discarded. If the bolts are loose or the head was overheated consider replacing the bolts as a precaution. Make sure the bolts threads are clean and lubricated with motor oil. Tighten only in the specified order and in stages or you may warp the head. If the surfaces are clean and oil free don't apply any engine sealant unless it is specified in the work instructions. If your work is done correctly it will seal on the first warm up, and as the cylinder gasket compresses over time the stretch bolts maintain tension and the seal. The first warm up is critical as the gasket really isn't sealed yet. Don't install the coolant pressure cap until it is at operating temperature and fully bleed the air with the heat on. If you pressurize it before it is fully warm one problem you can encounter is coolant forced between the gasket faces and this can inhibit proper sealing as the gasket face has temperature activated sealants on it. Run it up to operating temperature and let it cool off then install the coolant pressure cap.          

A common timesaver on M104 head gaskets involves blocking the chain tensioner with a wooden or soft wedge, so the tensioner is not removed or reset. Avoid this technique because the tensioner is a ratcheting type and even if wedged it could slip outwards slightly and the chain will end up too tight, upon reinstallation. A tight chain will be slightly noisy, and can wear and stretch causing the timing to be off. Removing the tensioner should be done when removing the heads. Make sure to reset it on installation or the chain will be way too tight. It has to be dissembled in order to be reset. If this is done correctly you will hear the chain tensioner advance on first start and oil pressure is applied as it is hydraulically driven, causing the sound of the chain to quiet as soon as this happens.  

The best bet here is to call or email us before trying to replace your head gaskets.