I'm starting to piece my 1987 420SEL back together, and thought that since there are very limited resources to fellow 420 owners (or W126 owners in general) in terms of rebuild/ repair work, a posting of my processes would be an asset to anyone considering to labour on thier own vehicles.
I'll see what I can do in terms of a better introduction, additional tasks to add, and part no.'s used, but for now, this will have to suffice. Best wishes to anyone with a Merc; I hope that this helps.
As most will be able to tell, I'm doing most of this work on a garage floor with a small forklift skid as my engine stand and an inverted carboard box as my workbench. It really isn't how elaborate of a setup you have in your garage, but the quality and expanse of the smaller tools and knowledge that aid in a rebuilding procedure such as this. Plus, it's a very good excuse to buy new tools. More on that as we progress.
NOTE- The date on the camera is not the actual date...Rest assured I couldn't bear to hold back such DIY information!
Oh, and I'm not responsible for the malfunction, decimation, deformation, vacation, explosion, combustion (spontaneous or otherwise) of your engine/vehicle/neighbourhood due to your mis-interpretation of my interpretation of mechanical advisory.
Last edited by Benz Prodigy : 08-20-2008 at 10:01 PM.
The removal of the engine is a fairly straight forward affair; simply label any connections you're not sure of and neatly tuck everything away in places that it cannot get snagged or torn off. Work in an organized manner, sequentially labeling everything and placing the related nuts, bolts and assorted fasteners in containers of your choice. Try to clean things as you take them apart too, as this makes the whole process more enjoyable rather than a greasy, utterly chaotic mess.
Any type of "W126" CD-ROM "Repair Manual" is not needed at all, and, as I've found, is useless even as fire-starter (If only because of its plastic composition). If one is looking for an excuse to purchase such an item, I suppose a reason could manifest itself in the form of torque values for various fasteners, but, even if relied on for this task, the manual falls inexcusably short.
Do a little research in aluminum metalurgy, thread pitches and the physical aspects of a torqued fastener; I assure you more useful, versatile and realavent knowledge will be gained to assist in the calculation of torque values. With that said, I will do my best to post as many torque values as possible.
The most difficult task I encountered in removing the engine from my car was the delicate balance of keeping the garage door opened enough for the engine hoist to clear the door itself, while keeping the hood of the car at the optimum angle for the engine to be lifted high enough to clear the radiator core support.
Equiped with light alloy (cast aluminum) engines, the V8 W126 engines have some inherent problems unique to thier construction. One of these anomalies is the tendency to strip out the cylinder head bolt threads from the block should the head bolts have to be removed for any reason. It is well docummented that many a cylinder head has been removed without any such occurance, however, we'll take a look from the most unfortunate of scenarios and assume that every single (Yes, all 36) threaded bore has been part of a metalurgical massacre.
What happens is the following: the head bolts are torqued from the factory, and, over time, succumb to the stresses of being, well, head bolts. Due to the fact that they're tightened in a less than desirable base metal, the maleable alloy steel external threads of the bolts cold-weld and fuse themselves to the internal threads of the engine casting, resulting in catasrophic failure of the aluminum threads should the bolts be removed.
The solution to this problem is to fit steel threaded inserts to the bores inside the engine block. There are many ways to go about doing this; I've illustrated the route that I felt was the utmost accurate way of accomplishing the task. I used Timeserts, 24.5mm long black oxide steel inserts with a thread pitch of 12mm external, 10mm x 1.5 internal. One can purchase a drilling jig from thier local Mercedes-Benz dealership (nothing to do with or including the Timeserts) for about $800-$1100, and use a simple hand drill to drill out the threaded bores. The jig basically bolts to existing head bolt holes (assuming they still exist) and allows you to use precisely reamed bores as guides to drill squarely into the engine block.
For $800-$1100, you could buy a Bridgeport-style vertical mill in excellent condition, get it delivered, and not only make 100 drilling jigs, sell them on E-bay and have the machine pay for itself, but mount the engine block directly onto the table and drill the thing sans jigidy jig.
The first thing to do is to mount the engine block directly on large parallels on the milling table (two average guys can lift the engine in this state), after the upper oil pan has been removed to provide a perfectly square mounting surface. Lightly draw stone all affected surfaces to assure a flush fit. TAKE CAREFUL NOTE of how the engine is mounted here. The block rests on parallels, the engine mounts are left installed and TIGHT, then, support is provided under the farmost point of the engine mount as not to cause stress, then the engine is actually clamped near the inner side of the engine mount as to ensure leverage here, using the outer support as a fulcrum. Any other way and the engine mounts will crack and tear out of the block; Yes, it's clamped down *that* tight. Engine 2.jpg
Luckily, this is a 90 degree V8, so the head on the machine can be tilted 45 degrees to do the first (starting from the bottom), second and fourth rows of threaded bores. These holes are at right angles to the cylinder head mating face. Indicate the block square in relation to the milling head and table by using the rear main seal cover mating surfaces and the cylinder head mating surfaces themselves. I used a 0.0005" indicator; there's nothing quite like precision! The drill itself will run out more than that in the chuck, so anything more accurate is overkill really. Engine 3.jpg
The holes are drilled using a simple 13/32" drill, 30mm deep. The actual holes are about 35mm deep, but I'm only using a 24mm insert, so as long as the insert clears, it's all good. The Timesert drill comes with flats milled on it so it can be used in a tap-wrench. I'm not going to go off on any more of a tangent explaining how fundamentally wrong that entire concept is. Engine 4.jpg
The engine block has to be drilled in stages, because the X travel of the mill table only allows one bank to be done at a time. As for the 3rd row of drilled holes, they're actually at 1 degree, 52 minutes from vertical, (I rounded up to 2 degrees; the 8 minute error will be more than taken up in the thread play) so the milling head has to be tilted further to 47 degrees to accomodate them. Thus, the milling of the entire block is done in four stages: drill first bank of all right angled holes, tilt head further and drill remaining offset holes, remove engine from mounts and re-indicate square before drilling second bank of offset holes, drill remaining right angled holes. Since there are four rows of holes for each cylinder bank, a reference point is needed to indicate the X position of each individual row. A head bolt with the cap ground off works very well, simply screw the bolt into the hole to be drilled (there are usually some salvaged threads left near the bottom of the holes, not enough to hold torque though) and indicate about the axis of the bolt to locate the exact centre position of the bore. As all the bolts in a row are on the same plane, the table can then be moved and indicated in Y only to drill the rest. Whether you do this yourself or have someone do it, bring along a head bolt- at least that way they/ you can test it to make sure the angle of whatever hole you're drilling looks somewhat right.
Now the drilling part is complete. Step one. This is a 4 step process if you decide to use Timeserts. Drill, Tap, Counter Bore, Install Threaded Inserts.
Last edited by Benz Prodigy : 08-24-2008 at 07:32 PM.
Reason: ANGLE ERRONEOUSLY STATED AS 12 DEGREES, UPDATED
The engine is comprised of 5 major components: the block, left & right cylinder heads, rear timing cover, front timing cover and the upper oil pan. Between the block and the upper oil pan/heads, there are specific gaskets in place, however, the front and rear timing covers have no such luxury and thus require the use of Room Temperature Vulcanizing (RTV) sealant.
My goal was to get the mating surfaces of these parts so smooth and consistent that there would really be no need for a gasket of any type. This way, it's very unlikely that any leak should occur once a gasket making medium is applied.
DCFC0002.jpg
The engine should be thoroughly stripped to its basic parts, ideally this would include the pistons, rods and crank shaft, however, that is beyond the scope of what will be conveyed here. My engine only has 113,000miles on it anyways. Having the parts and the block steam cleaned will make this whole process more enjoyable and much easier. Remove the front timing cover, no special attention needed to anything, really. Worst case scenario is that you forget the location of each bolt, in which case it's very easy to rectify by simply looking at the counterbore depths on the cover and matching up the short and long bolts to suit.
If the parts are still dirty after cleaning, scrub the surfaces with a brass wire brush. No steel bristles here; it will scratch the aluminum.
DCFC0003.jpg
After brushing, remove any sticky gasket material deposits with a razor blade. This step has the potential to destroy the mating surface in short order. Razor blades are made from hardened steel and have the ability to remove material en masse if not gingerly handled. Scraping the surface with the blade at an backwards angle is the safest method. Pushing the blade can result in it digging in and creating gouges. If there are any burrs on the front cover casting, this process will remove them.
The oil and coolant O-ring grooves are cleaned and refined with a finishing stone to remove any burr left by the foundry and to remove tool marks. Finish the whole process by oil-stoning the entire surface with something like a 2"x6" stone, using a light cross-hatch pattern. Depending on how your surface looks at this point, use the roughing side followed by the finish.
On the engine side, preparations are made to fit the cover. After repeating the above re-surfacing process for the block side, lightly oil and fit the pins for the timing chain tensioner, intermediate gear and distributor gear. DCFC0004.jpg
Last edited by Benz Prodigy : 08-13-2008 at 11:33 AM.
Inspect the bore the seal seats in and lightly stone the sides with a finishing stone should there be any ridges or rough areas. I put a light film of grease on the outside of the seal, to aid in installation. Be sure to put a coating of grease on the inner lip of the seal, so it won't burn out once the crank shaft makes one rotation against the rubber without oil pressure.
With the front main seal in place, the coolant O-rings and oil gallery O-rings can be placed in thier grooves. Smear a light coating of RTV sealant around their circumference to keep them in place when the cover is flipped over to bolt it on the block.
Don't forget to put on the oil pump chain tensioner rail and circlip. I forgot to put the circlip on after sealing the timing cover and bolting it up, and although it is possible to get it on with a screwdriver and a bit of fiddling, it's not fun by any means.
Put a new oil pump chain on at this point, and bend it around and out of the way when setting the timing cover back on. As far as those bolts for the right lower timing chain guide, I snugged them up fairly well with great caution as everything here is aluminum. The crank bolt and spring washers were removed prior to fitment of the cover.
This is an example of how NOT to route the timing chain. The chain must go below the middle intermediate gear, not above it. By the time I realized I only had this one picture, the cover was sealed and dry. Note that the chain is open on one side, where the left cam sprocket would go, and there is more than enough slack to put the right cam sprocket in on the other side.
A small bead of RTV sealant around the entire web of mating surface is needed to seal the front cover and block together. Make sure the surface is free of oil before starting, and try not to glob the sealant on, especially around the O-ring seats. A smooth, consistent bead works best. Two sets of hands helps at this point, as there are locating dowels on the block that need aligning all the while trying not to smudge sealant on everything. If your cover is like mine, it will have been warped to the point that it looks quite scary, but everything will buckle down and lie flat once the bolts are snugged.
Place only the bolts shown in the next post on the timing cover; the others are needed for the various attachments the engine runs. Finger-tighten these bolts in a criss-cross pattern and only snug them after 24 hours, as the sealant must dry first. Great time to put on the water-pump...next up.
Last edited by Benz Prodigy : 08-13-2008 at 12:30 PM.
There are two choices when considering a waterpump replacement for a W126: a new unit sourced from a supplier such as Graf, or a rebuilt core from the Mercedes dealership. When purchasing a rebuilt waterpump, it is advisable to return the core of the old pump to the dealer; somewhat of a discount can be had in most cases. A rebuilt pump has such dispensible pieces as the shaft seals and bearings replaced, while a new unit (some will argue that only Genuine will do and won't consider this route) is a brand new casting, fitted with internal parts and tested.
The installation of a new, bolt-on waterpump is very straight forward. Clean and draw stone the mating surfaces (as if we haven't had enough practise already), set the gasket in place with no sealant of any type, and match the bolts to thier respective bores in terms of length after giving thier threads a generous dose of anti-seize. Tighten the bolts to 25Nm, in as much of a criss-cross pattern as possible. It is definitely recommended to finger tighten all bolts and then torque.
If tightening bolts in aluminum is not exactly a forte you possess, try torqing the bolts to a slightly lesser torque value. 21Nm's should be plenty in this case, as it's much easier to tighten them should they leak than...order...more...Time...serts...
Fitted pump. I think I'm missing a bolt, I seem to have lost it somewhere. Theoretically there should be bolts all the way around the flange. If you re-sealed the timing cover, the waterpump has to be installed while the sealant dries, or partially installed, as the top bolts clamp the timing cover. Mercedes420SEL- Engine 098.jpg
The gasket for the upper piece. The better the surface quality of mating parts, the less the gasket has to compensate. The goal is to make it as effortless as possible.
If you'll direct your attention to the first photo in this section, it will be realized that the upper portion of the waterpump is infact a rough casting not finished nor suitable for a gasket. I'm not sure if all the new pumps are shipped this way, but mine required manual finishing. Use a very good file and be sure to cover all the ports with a rag, the waterpump and passages will still require a good vacuuming, but try to keep as many filings out as possible. Avoid getting filings in and around the front bearing, brush it off thoroughly afterwards.
Use the double-cut, coarse side of the file to rough, followed by the single-cut fine side, followed by the rough side of an oil stone and finished with the remaining side. Be mindful to keep the file as flat as possible and work with slow, even strokes. Material is removed very quickly when cutting cast aluminum; avoid taking the casting down anymore than is needed to just clean up the mating surface. When the file clogs, a wire brush works excellent in removing the chips.
The final seal will only be as good as the worst surface.
Bolt the upper connection on without the aid of a sealant, and torque to 25Nm.
Install the thermostat housing in the same manner, except that this side is already finished from the manufacturer. Simply place the thermostat in the bore with the ball check valve in the top position and torque the bolts, post placement of the gasket, O-ring and thread anti-seize, to 25Nm.
Last edited by Benz Prodigy : 08-20-2008 at 09:15 PM.
Since the front timing cover is installed, prior to fitment of the upper oil pan, it's a good idea to fit the rear cover and main seal. As previously mentioned, the engine block consists of various large parts and thier sequential assembly is essential to ease of installation of generally everything. The front and rear timing cover are located with dowel sleeves, no doubt to keep thier relative positions somewhat consistant as the seal of the upper oil pan relies heavily on thier square and exact location. It is therefore necessary to install these pieces prior to fitment of the upper oil pan, as then the upper sump has a complete mating surface to mount flush with.
Clean the cover thoroughly, by any prefered method. I opted to use foamy engine cleaner and a brass brush the size of a miniature tooth-brush. Considerable frustration ensued, and thus this is not a recommended course of action.
Draw stone the mating surfaces. Keep everything square, use progressively finer abrasive sides, don't be a caveman/woman and try to remove 0.100", yada, yada, yada...
To install the rear seal, one could employ something called a "seal driver", but if you don't happen to have one stashed away in the tickle trunk, a C-clamp and two pieces of somewhat flat ferrous metal will work. I used a set of parallel's I made for my newly acquired machine (details of which are coming up in a section entitled "Why I bought a Jig-boring machine and how rebuilding my engine has enabled me to further justify and continue practising my obsession with tools").
Just be sure to use even pressure on the clamp, and move it about the radius of the seal to ensure square, even seating. The seal is fully seated when it is just a touch below the surface surrounding it. Be very careful not to crush it; C-clamps can generate hundreds of pounds of pressure. Also beware of the lip on the seal with the back of the clamp.
Apply sealant in the same manner as when sealing the front timing cover, that is, make sure the surface is absolutely clean in every respect, stone, and then wipe it down to remove any oil or grease. If you're using the sealant I have chosen, it is to your benefit to know that there are two types of Black sealant made by Permatex. One is just Black, and the other is Ultra Black. Use the Black sealant; Ultra Black remains gummy forever and doesn't quite dry. It looks as though I'm trying to fill the void in the seal with sealant, rest assured, not the case, just less than optimal posing. Put a thin bead all over the entire web.
Install the rear cover, and, after an elapsed time of 24 hours for the sealant to cure, torque the bolts to 10Nm in a criss-cross fashion. Be extremely careful when seating the seal; it will twist and deflect every which way and requires a gentle seating on the flange. Use only your fingers, nothing metallic.
It can be seen at this point that I have installed the upper oil pan prior to fitment of the rear timing cover. Trying to proceed in this manner makes for an absolutely brutal experience. Do not try to do this, you will destroy the upper sump gasket- I almost did- and then you'll have to remove all 3000 of those little cap screws anyways. The only way I saved it was by stuffing layers of 0.002" shim stock over the gasket and guiding the cover down while trying to find the position of those dowels.
Last edited by Benz Prodigy : 08-20-2008 at 10:06 PM.
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