If you can link to or post repairs, please do so in this thread. I will be "polishing up" the thread later to make it easy to follow.
Is there a link for seat removal that works? I need to readup on this!http://www.benzworld.org/forums/forums/thread-view.asp?tid=1213744&posts=2
The new timing chain, upper chain rails and tensioner are finally in the 1989 420sel with 98Kmi. This report summarizes some thoughts that might be useful to a first-time DIY'er.
A. This job took at least twice as long as I thought it would, and that didn't include time studying the manual and other sources. If you are in a hurry or do not enjoy the work, this may not be a job for for a DIY'er.
B. Information--Search the List archives for useful information. Especially useful threads were "timing chain and rail replacement questions for a 117 engine" (Nov. 28-30, 2004) and "420sel timing chain" (October 29-30, 2003). I also bought an illustrated booklet ("preventing catastrophic V-8 engine failure") from Mercedessource.com (no connection) and this explained most of what the manual does not. There are photos of the "3 vice-grip method" of chain replacement at Mercedesshop.com ("M116/117 timing chain replacement"). Also, the recent book "Mercedes Benz Technical Companion" has several useful articles on timing chains and chain replacement.
C. The manual is essential reading of course, but I found several possible discrepancies. First, the camshaft sprockets should (must?) be removed to replace the upper chain rails. Second, it appears possible to wind in a new chain without removing the right side rockers. I did remove them, as recommended, to eliminate the problems of the right cam possibly jumping time. Third, I used a bit less than the manual's 25nm of torque on the 8mm bolts that attach various parts to the heads. The 1991 Technical Data Manual seems to indicate 21nm of torque for most of these. Also, I left the old tensioner partially inserted in the right head (held with a long 8mm bolt and nut arrangement) while winding in the new chain, so most of the slack on that side of the chain would be taken up.
D. Unanticipated problems--the 4 left-side chain rail pins were more or less rusted inside. A 6mm bottom tap cleaned them up. All 6 pins came out relatively easily after soaking in Kroil. I also "chilled" them with Radio Shack component cooler, and that might have helped. I bought 4 new pins. An old pin is the perfect drift for replacing the pins recessed in the heads. One of the cap screws holding the alternator mount on the right head was corroded and I bought a new one. The distributor retaining bolt was hard to get out. That bolt goes down through a 2-inch long spacer under the distributor and you need to get the Kroil down in there. Remember to counter-hold the fuel line fittings at the fuel distributor when disconnecting the fuel lines. Be prepared for a minor fuel spill. Relieving the gas tank pressure at the gas cap may alleviate this.
E. Other jobs--While the front of the engine is clear, it is easy to replace the thermostat, the radiator hoses and the voltage regulator. (Mine were original and due for replacement.) Also, with the top radiator hose off, the fan shroud can come out making it a little easier to work on the front of the engine.
F. Parts--I used genuine MB parts (chain, rails, tensioner, oil tube kits, valve cover gaskets and bolt washers). There are reports of aftermarket rails (and chains) failing relatively quickly. The new chain came with the "fish head" clip on the connecting link favored by Stu. (The clip must be installed so that the "head" points in the direction of chain travel.) I replaced the tension rail lining (available from MB as a separate part) and re-used the tension rail itself. I did not replace the cam sprockets, which looked fine.
G. Tools--Nothing truly exotic is required. A special valve spring compressor if the rockers are to be removed, a good 6mm hex bit socket, and a tool for pulling the chain rail pins. For the latter I ordered online a few long, fully-threaded 8.8 strength 6mm bolts, nuts and fender washers from the Bolt Depot (again, no connection), and used them with various sockets, large nuts, shim washers, etc. to replicate the official pin puller tool. The websites of Assenmacher, Zdmak, etc. illustrate the factory tools.
H. Although there may be some short-cuts, my order of battle was as follows (refer to the manual and/or the Mercedessource booklet for the job procedure details):
1. Disconnect battery. Remove air cleaner. Remove fan together with visco fan clutch (store in upright position), leaving 1 bolt in the fan pulley. I used a strap wrench on the pulley hub to keep it from turning. A long 10mm box wrench is helpful.
2. Unbolt the cruise control actuator and put to one side. Remove alternator and then alternator mount from right head.
3. Remove spark plug wires from plugs. Remove distributor cap together with plug wires. Set engine to TDC. On the engine, mark orientation of the distributor mark/rotor so the distributor can be replaced in the same position. Remove distributor.
4, Unbolt P/S pump and tie off to the side (the hoses remain connected). Unbolt P/S pump mounting plate from left head.
5. Disconnect fuel lines at fuel distributor and tie up out of the way. Remove valve covers. Gently pry off oil tubes and clean any plastic bits out of the cam bearing towers with picks and tweezers. If you are going to remove the right side rockers, now is a good time.
6. Check 3 upper chain rails for damage. Assuming no broken rails, pull all 6 chain rail pins 1/4 of the way out (about 3/8 of an inch). if you are going to have a problem with these, now is the time to know.
7. Slightly loosen both cam sprocket bolts. Reset engine to TDC and confirm/evaluate camshaft timing.
8. Mark and remove left camshaft sprocket, keeping tension on the chain (I used 2 bungee cords with hooks suspended from the garage overhead). Pull pins and fit new left side chain rails, one at a time. Refit left sprocket, observing marks, and the bolt.
9. Remove old chain tensioner. Mark and remove right camshaft sprocket, keeping tension on the chain. Pull the 2 rail pins and remove the old right side chain rail. Remove tension rail, replace lining, and refit the tension rail. Fit new right side chain rail. Replace right side cam sprocket, observing marks, and the bolt. (I deferred retorquing the cam sprocket bolts in case the timing needed to be adjusted after the new chain is in.) Reconfirm camshaft timing.
10. Wind in the new chain using the 3 vice-grip method or, with an assistant, using the method in the manual. It is easier if the spark plugs are out, but I would remove them only if new plugs are going in.
11. Fit new chain tensioner. Double check camshaft timing. Torque cam sprocket bolts to 100nm.
12. Put everything back together in reverse order. Use new oil tube kits and valve cover gaskets. Remember to set the distributor timing precisely according to the manual (before installing valve covers to make TDC identification easy). Do oil and filter change.
The main thing is to work patiently and carefully, and always use shop rags, etc. to prevent anything from falling into the engine. Be especially careful with the timing chains, as it would be a disaster if a chain fell into the engine.
All in all, the job was challenging and fun (and successful), but time-consuming as constant vigilance is necessary to avoid an expensive mistake.
At 98Kmi and 15 years, the old chain and rails were in OK shape (right cam retarded about 5 degrees), and didn't yet make noise on start-up. They probably would have gone a few more years. The plastic oiler tube clips were very brittle and definitely needed to be renewed.
I hope this report is of some help to other W126 DIY'ers. Thanks to Stu, Bob and others for their helpful advice. Charlie Wagner, 1989 420sel 98Kmi.
New valve stem seals are (finally) in the 1991 420sel at 120Kmi. Many thanks to Stu and others for their help on the project. This report summarizes information that may be useful to a first-time DIY'er.
Reading: Search the archives for postings on the subject. Particularly useful were "Valve Stem Seals" (March 4, 2001). The manual sections on the valve train are essential reading of course. If you have no experience under the valve covers, I would recommend a Mercedessource.com (no connection) publication "Preventing Catastrophic V-8 Engine Failure", which is directed to changing timing chains and rails, but also addresses oil tube clips and other points.
Tools: For this job you need (i) the factory valve spring compressor or equivalent from SIR Tools, Assenmacher, etc, and (ii) the genuine MB "magnetic finger" tool. Also very helpful are (i) valve seal pliers to remove the old seals and (ii) the valve seal mandrel tool (for 9mm stems) to press on the new seals. The "magnetic finger" tool makes it easy to get the valve cone halves (keepers) out, and I can't see how you can do the job without this inexpensive tool. Long needlenose pliers (or hemostats from your fishing vest) are needed to replace the keepers. You will also need a pair of curved, pointed tweezers to manipulate and position the keepers as you replace them. Also needed is an air compressor and a way to pressurize the cylinders. I used the inexpensive KD Tools version (# 901 Air Hold Fitting Set). It has a spark plug thread on one end and a female thread in the other end. Screw in a male quick-release air fitting and voila! Also, this rig is about the same size as a spark plug, so you can use your spark plug replacement technique to install it. (I use a 3-inch long piece of 8mm ID rubber hose to screw plugs in by hand several turns.)
Parts: I used genuine MB valve seals (and they are not expensive compared to aftermarket). I also replaced the oil tube clips, as it is easier to work with the oil tubes out and you could break the clips during the job. New valve cover gaskets too.
Procedure: The procedure is fairly straightforward and is described accurately in the manual, with the warning noted below. Note the following points:
1. Disconnect battery; remove air cleaner; remove fan and visco clutch, leaving 1 or 2 bolts in the pulley hub (and store fan/clutch in vertical position); remove cruise control servo, plug wires and valve covers.
2. Relieve gas tank pressure at the gas cap before removing gas line connections. Counterhold fittings with a thin 17mm wrench.
3. The manual has a page (in section 05-250) which shows the amount of crankshaft rotation after #1 TDC for each piston to be located at the top. This can be double-checked with the position of the rotor under the distributor cap. The harmonic balancer is marked every 90 degrees. See manual section 15-535 if you are unsure of cylinder location or firing order.
4. Prevent the possibility of rotation of the crankshaft while a cylinder is under air pressure with a 22mm socket/ratchet rig on the right camshaft nut and the 26mm socket/ratchet rig on the crankshaft nut. Brace each with the ratchets set to prevent rotation in opposite directions. I experienced no crank rotation when the timing marks were used precisely to set each cylinder to TDC.
5. The springs have a top and bottom and must be replaced as they were. On my engine, all the large/outer springs were marked with red paint on the bottom. Some inner springs were not marked.
6. Of course use shop rags to prevent anything from falling into the engine near the cam sprockets. I used a large cloth to cover the intake manifold and the side of the engine not being worked on, and this provided a place to put tools and parts.
7. On my engine both intake and exhaust valve stems are 9mm diameter. So only one size mandrel is needed to press the valve seals home. Also, the valve springs can be depressed as far as necessary to remove the keepers. On some engines, one valve stem is 11mm diameter, which makes for 2 sizes of valve seals and mandrels, and makes it much harder to remove the keepers from the larger stem.
8. I used a 20 gallon air compressor generating about 5 scfm at 90psi with a standard quick-release fitting on the hose. It wanted to operate at its set pressure of 100-125 psi,and this worked fine. It generated much more air than I needed, and most of the air was lost to a slight leak at the quick-release connection to the air hold fitting. A smaller compressor probably would have worked.
9. WARNING--While replacing the rockers after installing all of the valve seals, I had one valve stick in the closed position while depressing the valve spring retainer with the compressor tool. I saw the "thrust piece" rise a bit, and was able to stop before the valve keepers fell out of place. This would have been a disaster, as the valve would then have been free to drop into the engine. The problem is that the valve keepers are set gently into the spring retainer when the springs are replaced after the new valve seal is installed. To make sure that there is enough friction between the keepers and the spring retainer, just before you replace a rocker (i.e., while the related cam lobe is pointed straight up) and before putting the thrust piece in place, either (i) give the valve stem tip a gentle whack with a drift/hammer, or (ii) use the valve spring compressor to check the friction between the valve keepers and the spring retainer, and let the valve close smartly. Then go ahead and replace the thrust piece and the rocker. Turn engine and repeat for the next valve.
As usual for me, the job took much longer than I anticipated. And I replaced a number of things (chain rails, tension rail lining, P/S pump seal, radiator hoses, thermostat) whose time was up. But it was fun and with the right tools fairly easy.
Charlie Wagner, 1991 420sel