W123 wagon sls strut rebuild - Mercedes-Benz Forum
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post #1 of 5 (permalink) Old 07-11-2015, 12:36 AM Thread Starter
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Date registered: Aug 2009
Vehicle: W114/115 220d (1972) [sold] and W123 300tdt (1984) and two L406DG's (1967)
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W123 wagon sls strut rebuild

i just finished successfully rebuilding a leaking sls strut on my 1984 300TD. despite warnings that these struts are not user serviceable, i discovered a step by step approach to accomplishing this repair on a forum here: 84 300td rear struts | Mercedes-Benz Club of America

i would rate the article as excellent with two exceptions—1) i would recommend that you drill a hole behind the upper circlip rather than in the space between the two ends of the clip. in my case the circlip was rusted in place and refused to turn in its groove enough for me to push it out. i ended up drilling a second hole behind but near the end of the clip. this allowed me to push the clip out of its groove and pull it out.

exception number two—the writer of this how-to gives the specs of the two o-rings that must be replaced as 4mm X 39mm and 2.5mm X 26mm. he is correct about the size of the larger one. but the smaller of the two should be 3.5mm X 26mm. i ordered both o-rings without measuring the smaller one only to discover the size was wrong.

the only other tips/tricks i might add to the diy listed above involve the difficulty you may have in removing the lower plug or "gland" as it is called in the repair document. in my experience there was enough corrosion between the plug and the wall of the strut case to prevent me from pulling the plug out by hand. i ended up bolting both ends of the strut to separate blocks of 2" X 4" and used a fulcrum and bottle jack to extract the plug.

the former mbca member who wrote the guide stresses the need for cleanliness and to avoid scuffing/scratching any part of the plug, piston, and/or inside wall of the strut. as he suggested i used a fine 3m scuff pad to hone the interior walls of the strut before final cleaning and reassembly.

the strut i rebuilt first began leaking slightly in the hot summer months here in ohio last year. the leak had worsened over the winter prompting me to consider chasing down a good used one or biting the $300+ bullet for a new one. i remembered seeing some posts about rebuilding struts and finally tracked this one down.

as it turns out the procedure is quite straight forward and it works! this post is my effort to pass on what i learned in the process and add what i can to this excellent article.

Last edited by barnumlives; 07-11-2015 at 07:07 AM. Reason: sentence structure, ease of reading
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post #2 of 5 (permalink) Old 07-11-2015, 04:30 AM
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Vehicle: 2002 MB C230K, 1985 MB 300T Turbodiesel
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Thank you for posting the original article as well as your modifications to his procedure! Given the cost of replacement struts, this is highly useful and will certainly come in handy for me and others down the road.
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post #3 of 5 (permalink) Old 07-11-2015, 07:03 AM Thread Starter
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Date registered: Aug 2009
Vehicle: W114/115 220d (1972) [sold] and W123 300tdt (1984) and two L406DG's (1967)
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you are welcome, ptbowman. after reading that these struts are not serviceable time and time again it was a pleasant surprise to learn that we can rebuild them without a machine shop. thank you for your comment.
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post #4 of 5 (permalink) Old 05-21-2019, 08:16 AM
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Hello and thank you for your time sharing this valuable information. I am not finding the link to the original strut rebuild post if you could help me with that. Thank you. Ray

Thanks, Ray Green
80 300SD / 85 300TD
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post #5 of 5 (permalink) Old 05-21-2019, 03:28 PM
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Here ya go! No more pics though

1984 300TD
300td Rear Struts can be re-built!

Mercedes 123 Touring rear Strut Rebuilding

Tools needed:
Jewelers screwdriver, ground to a round, blunt point
Adjustable spanner, or large Channel Lock Pliers, or medium size pipe wrench
Drill motor
5/64 + or _ drill bit just larger than screwdriver shank
¾” thin open end wrench
Dremel grinding tool or equivalent
Dremel # 199 cutting saw or similar
2 Buna-N O-rings; 1 ea. 4 mm x 39 mm, 2.5 mm x 26 mm
Smooth On PMC-744, 2 part urethane, if needed, to repair rubber boot
Loctite medium strength thread locker

I love a challenge, especially when the easier way out will cost me more money than am willing to spend. When the rear hydraulic strut on my daughter’s 1884 300 td started leaking several years ago, I went to my local MBZ wrecking yard and bought a used one to replace it. The original was considered un-repairable and a new one was going to set me back $400 or more. Less than $100 for a used one looked pretty good.
Then, recently, that same strut started leaking also. With the original strut still available, I figured there could be no loss in trying to disassemble it and see if it could be fixed. I would like to share with you what I found and hope if you find yourself with the same problem I can spare you unnecessary time and expense. After rebuilding the original strut, I went on to do the second one which is now a working spare.
The rebuilding of the strut is what I will concentrate on. The removal and installation is pretty straight forward and I will assume if you have read this far that you have the tools and inclination to do that part without further description. It is an easy task and there are many articles and suggestions out there to guide you if you need help.
Once the strut is out of the vehicle cleaning it will be one of the most important things you need to do. When working with hydraulics, cleanliness IS Godliness. I have found a good scrub with some Simple Green or other such cleanser to get the loose gunk off is a good start. Be sure to drain out as much of the hydraulic fluid as possible by stroking the rod in and out. There is oil on both sides of the piston inside. The rubber boot that protects the cylinder rod might be cracked and dry like mine. If so, be very careful to not damage it any more than it is. We will cover salvaging it later. The boot is attached to a large perforated washer which is in turn sandwiched between the lower ball joint and the rod. Carefully peal the rubber boot off of the washer. If you find more dirt and grease in this area, clean it up now. You should be able to see two flats ground on the rod end just above the washer.

If you have a spanner wrench that will fit in the holes of the washer, great. I did not. What I did instead might upset some purists, but it worked. With large Channel Locks or a small pipe wrench, carefully grasp the outside of the washer. Using a thin ¾ inch open end wrench on the ground flats, break loose the “Loctite” coated threads between the [male] ball joint and the [female] cylinder rod. (Photo 1)

Once unscrewed, the rubber boot will now come off completely. At this point, all of the external surfaces of the strut assembly will be exposed and it is a good time to clean the worst of the road dirt and grime off of everything. Make certain the rod end is smooth and free of any burrs. This will be important when you slide the end plug off of the rod. I recommend running a tap (M8-1.0) into the rod end to clean out the threads and use “Loctite” Blue or Purple (medium strength) when replacing the joint.

Again, be careful of the rubber boot if it shows signs of dryness. Clean it especially well, both inside and out with mild cleaners. I ended up using a small stainless steel “toothbrush” with paint thinner/ kerosene, followed by Simple Green and water. Pay particular attention to the inside lower area as dirt seems to build up in that area. Using a spray parts cleaner/brake cleaner, completely clean out all dirt from the bottom end of the cylinder. Blowing with compressed air afterwards will help. Discard the dirty gritty rags you have used up until here and start with clean ones.
Now, here is the tricky part and the key to success. This cylinder is comprised of a precision ground steel tube and an aluminum end plug ‘gland’ held in with two spring steel circlips nested in grooves in the cylinder wall. Removing the circlips allows the rod end to come out, exposing the innards. Locate the ends of the outside circlip wire. There should be a gap of about 1/8 inch between the two ends. Staying within this area, locate a mark .200 inch (between 3/16” and ¼”) below the edge of the cylinder. You are aiming for a line just below the centerline of the circlip. With a center punch, lightly mark this point. Using great care with a drill motor and a small (5/64” or .080”) drill bit, drill a hole through the steel wall of the tube (Photo 2).
I drilled slightly lower and angled the bit upwards but that is splitting cat’s hairs. Be careful not to drill into the aluminum plug when you exit the steel. Don’t have a heart attack if you do nick the aluminum since that area of the plug is not critical- but do be careful. De-burr as much steel from the exit wound as possible with a jewelers file or sharp deburring tool.
Now, if the area has been cleaned properly, you will notice that the ends of the circlip have been cut at an angle. Using the jeweler’s screwdriver or similar strong, small rod, rotate the circlip so that one end is now covering the newly drilled hole by about 1/8th of an inch. Rotate the clip in the direction that allows the spring steel to “ramp over” your hole, not dig into it. Ask me how I learned this. With the circlip over the hole, insert the screwdriver (ground down as necessary) from the outside and push the clip in and up out of the groove in the cylinder wall. Be careful to not allow it to fly away. Closely inspect where you drilled your hole again and make sure there are no burrs or sharp edges on the steel. This is important.
Pull the strut rod all the way out until it stops and push it back in about 2 to 3 inches. Holding the whole thing over a rag, quickly pull the rod to loosen the gland plug. It may take several attempts. Because there is an inner circlip, the piston will not come all the way out. If this does not succeed loosening the aluminum gland plug, retract the rod about 1 inch and give a quick blast of compressed air into the inlet port. When the plug does break loose slide it down the rod and remove it from the cylinder. If you are in a hurry, no further disassembly is required. However in the interest of thorough cleaning and your own curiosity, look back into the cylinder and you will see the second circlip in a groove of the cylinder wall. Without scratching the bore, push down on the center of the wire with your jeweler’s screwdriver. It should pop out of the groove. Pull it out of the cylinder (no scratching!) and remove the rod/ piston assembly. Now marvel at the simplicity of it all.

There are only 2 o-rings in this assembly and the only parts you will need to replace. The large one that seals between the gland plug and cylinder wall (4 mm X 39 mm) is obvious. You can either cut or hook it and remove it, being careful not to scratch or nick the aluminum.
The second, smaller o-ring (2.5 mm X 26 mm) is hidden and this procedure is one of the more important points of the whole job. There is a Teflon/ plastic rod wiper underneath a steel washer that must not be damaged. You will notice the washer has been staked in place which holds the rod packing seal (Photo 3).
I used a Dremel tool with a small rotary saw/ cutter (Dremel # 199) to remove the crimped aluminum material that was swadged over the steel. There are probably other ways to do it, but the goal is to loosen up the steel washer and allow it to be removed from its aluminum ‘nest’. Carefully remove the steel washer. If you must pry the ring up, stay inside one of the three reliefs on the ID. If you damage the Teflon wiper, good luck. I tried to find replacements but could not. They are a ‘Mercedes only’ piece and having a machine shop duplicate it might be your only option, so be cautious. If you look closely at the rod wiper, there is a small step on the ID. When you reinstall it, be sure to put it back the same way. The hidden, smaller o-ring should now be visible behind the seal.

Now is the time to clean thoroughly. Again spray cleaners/ brake cleaners and an air compressor are great things here. The piston assembly attached at the end of the rod does not need to be disassembled. I have included a picture of it (Photo 6)
to satisfy the curious. It is simply a cast iron piston with holes through it and a series of stepped spring washers acting as progressive valves. This is what actually does the dampening of the car’s suspension, not the air accumulators as I originally was lead to believe. There is oil on both sides of the piston and it is metered both directions as the rod moves up and down.

The strut has an iron piston ring similar to engines’ rings. Because this ring had been mated to the cylinder wall for 28 years, I felt that honing the cylinder might not be a good idea. Instead I wrapped a piece of green “Scotch Brite” to a rod chucked up in my drill motor (Photo 4) and gave a gentle burnish to the original surface.

Now for assembly. With clean suspension fluid, lubricate the small o-ring and set it in the bottom of the aluminum recess. Gently slip the Teflon wiper seal inside of it with the step facing to the outside/ bottom of the aluminum plug. Place the steel washer on top of the two and hold it in place. I used a socket and vice (Photo 5)
to make sure the washer was fully seated. With a hammer and punch, re-stake the washer in place while the unit is compressed. Do not punch the original locations since the aluminum needed to swadge over the steel was removed earlier upon disassembly. Double check that you have successfully trapped the washer forever. The staking is the only thing holding it in place. Lubricate and install the new large o-ring into its external groove.
Pour a small amount of suspension fluid into the cylinder and wash it around, coating all of the walls. Lubricate the piston and gently slide it back into the cylinder, compressing the ring until it pops in. Push the rod all the way in and re-install the inner spring circlip. No scratching allowed! Add a bit more fluid to re- lubricate the walls and push in the aluminum end plug just until the o-ring enters. On the second strut I rebuilt I noticed some rust “stains” had formed between the end plug and the cylinder wall, downhill from the large o-ring. I attributed this to moisture getting trapped below the rubber ring and above the circlip. I decided to smear some wheel bearing grease on the plug to fill the gap and keep water out. Maybe someone in another 28 years will see if that helped. Push the plug in until it seats against the inner ring and re-install the last circlip. In case I ever had to open the unit another time, I placed one end of the wire over the drilled hole to save a step. So far I have had no problems. The strut is now as good as new.
The next order of business is re-packing the lower ball joint. With it removed from the strut rod, you can see how easy it is to remove the rubber boot. The upper spring clip is easy to remove with either your jeweler’s screwdriver or a loop of fine piano wire. The bottom ‘key ring’ type clip can be un-wound with the same tools. I cleaned out what little was left or the original black grease and flushed the joint with a Teflon based spray lube before re-packing it with synthetic wheel bearing grease. Care is needed re-installing the bottom clip without tearing the rubber. Installing the top ring first will make things easier. This completes the mechanical rebuild but does not address the repair of the rubber boot which must be ready for installation before the ball joint goes back on.

My rubber boot was dried out and badly cracked (Photo 7). Since it is an integral part of the whole assembly it cannot be replaced separately. Upon the advice of “Villageidiot” in the MBCA Tech Forums I found a company Smooth On, Inc. (smooth-on.com) and purchased a one pint kit of PMC-744 2 part Urethane and black dye. This was enough to do at least 20 boots, for around $35. “Villageidiot” suggested I make a plaster mold of the inside of my boot and paint on the urethane. Fortunately my original boot was salvageable. I crazy glued the cracked pieces that were still there and taped up any holes left. After scrubbing my original rubber boot with sandpaper I painted the 40 durometer urethane on to the old boot (Photo 8).

Even though the instructions said 15 minute application time, I had to continue rotating the wet part for an hour before it set enough to stop sagging. I will add their accelerator additive if I ever do it again. The rubber boot for my lower ball joint also had a small tear in it so I did a little repair on it before the urethane set up. I love that stuff. I will use it again for many other projects.
After 24 hours the new boot was ready to be installed. That procedure is the opposite of disassembly. Slide the rubber boot over the strut and pull the rod all the way out. Dab a bit of Loctite on the ball joint threads and spin it on. Hold the large washer with the spanner or channel locks and tighten the rod with the thin ¾ inch thin open end wrench. Carefully slip the new and improved boot over the perforated washer and you are ready to install the new strut. Pre-filling the cylinder with oil will help with the bleeding process. I wrapped some electrical tape around the barrel to seal the opening and keep most of the oil in and the dirt out while working it back into the coil spring. Removing the tape involved spinning the cylinder, so do that before permanently mounting it in the rubber bushing and body.

The rebuilt strut has been in service now for 4 months with no sign of leakage. I have faith that it will be good for another 28 years. By rebuilding this strut and flushing out the entire system, I have noticed that the fluid in the reservoir is staying cleaner. I suspect this is because the dirty oil in the accumulators and struts never really gets re-circulated within the system, just pushed back and forth as needed.

I hope this helps others in my position. The key is paying very close attention to each detail and keeping everything clean. If and when the other strut begins leaking, I now have a spare to quickly replace it- and extra o-rings to rebuild the original.
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