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Torsion Bar (Sway Bar) Broken Tip Low Cost Fix

19K views 18 replies 14 participants last post by  SEC Meister 
#1 ·
I broke the end off my torsion bar. There are posts that describe this and suggestions what to do. A complete torsion bar replacement takes about 20 man hours and $300 in parts making a $3,000 job. A forum member suggested getting a part manufactured at a machine shop, cut off the end and replace the tip with the machined part. I lost my patience waiting for the part from the machine shop and decided to try another approach.

The tip of the torsion bar broke off at the point where the bolt ends inside (picture below at the point labeled 6). The threads end there but the hole is about 10mm deeper (point 22a). This is all at the end of the bar where the wall thickness is very thin and it a shame Mercedes designed it that way. If the threads went about 10mm deeper (point 22b) or even deeper (point 10) they would catch the full thickness of the torsion bar and it would likely never break. With that in mind I thought I could drill the hole deeper, tap it and put in a longer high strength bolt.

I drilled a hole deeper into the end of the torsion bar. I was surprised how soft the metal was. I expected spring steel and a very difficult job. I used a very small bit to start and worked my way up using 5 bits to an 8.5mm bit (I tried to go straight to a big bit and it just spun and heated). I was able to drill 40mm into the end from the break. Then I tapped the hole with a 10x1.5tap ($5.00 from Levine Automotive or Sears) for about 30mm deep. I carefully tapped and tested several times, patiently and slowly. I bought a 10mmx1.5mmx60mm grade 10.9 bolt ($0.80 from www.boltdepot.com). Then I reassembled the two bushings into the upper control arm front and back, the tip that broke off and the new bolt. The shaft of the bolt was too long (the threading on my bolt was too short) so I had to drill the tip inside shaft a little (point 22c) to let the longer bolt fit all the way in. I used a crow bar to place the control arm in the right place and carefully threaded it all back together. Cost is less than $10. Time took me about 2 hours. It feel and looks like it will be a great permanent fix. It is done! What do you think?

It is a little late, but I would suggest Mercedes retrofit deeper holes and longer bolts.
 

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#2 ·
In the original torsion bar (stabilizer bar, sway bar) the spacing between the large washers is about 60mm, and add another 10mm for engagement into the sway bar gives 70mm. This defines how long the bolt should be at a minimum. The Mercedes original bolt was:
10mmx1.5mmx33mm - 10.9 grade metric hex bolt (17mm head)
- (10mm diameter shaft)
- (1.5mm thread pitch - coarse thread)
- (33mm long, bolt shaft length does not include the head dimensions)
- (10.9 Grade metric high strength steel, as opposed to 8.8 mild steel.)
- (17mm hex head, this has little to do with the actual bolt strength, etc)
10.9 metric grade bolts have the same strength specifications as Grade 8 American bolts (marked with six radial lines on the head). 10.9 bolts are made from alloy steel quenched, tempered, and zinc plated (usually yellow). Best for automotive use and other areas where high strength is needed.

The first replacement bolt is:
10mmx1.5mmx55mm - 10.9 grade metric hex bolt (17mm head) adding 22mm in length.

Just to be sure, I have ordered an even higher strength and even longer bolt:
10mmx1.5mmx70mm - 12.9 grade metric socket head.
This will add another 15mm to go even deeper into the torsion bar, and the higher strength to limit deflection and possible breakage under high loads. 12.9 grade steel bolts are made from heat treated alloy steel. Best for applications where extreme strength and hardness are required. Black finish. They have a round head with an inside hex (Allen) metric socket head. I think the higher bolt strength is unnecessary. I also think the 70mm length is unnecessary. But I don't want to have to redo this work.

Can you see in the top diagram above that a longer bolt is a great idea, and than an even longer one would be the best? Such a pity Mercedes came so close but missed the mark by so little on a part so difficult (expensive) to replace.
 
#3 ·
Bolt Specs from http://www.boltdepot.com
Socket Head
Hex Head

Bolt Grades http://www.boltdepot.com/fastener-information/Bolt-Grade-Chart.aspx
2 (no markings)
5=8.8 (3 radial lines)
8=10.9 (6 radial lines)
 

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#5 ·
Nice repair!

The sway bar as designed is the most complicated I have seen, most have a flat area with a stamped hole in it that gets a bolt and nut through with the rubber bushings. It had to cost a lot more to neck down the end, drill and then tap the hole. Plus it created the stress riser that caused the failure. I sure hope this is not common!

The other expensive part of this is where it is mounted...yikes looks like you would have to remove the engine!

The other way to have repaired this would be to purchase from a race shop a clamp that grabs around the bar. This could be used to get a little more stifness out of the bars and reduce the body roll without replacing the bars.
 
#6 ·
Hello,

I have 1991 300 SE with a busted Stabilizer Bar tip, just as you posted. I am thinking about travelling down the same path as you did with the tapping and a replacement bolt, but was wondering how it has held up? Also, I do not have the original washer, as that has found its way to some roadway in Dallas, what do you think about two large flatwashers? I picked up a hex socket head , 12.9, 10x1.5x80 bolt. In your opinon, should I have gone with a 65 (as there was no 70 available),or would the extra depth into the end of the Stabilizer be too deep?
 
#8 ·
I did the same repair to a 500 SEL I had, and it was still fine when I sold it, years later. I remember the tie rod end being more difficult to drill, though...

Best Regards,

e

.::.
 
#9 · (Edited)
Update on the fix...

UPDATE

Not all was perfect, but she does ride beautifully now.

Be sure to keep the following in mind if you are going to do this job:

1.) Always wear protective eye gear when drilling metal.
2.) Small pieces of sharp metal everywhere WILL make you bleed.
3.) Drink the beer to STAY COOL WHILE working, and to ALTER MOOD AFTER the job is complete.

and finally,

be extra sure to:

4.) Keep a running vehicle parked where it is NOT BLOCKED IN by the unmoveable car that you are working on!

(I in fact DID abide by all of these points while the fix was in progress)

Anyway, my technical differences included starting with that 80 mm bolt, turns out the 70 mm was better, but a shorter one might have done just fine too, however, I did also purchase a couple of different diameter metal sleeves to fit inside of the bushings to keep the bushing and bolt surfaces from meeting. My preference, probably not necessary, and I think that the inner sleeve might be a bit longer than I needed, but it seems to be working alright.

So, on to the meat of this endeavor...

Now, I have no idea what Ben does for a living, but he must do it really well to do this job in 2 hours. Granted, I did work with a friend, have two beers, break some tools, and run to the hardware store once, but, I learned a helluva lot in 6 hours, and had fun too. Oh, and, it is a real pain in the ... to keep that drill exactly where it needs to be, and at the proper angle; I mean you are pretty much inside the fender well, and you can't actually get your head behind the drill to see what you are doing, so you just watch the side and try to keep it straight and parallel to the flange on the end of the bar.

First, I actually hacked the majority of the twisted, broken end off, leaving the protruding threaded portion about 5 mm in length, planning to drill into the end of the bar for 20 mm. After filing the cut outside edge for safety, I started with the first drill bit (the first of TWELVE), it seemed to take forever and great energy to get through the end of that bar, but when I finally drilled through the end portion, the rest of the bar went a bit easier.

As I said it took 12 different sized drill bits to get the hole I was looking for. I figured 5 should work, but it turns out that those larger jumps in bit size made it just as difficult to widen the hole as it was for the first bit to get started. So, the smaller jumps worked better. --Important Note: be sure to use plenty of oil while drilling this out; I also recommend some compressed air to blow the metal shavings out of the hole frequently. Cleanliness is a key to success on this job. And, be ready for the drill to bind up as you move up on bit sizes, at least until you can break through the first tough outer portion (don't forget, lots of oil...).

Next, once you have sufficiently tired yourself of drilling for a couple of hours, you are probably at the point of tapping the new threads (you may need a beer first). I used Ben's recommended 10 x 1.5 tap, and standard drill bits, the last one being 11/32, before I tapped the hole. -TAPPING... I only had one shot, and have never before tapped out new threads so I was definitely as nervous as I could get. I figured if I was going to screw anything up, this would be the thing. As it turns out, it is not that tough to do a good tap job, just check it and clean it out frequently; key terms would be SLOW and STEADY. It tapped out GREAT!

Being proud of my accomplishment, it was now time for the test...would the bolt go in well and fit properly??? YES, true success!! I did of course check the fitment each time I removed the tap to clean out the hole. But, I was very satisfied. Now it was time to check the whole setup and see if it was going to work, and it did!!...BUT...80 mm is too long of a bolt,, even if you drill out 20 mm of sway bar and tap it, BECAUSE I never made allowance for the compression of the bushings, DOH!!! ( Go turn on music for interlude while I go to the store for a shorter bolt...) Okay, I am back... Turns out the 70 mm is fine (10x1.5x70, 10.9, 15mm head). Be sure to get a couple of LARGE steel washers for the end of the bolt because not only will the busings compress, they also flatten out quite a bit, mine did to the size of the washers, about an inch or more ( I forget the washer size now), if you have a question, pick the bigger one.

The moment of truth...
Well, okay, not yet, there is still one more part to let you in on.

To actually get the bushings in place, and to be sure the drill could get to the spot we needed it, we used a hydraulic jack under the suspension gear, basically allowing us to line up the opening in the end of the control arm to the center of where we needed to drill; this also turned out to be a good thing when it came to trying to get the rear bushing in place, AND when it came to positioning the front bushing and bolt with the rear. To actually insert the bolt, however, was a two man job; I fitted the pieces together (blindly, of course), while my friend used a large piece of spare 2x6 to push the stabilizer bar into place so that I could in fact seat the bolt and get it in as far as posible.

Once that was done I tightened it up and she drives much smoother now, also, I can now brake safely too!! YIPPIE!!!

I was only sore everywhere for a day or so, but it doesn't hurt so bad on those big comfy leather seats!!

BEN, thanks for the post, it really helped me out and saved me quite a bit of unavailable cash. DIY!!!

NOW I CAN FIX THE...
 
#11 ·
I replaced the sway bar on my 420 SEL about 5 years ago. A new bar was about $60 from the Benz Bin--much cheaper than I had anticipated. Replacement requires some fiddling includidng removing the battery and left front fender to slide the old bar out and the new one in. But it is not a twenty hour job. As I recall it took me 4 or 5 hours.
 
#12 ·
Torsion (Sway Bar) Repair W126

I have a 1987 420 SEL that has a bunch of miles (as in over 300K). I bought it cheap and found out why when I tried to change the upper forward sway arm bushings. Guess what? The right side broke and the left side darn near did. I am an old (well not that old) aircraft mechanic and don't give up easily. After several bad words and a little research, I came up with a repair also. It copies the before mentioned repairs to some extent, except I simplified it considerably.

The long and short of it, I may post a detailed repair at a later date, if I see any future interest.

Good luck and I would suggest being ready to make this repair, or ready to take it to a shop before attempting the bushing replacement. The dirty atmosphere of the wheel well is an excellent place for oxidation (rust) of the metal under the bushing, and the internally threaded thin piece of rusted metal can easily break.

I still need to do the left side. It has slop in it, due to the void between the bushing and rusted bushing mount. This will cause the frontend to feel like it is loose and sounds pretty bad - like a loose clunk or thud - that's airplane talk for something not being right.

Anyway, I made this repair on the right side yesterday and it helped considerably. I will make the left side repair later and have the car aligned to see if everything is good.


Additionally, my repair does not have any internal threads, instead I had the repair piece made solid with external threads at the forward end for the bushing.

More later (maybe) - Ken.
 

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#13 ·
I had the same problem with my 89 300se torsion bar (driver side). Upon inspection I noted that the failure was caused by corrosion, I have also noted that the passenger side has undergone some corrosion as well. I had thought to do as the post describes however as an ASU student I have access to a very nice machine shop and as a welder I have good metal suppliers. I intend on cutting the end of the bar off at one half of an inch behind the shoulder and machining a replacement out of 1 ½‘’ 403 stainless round stock. If the torsion bar is exact enough I will make it more or less a press fit with four to six set screws perhaps 1/4 to 3/16’’ in diameter. If the end of the torsion bar is not exact enough for a press fit I had thought to cut a 1/8’’ grove down one side of the collar of the machined part and use a very strong clamp to secure it. If anyone has any suggestions or ideas they would be appreciated. I will probably get started on this project tomorrow or the next day, and I will keep everyone posted on how it goes, and when I get the measurements I will post them as well. Noticing that this thread is quite old I would like to know if these fixes have withstood the test of time so please let me know on that
 
#15 ·
Slip on replacement.

I machined the parts that i mentioned previously and installed them today, it was very fast and easy when compared to the other optiones. However machining took the better part of six hours since there was a lot of material to cut. I made replacement ends from 403 stainless steel, and welded them onto the shaft end with a tig welder at 100 amps using 1/16'' tungstun and stainless filler rod, the welds turned out good despite the dissimilar materials and the tight space. I will post some pics.
 
#16 ·
I machined the parts that i mentioned previously and installed them today, it was very fast and easy when compared to the other optiones. However machining took the better part of six hours since there was a lot of material to cut. I made replacement ends from 403 stainless steel, and welded them onto the shaft end with a tig welder at 100 amps using 1/16'' tungstun and stainless filler rod, the welds turned out good despite the dissimilar materials and the tight space. I will post some pics.
Welcome to the forum.
Please complete your profile with the car mod and location, by clicking 'User CP' on the top tool bar, to make it easier for members to offer advice.
 
#19 ·
Repairs to my 560 SEC ('92)

Hey,
After reading all the input from various contributers I find that it's a dooable job for me and it'll save the car I've always wanted from the scrap pile. Had a few M-B's over the years and this is the one I've always been headed toward. Thanks to those who have "gone-before" I'll enjoy retirement in style. Thank you very much for sharing.

John
 
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