500E Snapped engine mount bracket bolt in block (M119) - Is 3/4 bolts a "no-go"?

[dinbirb]

Hi everyone,

I’m in the middle of a nightmare while refreshing the engine mounts on my W124 500E. While reinstalling the engine mount bracket (arm), one of the bolts snapped off flush inside the engine block during tightening.

I’ve attached a photo of the situation. As you can see, the access is extremely restricted. There is absolutely no room to get a drill, an extractor, or a welder in there without potentially dropping or pulling the entire engine.

I am planning a full "engine out" overhaul and restoration in about 5 years. My question to the experts here:

Can I safely run the car with only 3 out of 4 bolts on that bracket for now? Does the M119 bracket have enough redundancy, or am I risking a cracked block or a catastrophic failure of the remaining bolts due to the torque of the V8?

If this is a "total no-go," are there any "voodoo" tricks to get that bolt out in such a tight space without pulling the motor?

Thanks in advance for your help!

 

[gsxr]

Ouch. This is on the passenger side, where more stuff is in the way. However, if you remove the main exhaust/catalyst downpipe, and/or the starter if it's in the way... there may be just enough space to use a small right-angle drill adaptor, if you lift the engine upward. Or not. As you said, space is super tight with the engine in the car.

If this was one of the factory bolts, it must have been massively over-tightened to snap like that. I'm surprised it broke, rather than strip the aluminium threads in the block. Make sure you are using the FSM (Factory Service Manual) for torque specs, those bolts are only 25Nm (link), which is barely "snug".

Anyway. For a 5.0L engine and normal street driving, I'd have no issues with only 3 bolts holding the bracket. It should be fine.

Welcome to the forum!

 

[dinbirb]

Ouch. This is on the passenger side, where more stuff is in the way. However, if you remove the main exhaust/catalyst downpipe, and/or the starter if it's in the way... there may be just enough space to use a small right-angle drill adaptor, if you lift the engine upward. Or not. As you said, space is super tight with the engine in the car.

If this was one of the factory bolts, it must have been massively over-tightened to snap like that. I'm surprised it broke, rather than strip the aluminium threads in the block. Make sure you are using the FSM (Factory Service Manual) for torque specs, those bolts are only 25Nm (link), which is barely "snug".

Anyway. For a 5.0L engine and normal street driving, I'd have no issues with only 3 bolts holding the bracket. It should be fine.

Welcome to the forum!

Hi gsxr,


Thank you so much for the feedback, that makes me feel a lot better!


I actually did the driver's side first and torqued those bolts to 40 Nm because I found a document online with that spec. Clearly, that was a major blunder. I was reusing the old bolts; luckily the driver's side held, but the passenger side obviously didn't.


Since I over-torqued the driver's side: Should I loosen them and retighten them to the correct 25 Nm, or is it better to just leave them alone now to avoid risking another snap?


Also, regarding the assembly sequence: Is it better to tighten the single top bolt (mount to bracket) first, or the four bolts that go into the engine block?


So, just to confirm one last time—as long as I'm not driving like a maniac, you think I'll be fine with just the three bolts on the passenger side?


Thanks again for your expertise!

 

[FU2]

Hi gsxr,


Thank you so much for the feedback, that makes me feel a lot better!


I actually did the driver's side first and torqued those bolts to 40 Nm because I found a document online with that spec. Clearly, that was a major blunder. I was reusing the old bolts; luckily the driver's side held, but the passenger side obviously didn't.


Since I over-torqued the driver's side: Should I loosen them and retighten them to the correct 25 Nm, or is it better to just leave them alone now to avoid risking another snap?


Also, regarding the assembly sequence: Is it better to tighten the single top bolt (mount to bracket) first, or the four bolts that go into the engine block?


So, just to confirm one last time—as long as I'm not driving like a maniac, you think I'll be fine with just the three bolts on the passenger side?


Thanks again for your expertise!

You should definetly get new bolts,and then replace them all,tightening them to specified torque.An m8 bolt, torqued to 40nm is now a " stretched/weakend" bolt,all depending on which strength bolt they are.

 

[gsxr]

Since I over-torqued the driver's side: Should I loosen them and retighten them to the correct 25 Nm, or is it better to just leave them alone now to avoid risking another snap?

I'm torn either way. Only 40Nm shouldn't strain anything, but 25Nm would be better, assuming you can get a torque wrench on them.

Also, regarding the assembly sequence: Is it better to tighten the single top bolt (mount to bracket) first, or the four bolts that go into the engine block?

The 4 bolts to the block should be tightened first. Then the top/bottom bolts on the mount itself. Make sure the locating tab is in the notch in the bracket, and the heat shield is in place too.

So, just to confirm one last time—as long as I'm not driving like a maniac, you think I'll be fine with just the three bolts on the passenger side?

Yes - I think it will be fine with normal use, thanks to MB over-engineering.

 

[dinbirb]

I'm torn either way. Only 40Nm shouldn't strain anything, but 25Nm would be better, assuming you can get a torque wrench on them.



The 4 bolts to the block should be tightened first. Then the top/bottom bolts on the mount itself. Make sure the locating tab is in the notch in the bracket, and the heat shield is in place too.



Yes - I think it will be fine with normal use, thanks to MB over-engineering.

Thank you for the advice!


Regarding the driver's side: I’ve decided to leave those bolts as they are. I’m honestly afraid that trying to loosen them might cause another bolt to snap, especially given the limited space and the fact that I didn't have a perfectly straight angle when I torqued them initially. I’ll keep a close eye on them during my regular inspections.


For the passenger side, I managed to get new OEM bolts from Mercedes. However, they didn't supply any washers with them. Is it safe to use generic hardware store washers, or is it strictly necessary to use the original Mercedes washers for this application? I want to make sure I’m not compromising the integrity of the setup.


Sorry for the follow-up questions, but I really appreciate your patience and expertise!

 

[FU2]

I'm torn either way. Only 40Nm shouldn't strain anything, but 25Nm would be better, assuming you can get a torque wrench on them.

They are only 8.8 bolts ,and if they are indeed m8 ,they are definetly weakened after beeing tightened with 40+nm. So they should definetly ideally be replaced, to avoid them snapping during operation.

 

[daantjie]

Indeed unusual for the original hardware to snap like that. In my experience these Kamax or similar branded bolts are hella stout. New hardware is trash compared to the old stuff.

 

[gsxr]

For the passenger side, I managed to get new OEM bolts from Mercedes. However, they didn't supply any washers with them. Is it safe to use generic hardware store washers, or is it strictly necessary to use the original Mercedes washers for this application? I want to make sure I’m not compromising the integrity of the setup.

Hardware store washers should be ok if they are at least Grade 8 / equivalent / better. This is a relatively low-stress application so I wouldn't lose too much sleep over it.

Indeed unusual for the original hardware to snap like that. In my experience these Kamax or similar branded bolts are hella stout. New hardware is trash compared to the old stuff.

That's what I was t hinking as well. The Kamax stuff is top shelf. I'm wondering if the torque wrench calibration is off. No way that 40-50Nm should have broken the bolt. I'd expect it would need >100Nm to shear like that!

 

[FU2]

You guys need to remember, This is NOT a " low stress" application. It is the engine mount after all. It is constantly exposed to vibration,and massive weight transfer in motion all the time. 4 bolts clamping down the massive engine mount, is quite a force ,and the pulling forces on the bottom bolts, is significant,so if one of the lower bolts are missing, the last bottom bolt takes alot more stress.So to make the situation better, replacing the bolts that also has been overtightened is definetly the best way to go. Will they break if not beeing replaced?one really cannot tell,and most likely not.But that doesnt change the fact that it can,and it will make the situation even worse if they do.All this makes for an important "area/part to be done up correctly,and it is no wonder that the bolt might have broken after beeing tightened to 40+++. The bolts are old and have been exposed to the above mentioned forces for 30years.Threads are not as smooth as when new,which also disturbs the torque applied to the bolt. All this adds up to why the bolt snapped/sheared off.

 

[gerryvz]

I would try to see if you can get access to the broken bolt, to extract it, as said by removing equipment and lifting.

While three out of four bolts is probably going to be OK, because of limited movement of the engine (basically rocking in one direction under acceleration), would you want four out of five lug bolts on each wheel, or would you want all five of them to be in place and torqued down to the proper tightness?

I think we all know the answer. So I would make a strong try to get that bolt out and replace it with the equivalent.

Also, @dinbirb, you mentioned that you found 40Nm torque spec for this bolt "in a document online." DO NOT TRUST ONLINE MANUALS, OR PARTS DIAGRAMS. EVER.

ONLY use MB factory repair and maintenance information. You can find that on this site: Model 124 Maintenance Manual Index

And only use MB EPC or ISPPI for part diagrams and part numbers.

Factory service manual diagram, from above link (Section 22, Engine Mounts):

 

[nocfn]

Just a bit or real world for your thoughts. I noticed during my exhaust replacement that I only had two bolts on the left side holding the bracket to the block. After much back and forth with the GSXR I got proper pitch and length and installed them. I replaced the mounts when I bought the car but never even looked at the attachment points. I removed the connection top side to the brackets raised engine and installed new mounts, reversed. There was more than an 7 year span of 10-20k put on the car with just 2 of the four bolts on the left. Sleep well, I am sure there is a need above redundancy - perhaps I was lucky.

 

[dinbirb]

I would try to see if you can get access to the broken bolt, to extract it, as said by removing equipment and lifting.

While three out of four bolts is probably going to be OK, because of limited movement of the engine (basically rocking in one direction under acceleration), would you want four out of five lug bolts on each wheel, or would you want all five of them to be in place and torqued down to the proper tightness?

I think we all know the answer. So I would make a strong try to get that bolt out and replace it with the equivalent.

Also, @dinbirb, you mentioned that you found 40Nm torque spec for this bolt "in a document online." DO NOT TRUST ONLINE MANUALS, OR PARTS DIAGRAMS. EVER.

ONLY use MB factory repair and maintenance information. You can find that on this site: Model 124 Maintenance Manual Index

And only use MB EPC or ISPPI for part diagrams and part numbers.

Factory service manual diagram, from above link (Section 22, Engine Mounts):
View attachment 233420

Thank you for the feedback. I’ve definitely learned my lesson about trusting random documents or diagrams found on the internet.


However, I am currently under significant time pressure, and I don’t have access to a lift or a wide range of specialized tools to perform an extraction right now. My plan is to proceed as discussed—using the three remaining bolts on the passenger side and leaving the driver's side as is for now.


I will monitor the situation very closely and perform regular inspections to make sure everything stays tight. I plan to do a proper repair and extract the broken bolt by next winter at the latest when I have more time and better resources.


Thanks again for your patience and for pointing me toward the official factory manuals.