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Discussion Starter · #1 ·
I am so amazed that the bolts holding the engine in the car would be inadequately engineered to the point of frequently breaking that I did some investigation on my own car. This is what I found. The four engine mount bolts which hold the engine mounts on each side to the engine block are undersized. The size is M8, which is about 5/16 of an inch. That is too small for the load that the bolts are subjected to. That is the first problem - use of the wrong size fastener for the application.

Here is the second problem, and the explanation of why they break on the passenger side. All of the bolts on the drivers side and two on the passenger side look about like this (see arrow on photo):



The front two bolts on the passenger side look like this. Photo is taken from underneath the car, looking upward. The black painted object on the right side of the picture is the engine block.



You can see that, in addition to the engine mount itself, they have added a spacer and the bracket for the VANOS accumulator, which have made the bolt significantly longer and weaker. There is about one inch of bolt sticking out of the block unsupported by threads, which is constantly subjected to bending and stretching with the situation worsened by adding extra washers and brackets to an already undersized fastener. Looking at this photo and thinking that this is carrying the weight of the engine and transmission in a 333 HP automobile, it is hardly surprising that there are frequent failures.

Using higher specification (10.9) fasteners is a band aid solution to an engineering mistake. Longer bolts should never have been used as a convenient place to attach the VANOS accumulator. Surely it could have been fastened somewhere else. That part could be corrected by some retro engineering away from the factory design. The use of inadequately sized bolts is impossible to correct, however.
 

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Bad Lieutenant
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I had read somewhere that another cause of the bolt breakage was that they were subject to abnormal and high heat during the build process and became weakened. I have my dealership check those bolts every time it's in for service, ~ 5k miles, as well as re-torque the strut braces and the sway bar bushing bolts. Great pics by the way.
 

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Discussion Starter · #3 · (Edited)
I had read somewhere that another cause of the bolt breakage was that they were subject to abnormal and high heat during the build process and became weakened. I have my dealership check those bolts every time it's in for service, ~ 5k miles, as well as re-torque the strut braces and the sway bar bushing bolts. Great pics by the way.
That is the explanation given in the factory service bulletin that they don't want owners to know about. Honestly I don't believe any part of that explanation and I think it is something someone made up to use as an excuse.

Having the bolts retightened periodically would not be very reassuring to me because they will just continue to stretch until they finally break.
 

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S54=Living on the Edge
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IIRC, most reported failures were on 2007 MY cars which may indicate a problem with supply quality or along the line, no? Regardless, your theory is pretty good. I am running the band aid fix with Zinc plated 10.9 bolts.
 

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That is the explanation given in the factory service bulletin that they don't want owners to know about. Honestly I don't believe any part of that explanation and I think it is something someone made up to use as an excuse.

Having the bolts retightened periodically would not be very reassuring to me because they will just continue to stretch until they finally break.
As of 03/08 no issue in the bulletin.
E46 M3 uses the same set up (parts) with no issues I've heard of.
 

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IIRC, most reported failures were on 2007 MY cars which may indicate a problem with supply quality or along the line, no? Regardless, your theory is pretty good. I am running the band aid fix with Zinc plated 10.9 bolts.
You think it is a band-aid?
 

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I am so amazed that the bolts holding the engine in the car would be inadequately engineered to the point of frequently breaking that I did some investigation on my own car. This is what I found. The four engine mount bolts which hold the engine mounts on each side to the engine block are undersized. The size is M8, which is about 5/16 of an inch. That is too small for the load that the bolts are subjected to. That is the first problem - use of the wrong size fastener for the application.

Here is the second problem, and the explanation of why they break on the passenger side. All of the bolts on the drivers side and two on the passenger side look about like this (see arrow on photo):



The front two bolts on the passenger side look like this. Photo is taken from underneath the car, looking upward. The black painted object on the right side of the picture is the engine block.



You can see that, in addition to the engine mount itself, they have added a spacer and the bracket for the VANOS accumulator, which have made the bolt significantly longer and weaker. There is about one inch of bolt sticking out of the block unsupported by threads, which is constantly subjected to bending and stretching with the situation worsened by adding extra washers and brackets to an already undersized fastener. Looking at this photo and thinking that this is carrying the weight of the engine and transmission in a 333 HP automobile, it is hardly surprising that there are frequent failures.

Using higher specification (10.9) fasteners is a band aid solution to an engineering mistake. Longer bolts should never have been used as a convenient place to attach the VANOS accumulator. Surely it could have been fastened somewhere else. That part could be corrected by some retro engineering away from the factory design. The use of inadequately sized bolts is impossible to correct, however.
I believe there is plenty of strength in the M8 8.8 bolts for the engine capabilities. At maximum torque of 365Nm, close to half of the torque is reacted through each of the two main engine mounts. Each mount is about 200mm from the engine rotational centre, so the shear force applied at each mount is about 913N, or a shear of about 250N at each bolt. An M8 8.8 bolt has a failure shear strength of about 12000N and a tensile strength of about 24000N,

If the bolts are torqued to spec (21Nm), a M8x1.25 bolt with a dry thread will give a maximum clamping force of about 9000N. A reasonable estimate for the coefficient of friction between the engine mount and machined aluminium surface is about 0.2, so the shear resistance provided by each bolt will be about 1800N, or about 7200N of shear resistance at each mount.

What this all means is that the maximum engine torque in steady state will only apply about 14% of the torque required to overcome the M8 bolt clamping force if the torque is maintained at 21Nm. If there is a shock load on the driveline, this could go higher but will be quite limited by the shock absorption of the rubber in the engine mount (so a high multiplication is unlikely, I don't think the torque would exceed about 20% of clamping force). Even the use of spacers and washers would not be sufficient to allow the clamping forces to be overcome. I would surmise that the failure mode has to be due to the bolts backing out due to vibration or incorrect torque at the factory, with fatigue failure possible or the bolt just falling out. The bolts may also snap due to softening of the metal reducing the maximum tensile strength, causing the specified 21Nm torque to be sufficient for the axial clamping force to be close to the yield strength of the bolt.

I would say the most important thing for these bolts is to make sure that they are sound (by replacing them) and using a locking mechanism to maintain the specified torque. Therefore either the standard 8.8 or upgraded 10.9 bolts with a means of locking the bolt to mainatin torque is a totally acceptable solution, not just a band-aid.
 

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S54=Living on the Edge
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You think it is a band-aid?
Nope ... was just stating that in the context of the OPs post; should have put that in quotes ;) My thought is along the lines of a bad batch of bolts of a problem on the line that did not torque the bolts to the minimum needed.

Looks like jsc did some calculations for us as well :thumbup:
 

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The question I have is...

How many owners have had this issue happen twice on their cars? I'm interested to know if the "updated" OEM bolts and / or aftermarket bolts used to remedy the situation have still resulted in a failure?
 

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Discussion Starter · #10 ·
The calculations are interesting but don't take into account the static weight of the engine or the bending torque applied to a longer bolt compared to a shorter one. You could claim that a wheel attached with spacers is just as strong as one without spacers, but experience is that long wheel bolts and thick spacers fail more often than wheels bolted directly to the hub.

Why do these marginal bolts work in the M3? M3 was made in Germany.
Marginal engineering + better German parts + better German workers = barely adequate
Marginal engineering + inferior parts + inferior workers = pattern of failures.
 

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S54=Living on the Edge
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What empirical evidence do you have to support the "marginal engineering" claim? The Z4Ms are driven hard at the track by more owners than probably many new cars out there and the few people that did have the broken bolts issue have not complained since the fix. That to me sounds more like an assembly line/parts quality issue than poor engineering.
 

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Discussion Starter · #12 ·
I am following the conventional wisdom and just replacing the bolts with grade 10.9 equivalents, using loctite, and making sure they are torqued properly. So far 4 of the 6 bolts that I have been able to get to were loose. That would support an assembly problem. By the way, my car only has 2000 miles on it.

I would appreciate some help on how to get to the front two engine mount bolts on the driver's side. Access from above and below seems almost impossible. Perhaps by removing the airbox and the power steering reservoir they could be reached from above. If someone could assist me with the best way to get to the two front bolts on the driver's side, I would be very grateful.
 

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I am following the conventional wisdom and just replacing the bolts with grade 10.9 equivalents, using loctite, and making sure they are torqued properly. So far 4 of the 6 bolts that I have been able to get to were loose. That would support an assembly problem. By the way, my car only has 2000 miles on it.

I would appreciate some help on how to get to the front two engine mount bolts on the driver's side. Access from above and below seems almost impossible. Perhaps by removing the airbox and the power steering reservoir they could be reached from above. If someone could assist me with the best way to get to the two front bolts on the driver's side, I would be very grateful.
TIS says to remove the air collector and move over the steering reservoir.

 

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The calculations are interesting but don't take into account the static weight of the engine or the bending torque applied to a longer bolt compared to a shorter one. You could claim that a wheel attached with spacers is just as strong as one without spacers, but experience is that long wheel bolts and thick spacers fail more often than wheels bolted directly to the hub.

Why do these marginal bolts work in the M3? M3 was made in Germany.
Marginal engineering + better German parts + better German workers = barely adequate
Marginal engineering + inferior parts + inferior workers = pattern of failures.
The static weight of the engine puts very little shear on the bolts due to the angle of the engine mount, mainly compression in the mount around the top 2 bolts of each mount and tension in the bottom 2 bolts (taking them closer to their tensile limit, which is a lot higher than the shear limit).

With wheel spacers, the issue is not the length of the bolt, but the change in lever arm. If you added a 20mm spacer, but also used a wheel with 20mm more negative offset, then you shouldn't say any change in the rate of failure.

These bolts likely work in the M3 due to the fact that they aren't marginal from an engineering standpoint (marginal is usually putting the component above 50% of it's yield limit, in this case, the M8 bolts seem to be at a much lower stress than that), it really does seem to be a manufacturing issue on the Z4M, so the BMW fix should work fine.
 

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jsc. Don't even bother, he'll think what he wants to think...

I think the true cause is, as specified in the service bulletin, bad parts. How many failures do you see in aircraft and other vehicle worldwide the come down to bad part made be a cheap company trying to extend their profit? It's not poor engineering design, but poor execution by the companies that supply the parts for the lowest cost.
 

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I thought these failures only happened on the passenger side mount?

I replaced my 4 bolts this weekend, Thankfully none of mine were loose or damaged but the 10.9 bolts went in,

Ron/Guys
Did you replace you drivers side mount bolts too? :dunno:
 

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I thought these failures only happened on the passenger side mount?

I replaced my 4 bolts this weekend, Thankfully none of mine were loose or damaged but the 10.9 bolts went in,

Ron/Guys
Did you replace you drivers side mount bolts too? :dunno:
I didn't replace any. Will just do the passenger (right) side when I do.
 
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