Z4M - Cause of breaking engine mount bolts found

[car62]

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):

http://i276.photobucket.com/albums/kk27/z4mc/enginemountbolts2.jpg






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.

http://i276.photobucket.com/albums/kk27/z4mc/enginemountbolts1.jpg

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.

[thekurgan]

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.

[car62]

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.

[pal]

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.

[Ron Stygar]

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.

[Ron Stygar]

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?

[jsc]

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):

http://i276.photobucket.com/albums/kk27/z4mc/enginemountbolts2.jpg






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.

http://i276.photobucket.com/albums/kk27/z4mc/enginemountbolts1.jpg

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.

[pal]

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:

[Rick Hunter]

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?

[car62]

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.

[pal]

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.

[car62]

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.

[Ron Stygar]

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.

<img src="http://www.unofficialbmw.com/images/gaugesz4-3.jpg">

[jsc]

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.

[Dammmittt]

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.

[blau335]

Poor thread.

[car62]

TIS says to remove the air collector and move over the steering reservoir.

<img src="http://www.unofficialbmw.com/images/gaugesz4-3.jpg">

Thanks very much. The front two on the left seem to be the most difficult to access.

[Starscream88]

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:

[Ron Stygar]

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.

[Starscream88]

I didn't replace any. Will just do the passenger (right) side when I do.

:thumbup: Thanks Ron

[KenTenn]

I thought I read that this was just a coupe problem. Is that the case or should roadsters be checked?

[Ron Stygar]

I thought I read that this was just a coupe problem. Is that the case or should roadsters be checked?

Both. See bulletin.

http://www.bimmerfest.com/forums/showpost.php?p=3887169&postcount=39

[Randy Forbes]

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. The motor mount brackets on the Z3Ms (1998-2002) are all M8, and stay tight. The 01/02 models are rated at 315 bhp, so that's not far behind your 333 bhp.

I'm leaning towards an assembly line problem, because the M8 size bolts have been proven to work; especially considering a car with 2000 miles (!) has loose bolts. :rolleyes:

[Ron Stygar]

The motor mount brackets on the Z3Ms (1998-2002) are all M8, and stay tight. The 01/02 models are rated at 315 bhp, so that's not far behind your 333 bhp.

I'm leaning towards an assembly line problem, because the M8 size bolts have been proven to work; especially considering a car with 2000 miles (!) has loose bolts. :rolleyes:

I agree.

Bulletin says:
CAUSE
Prolonged heat exposure to the right side engine mount bracket, during the engine assembly process,
may cause the bolts, which hold the engine bracket to the block, to*** lose*** the specified tightening torque.

[car62]

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 replaced all 8 bolts yesterday. Two of the four on the right side were loose and two of the four on the left side were loose. The loose ones were the front two on the right and the rear two on the left.

[JCz04Bimmer]

I'll be under the car this coming Tuesday so I should probably have a look.

Are the bolts pretty readily available at dealerships? There isn't enough time to receive a package from Tischer...

[Starscream88]

I'll be under the car this coming Tuesday so I should probably have a look.

Are the bolts pretty readily available at dealerships? There isn't enough time to receive a package from Tischer...

No the dealer will sell you another set of 8.8 grade bolts,
You will need 10.9 grade with hardened washers,

As Ron posted the parts numbers of BMW Motorsport bolts but there is only a few dealers right now that can get you the motorsport parts,

Im not sure if anyone here managed to buy bolts from a local hardware store or Home Depot/Lowes?

That would be he only "walk in" stores I could think of,

You want 2 M8x25 1.25 Pitch 10.9 grade
and you need 2 M8x35 1.25 Pitch 10.9 grade
plus 4 hardened washers or you could use locking washers,

Its a long shot but you could try some auto part stores too, I wouldn't hold my breath at a chain parts store but I think an independent auto parts store might have them,

[jsc]

Im not sure if anyone here managed to buy bolts from a local hardware store or Home Depot/Lowes?

That would be he only "walk in" stores I could think of,

Its a long shot but you could try some auto part stores too, I wouldn't hold my breath at a chain parts store but I think an independent auto parts store might have them,
Here in Calgary there are a number of specialised fastener suppliers that are 'walk-in'. I would expect any larger city should have this type of store. They are typically only open 8-5 Mon to Fri, though, with the odd one open on Sat mornings.

[jmillet]

No the dealer will sell you another set of 8.8 grade bolts,
You will need 10.9 grade with hardened washers,

As Ron posted the parts numbers of BMW Motorsport bolts but there is only a few dealers right now that can get you the motorsport parts,

Im not sure if anyone here managed to buy bolts from a local hardware store or Home Depot/Lowes?

That would be he only "walk in" stores I could think of,

You want 2 M8x25 1.25 Pitch 10.9 grade
and you need 2 M8x35 1.25 Pitch 10.9 grade
plus 4 hardened washers or you could use locking washers,

Its a long shot but you could try some auto part stores too, I wouldn't hold my breath at a chain parts store but I think an independent auto parts store might have them,

If you plug those Motorsport bolt and washer part numbers into realoem.com you'll find the same 10.9 grade bolts and hardened washers have several other applications and are available from any dealer; I ordered mine from North Haven BMW.

[car62]

I'm sorry if some people didn't appreciate my point of view on this issue, but I think the post did serve to get out information to some owners who might have missed it otherwise. Changing all 8 bolts is not very easy and if someone wants to send me a private message with specific questions I will be happy to try to answer them.

Regarding sourcing the necessary parts, I recommend McMaster-Carr or the special BMW part numbers for the grade 10.9 bolts. Hardened M8 washers are also required. Blue thread locker is available at any auto parts store. If you wish to attempt this job yourself, here are some specific recommendations:

The car needs to be up on jackstands with the plastic belly pan under the radiator removed and also the aluminum belly pan under the engine removed. Rather than separate the two pieces of the aluminum belly pan, just leave them attached together and remove as a unit. Both front wheels should also be off the car.

Access to the passenger side is easier than the drivers side. The front two passenger side bolts are the longer ones. To get to them, you must undo and remove the fastening bolt that holds the VANOS accumulator bracket around the accumulator. Also it will be easier if you remove the attachment of the ground wire at the engine just next to the VANOS accumulator. After doing this, you can just push the VANOS accumulator up and out of the way, it is on a flexible hose. You now have fairly easy access to the two front engine mount bolts on the passenger side. The two rear bolts will also be fairly easy to get to from underneath the car with the aluminum pan removed.

On the driver's side, the two rear bolts are easier than the front. There is an electrical connector in front of the upper bolt that blocks access; look at this carefully so you can see how it detaches and then remove the electrical connector to get access to the bolt. The drivers side bolts must be worked on from above. The lower rear bolt is pretty easy but the upper rear bolt is not visible and you have to work by feel - you cannot actually see what you are doing. Remember to replace the electrical connector when finished.

The front drivers side bolts are the most difficult. I was able to gain access by removing the complete air filter box (upper and lower sections) along with the curved plastic pipe that attaches to the intake manifold. It is not necessary to remove the intake manifold itself. It also helps to remove the bolts holding the power steering fluid reservoir so that it can be pushed slightly out of the way. After removing these parts, you can use a very long extension and a socket to loosen the engine mount bolts. A magnet on a long wand will hep with pulling the bolts out, because you may not be able to get your fingers in there. Similarly, you can use the magnet wand to carefully place the new bolts in the empty hole, and then use your long extension and socket to tighten the bolts. Then reassemble and you are done.

Since the aluminum belly pan is a suspension reinforcement, I would have the car with the weight on the tires so the chassis will be under the correct stress before tightening the holding bolts for the aluminum pan.

Again I hope this information is helpful to someone with engine mount issues. Please send me a PM if you want to ask a question.

[Ron Stygar]

Since the aluminum belly pan is a suspension reinforcement, I would have the car with the weight on the tires so the chassis will be under the correct stress before tightening the holding bolts for the aluminum pan.

Again I hope this information is helpful to someone with engine mount issues. Please send me a PM if you want to ask a question.

You aware that the screws are one use and angle torqued?

http://www.bimmerfest.com/forums/showpost.php?p=3782138&postcount=10

http://www.bimmerfest.com/forums/showthread.php?p=4067789

[Randy Forbes]

You aware that the screws are one use and angle torqued?

http://www.bimmerfest.com/forums/showpost.php?p=3782138&postcount=10

http://www.bimmerfest.com/forums/showthread.php?p=4067789I have bags of those bolts left over, as they are included with every E-46 M3 bearing kit I install, and not used on the Z3Ms. I must have at least two hundred (200) pieces.

[pal]

JCz04Bimmer (http://www.bimmerfest.com/forums/member.php?u=5430) - IM me with your address and I will send you 2 25mm and 2 35mm bolts for the passenger side mount since those are the ones that appear to fail possibly because the motor leans that way. I have some left over of the McMaster Carr 10.9s.

[cruisingdog]

Are the bolts only failing on M's or are they failing on all models. I'm just curious as my car's due for a service soon and I think it might be worth mentioning and asking the service guys to check.

BTW, what's the spec.s on the new bolts ant the torque required ? Mcmaster Carr PN's ? Simply a replacement job whilst jacking up the engine ?

[Ron Stygar]

Are the bolts only failing on M's or are they failing on all models. I'm just curious as my car's due for a service soon and I think it might be worth mentioning and asking the service guys to check.

BTW, what's the spec.s on the new bolts ant the torque required ? Mcmaster Carr PN's ? Simply a replacement job whilst jacking up the engine ?

See the bulletin:

http://www.bimmerfest.com/forums/showpost.php?p=3887169&postcount=39

[cruisingdog]

Thanks. V. useful info.