Rear Tension Struts - stripped threads in body (warning)

[wss4]

Summary - BE CAREFUL USING THE M10X35-10.9ZNS3 AND M10X40-10.9ZNS3 BOLTS LISTED ON REALOEM IN THE REAR TENSION STRUTS. I do not think they are long enough for the application.

My afternoon putting my car back together after installing a new differential came to an abrupt and unpleasant halt. Upon reinstallation of the two rear "tension struts" that run from the side sills to the body protrusion behind the differential, the rear bolt began to twist in its hole well below the 59Nm torque spec (and definitely before another 90 degrees of torque after 59Nm). When I removed the bolt, a nice steel curly cue came out with it.

Unfortunately, I know this problem all too well, as I JUST had to repair the threads in my engine head with a time-sert at the left most VANOS M6 mounting bolt hole. I suspect the VANOS issue happened due to too little thread engaged (which was a result of the new "upgraded" VANOS accumulator pressure line, which includes an additional mounting tab at that bolt). The additional thickness from the mounting tab means too little thread is engaged with the original bolt.

I believe realOEM lists the incorrect bolt for the rear of the tension struts. It shows the same M10 10.9 TTY bolt used all over the place (namely the front aluminum reinforcement). For some reason, I remember the original bolt I removed way back when being slightly different. My guess is it was longer. I was already using a longer McMaster bolt in the front hole of the tension struts, I think because of difficulty finding the BMW bolt in stock at the time.

Interesting that I was able to torque these bolts to spec at least once or twice before (the bolts that came out were exactly the same length as those I tried to put in). Also of potential relevance is that these are the bolts that the rear of the car gets jacked from at the center. Seems like this could have been a fatigue failure, at least to a degree.

Also interesting that the tension struts were apparently revised in 01/2008. My car is an 02/2008 build.

Pre 01/2008 tension strut part numbers are:

51718040189 (left)
51718040190 (right)

Post 01/2008 tension strut part numbers are:

51617213525 (left)
51617213526 (right)

Does anyone know what changes were made to the tension struts? Maybe they changed the mounting bosses and forgot to update the bolts in the system?

Has anyone had this issue before? I have an M10x1.5 time-sert kit on order, and am praying that there is enough wall thickness on whatever threaded boss is tacked up inside the body.

For everyone that reads this - BE CAREFUL USING THE M10X35-10.9ZNS3 AND M10X40-10.9ZNS3 BOLTS LISTED ON REALOEM IN THE REAR TENSION STRUTS. I do not think they are long enough for the application.

[GuidoK]

I've had these tension struts off probably over 20 times now, and never had any problems.
The longer bolts you're talking about are used in the front holes of the struts (so near the sideskirts/jackpoints). Those bolts are 50mm long because the tension struts there have a piece of bushing on them.

The tension struts at the rear point at the back of the diff are I think less than 1cm thick at that point so that leaves over an inch for the threads which is I think more than enough?

However its important to either use new bolts when torquing it to spec, or use less than 90 deg when using the old bolts. The bolts are stretch bolts and the more you use the old bolts the stiffer (stretching stiffens the metal) they get so 90deg torque becomes more and more, which is a common trait of stretch bolts. Maybe that's the problem?

[wss4]

Reused TTY bolts is definitely a possible explanation. That being said, I never reuse those bolts more than once, and this was a new bolt that I was putting in. However, it's possible the threads in the body were already damaged/weakened.

The rough measurements I took were:

+ 32mm length of bolt under head and washer
- 12mm tension strut boss
- 10mm unthreaded portion of hole in body
- 1.5mm unthreaded point of bolt, large chamfer
= ~8.5mm thread engaged

Unfortunately I don't remember enough of my undergrad engineering to determine if this is sufficient. Appears to be borderline based on the online thread engagement calculators and the one diameter rule of thumb.

The holes are approximately 38mm deep, so the threaded portion of the hole is approximately 25mm in length. Assuming I can get this fixed, I think I'll try to use a 45mm bolt in the future.

Probably a result of the combination of several less than ideal circumstances.

[GuidoK]

My car is on the lift as we speak.
I'll measure what the effective used thread deapth is (number of turns or something like that) and see if you had a problem to begin with caused by previous work.
The 8.5mm thread you give seems exceptionally low.
Be careful with using longer bolts, if the deeper part of the hole is not up to spec and, say, runs into a slight taper, you risk splitting/cracking the piece of the chassis..
And like I said, bmw uses special stretch bolts. Using a longer 'normal bolt' with the tis specs will strip out the threads for sure.

[GuidoK]

So I measured how my bolts are seated in the threads.
They grab little over 10.5 turns so thats over 15mm. A healty thread lenght I think.

Also the rear strut ends are compressed from 10mm (stock thickness) to 4mm where the bolt holes are.


If your car is different, then I think something strange is going on.

[wss4]

Thank you for checking on your car, it is very helpful for me to try to figure out what's going on.

Very interesting, what year/model is your car? This could be a difference between M and non-M models, as it does list different part numbers on realOEM. If we are comparing M to M, then maybe the revision in 01/2008 was significant.

The boss on my tension struts at the rear holes is 12mm thick. That 8mm of difference in tension strut thickness accounts for the 7-8mm difference in thread engagement we are seeing between our cars.

[wss4]

There is definitely an issue with the bolts listed on realOEM for both front and rear holes on the tension struts for a 2008 Z4 M Coupe. I took some more measurements, and got the following:

Rear holes (body protrusion behind differential cover):

- 12mm tension strut boss thickness
- 10mm unthreaded depth of hole in body

The bolt listed for this location is:
Hexagon screw with flange - M10X35-10.9ZNS3 (31106772199)

Which has approximately 30.5mm of useful thread when you exclude the washer and bolt point chamfer. This leaves about 8.5mm of thread engaged.


Front holes (side sill by jack pad):

- 20mm tension strut boss thickness
- 13mm unthreaded depth of hole in body

The bolt listed for this location is:
Hex bolt - M10X40-10.9ZNS3 (07119905655)

Which I would guess has approximately 37mm of useful thread when you exclude the washer and bolt point chamfer (I don't have the BMW bolt on hand). This leaves about 4mm of thread engaged!!

Fortunately I am using a McMaster 45mm bolt in the front holes, which has approximately 43mm of useful thread. This still only leaves 10mm of thread engaged. I hope that I do not suffer the same fate at all 4 bolt locations.

[GuidoK]

Originally Posted by wss4

The boss on my tension struts at the rear holes is 12mm thick. That 8mm of difference in tension strut thickness accounts for the 7-8mm difference in thread engagement we are seeing between our cars.

Can you make a picture of the front bolthole and rear bolthole on your tension strut?
Theres something strange going on.

I have a prefl car but that shouldnt matter an those parts also have been fitted on the fl cars

[wss4]

Hopefully this works:

https://stanford.box.com/s/ksuwoynuy...7tv8dpcqpzcstr
https://stanford.box.com/s/8wcc1td1n...fnghgap2fe82u8
https://stanford.box.com/s/ocqvj92c6...wq8in50nggmc9x
https://stanford.box.com/s/0oj63guro...g4otoquwdkhddl
https://stanford.box.com/s/lv7oxh24t...f6irzcp05qillm

[wss4]

It looks like the M variants have an additional boss at the rear mounting point, versus the non-M cars. Maybe this is for additional differential or exhaust clearance?

Non-M part (pre 01/2008):

https://www.ecstuning.com/b-genuine-...t/51717159199/

M part (pre 01/2008):

https://www.ecstuning.com/b-genuine-...t/51718040190/

[GuidoK]

Yes, that small boss on the rear is definately something I dont have. (the rest is the same so the height of that small boss is the extra lenght you need)
The thick boss on the front seems to be the same.

The strange thing is, if you look in the ETK (realoem/bmwfans.info), on the normal z4 (so the non-M), in the body reinforcements diagram, there are 2 bolts listed that are m10x50mm and they go in the front thick boss of the struts, and the rest of the bolts (14 pieces m10x35) go in the various other spots like the rear of the tension struts and the front reinforcement plate.

However if you look at the reinforcement diagram of the z4m, there are only 2 bolts listed that are m10x40mm and the rest is m10x35mm
Can this be a mistake and that it should be 2 bolts m10x50 (going in the front thick bosses), 2 bolts m10x40 going in the rear thinner bosses (that gives you about 8mm+5mm=13mm of thread) and the rest m10x35?
So that its a fault in the bmw spare parts catalog?
Why else have that 40mm bolt in there in the first place?

The m10x40 bolt is part# 07119905655
The m10x50 bolt is part# 07149140997

Maybe in the past a mechanic got really confused and replaced the bolts during a previous job with the wrong ones? (or according to realoem the right ones, however you look at it )
Or those m10x40 bolts are now stuck in another place that he mixed them up...

For me it would make sense to use m10x40 in the rear, m10x50 in the front strut hole and m10x35 on the rest of the chassis (reinforcement plates etc) IF the m10x50 bolt doesnt bottom out/has lots of resistance in the end of the threads you obviously can use that too. But check that by hand in the hole without the strut fitted, you must at least be able to thread the bolt in 2mm deeper than the lenght fitted with the strut.

[wss4]

I reached essentially the same conclusion you did with a 40mm bolt in the rear and 50mm bolt in the front, but via a bunch of measurements and algebra factoring in the time-serts. Seems that an error in the parts database is the most likely issue. Thank you again for your help and persistence diagnosing!

Unfortunately the M10x1.5 time-serts are pretty hefty, and are going to require drilling out not only the messed up threads, but also the 10mm deep counterbore (from 11.8mm to 12.7mm diameter). I would guess there is enough material to do this, it is just going to require careful use of the drill and some drill stop collars for depth.

This is probably a hopeless endeavor, but I'm thinking about sticking a cheap small USB borescope through the smaller ~6mm holes next to the bolt holes to see if I can get any sense of how thick the threaded boss actually is, before I go on drilling out material.

Hopefully this thread can serve as fair warning to others. Given that these are TTY bolts, if people are being diligent, they will be ordering new bolts. And if they use realOEM for part numbers, like most do, it is an easy but painful mistake to make. It is definitely believable that the 35mm bolt could be the correct one in the rear holes, as it does thread in a decent amount. If one were to attempt to put a 40mm bolt in the front hole, I imagine it would rip the threads out of the body under very little torque.

[GuidoK]

If you want to drill without the counterbore, you can use helicoils as they dont have a collar.

BMW suggests in TIS using helicoils on messed up threads for the front suspension mountings, they have a special entry for that.
For messed up threads with the reinforcement bars they dont have an entry, but thats pretty much the same situation as those holes have a protruding guide bushing.

[StickMon]

Thanks for the heads-up.
I also have the problem with the threads in the head for the bolt that goes through the tab in the accumulator line.
I currently have that bolt glued in. Next time I take my car to Lang I'll ask him to put inserts in all 5 holes.

[wss4]

It was a nasty surprise. I had kind of forgotten about it for a while, as I was able to get the bolt to hold enough to stay in and keep the tab in place. Then I decided to track down a slow oil leak on that side of the engine, that I thought might have been the chain tensioner or top of the VANOS accumulator, but sure enough turned out to be the gasket between the VANOS and head, right at that corner.

Fortunately the timeserts went in smoothly, once I released the passenger side engine mount and jacked up that corner of the engine to get a clear shot at both top corner bolts with a longer drill bit.

The thread repair for the tension struts has me a bit more nervous. I really don't know how much extra steel I have to work with, and if something goes wrong I'm probably looking at cutting the piece out and welding a fabricated piece back in.

[Finnegan]

Originally Posted by wss4

It was a nasty surprise. I had kind of forgotten about it for a while, as I was able to get the bolt to hold enough to stay in and keep the tab in place. Then I decided to track down a slow oil leak on that side of the engine, that I thought might have been the chain tensioner or top of the VANOS accumulator, but sure enough turned out to be the gasket between the VANOS and head, right at that corner.

Fortunately the timeserts went in smoothly, once I released the passenger side engine mount and jacked up that corner of the engine to get a clear shot at both top corner bolts with a longer drill bit.

The thread repair for the tension struts has me a bit more nervous. I really don't know how much extra steel I have to work with, and if something goes wrong I'm probably looking at cutting the piece out and welding a fabricated piece back in.

Good news on the work done, I'm glad that proceeded well.

On the tension struts, that's not a series of happy prospects. I really dislike projects where it boils down to a coin toss that the fix "fixes" the issue or makes it dramatically worse. Which often makes me procrastinate, which ends up causes problems itself due to the lack of making the repair. Losing a bolt while driving would not end well.

I wonder if there's any way to get the technical / build schematics for the car to sort out what you really have to work with. It would probably be a treasure hunt with BMWNA, Germany, and the M division. But it would be eminently useful.

I guess the other idea is to find a specialized auto repair shop that works on "classic" cars and German cars and has the means/ability to do any reinforcement/repair of the repair should it go wrong. (I can think of one place near me, but I'm in Sacramento CA.)

[wss4]

I ended up just going for it, based on my gut feeling that the boss had to have a wall thickness of at least 3-4mm (combined with my terribly imprecise measurements done with the short end of a small allen key through the adjacent 5-6mm hole). The time-serts went in successfully, the bolts torqued nicely, and everything appears to be holding thus far (although I only drove the car 40-50 miles). Time will tell if the repair will hold.

I ended up having to do all 4 holes, as the thread in the front holes was in bad condition as well, after more careful inspection. I put in 20mm long M10x1.5 time-serts all around, and installed a 50mm bolt up front (would ideally use 55mm) and 45mm bolt in the rear. Getting the drill perfectly perpendicular was the most difficult part of the operation, and one of the holes did go slightly off center (maybe 1mm at the top of the hole, which is only a couple of degrees off axis at most). Drill stop collars for depth are an absolute must. I did have to drill both the threads and the counterbore out, and do some minor clean up and adjustment of the counterbore with the dremel.

I definitely do believe that, as GuidoK mentioned, BMW intended for the 50mm bolt to be used up front and the 40mm bolt to be used in the rear.

To top this entire bummer of a process off, after returning from the 40-50 mile test drive, I found the entire rear end of my car covered in gear oil. It appears the freshly rebuilt differential destroyed an output shaft seal quite catastrophically. I am going to refrain from going into further detail, as I am actively working with the builder to get to a resolution. But let's just say I am less than enthused with the entire situation.

[Finnegan]

Originally Posted by wss4

I ended up just going for it, based on my gut feeling that the boss had to have a wall thickness of at least 3-4mm (combined with my terribly imprecise measurements done with the short end of a small allen key through the adjacent 5-6mm hole). The time-serts went in successfully, the bolts torqued nicely, and everything appears to be holding thus far (although I only drove the car 40-50 miles). Time will tell if the repair will hold.

I ended up having to do all 4 holes, as the thread in the front holes was in bad condition as well, after more careful inspection. I put in 20mm long M10x1.5 time-serts all around, and installed a 50mm bolt up front (would ideally use 55mm) and 45mm bolt in the rear. Getting the drill perfectly perpendicular was the most difficult part of the operation, and one of the holes did go slightly off center (maybe 1mm at the top of the hole, which is only a couple of degrees off axis at most). Drill stop collars for depth are an absolute must. I did have to drill both the threads and the counterbore out, and do some minor clean up and adjustment of the counterbore with the dremel.

I definitely do believe that, as GuidoK mentioned, BMW intended for the 50mm bolt to be used up front and the 40mm bolt to be used in the rear.

To top this entire bummer of a process off, after returning from the 40-50 mile test drive, I found the entire rear end of my car covered in gear oil. It appears the freshly rebuilt differential destroyed an output shaft seal quite catastrophically. I am going to refrain from going into further detail, as I am actively working with the builder to get to a resolution. But let's just say I am less than enthused with the entire situation.

I'm glad the repairs you did went well. I'll just sigh and groan and hope the builder makes good on the output shaft seal. Coming on a rebuilt diff .