[justinisbxmb]

What's up guys I own a 2006 BMW 330i and over the weekend I was replacing my tensioner and idler pulley on my engine.

I'm sure everyone here has done this before at some point, very easy DIY. But upon removing the tensioner assembly, I notice that it is being held on to the motor block with a nut without a screw.

It seems as if someone maybe stripped the old bolt that goes in to the block and replaced it with a bigger screw, threw on the tensioner, then tightened it with a nut. I know that this is not normal but I wanted to see if any one else have ever seen this on the forum, it's confusing cause it looked factory. I will try to get pictures when I get home this evening. I was able to replace tensioner and everything runs fine, I just want to know if this is anything to worry about / bother to look in to. I had to use the existing screw and bolt, I could not use the new aluminum bolt that comes with tensioner because of the screw being in the block, I didn't want to mess with it or try to take it out.

[hassmaschine]

yeah, that is wrong. It should be a big aluminum bolt. It threads into magnesium, so using a steel stud & nut is a big no-no.

Either that or the head of the original bolt completely twisted off, which happens. Just remove the tensioner, unscrew the remains of the original bolt, and replace it.

[justinisbxmb]

Originally Posted by hassmaschine

yeah, that is wrong. It should be a big aluminum bolt. It threads into magnesium, so using a steel stud & nut is a big no-no.

Either that or the head of the original bolt completely twisted off, which happens. Just remove the tensioner, unscrew the remains of the original bolt, and replace it.

Thank you for the response, how can I remove it safely? I know I could maybe drill it out but I don't want to risk messing up the block.

[TheMidnightNarwhal]

Originally Posted by justinisbxmb

Thank you for the response, how can I remove it safely? I know I could maybe drill it out but I don't want to risk messing up the block.

I think I know what you mean but once you post a picture I'll understand better.

[justinisbxmb]

Sorry the image is so big, but as of now this is all I can show because it's raining and I'm unable to remove tensioner..but clearly you can see this is not the correct bolt that needs to be there. That screw the bolt is attached to runs all the way in to the block and I have no idea how it got there, previous owner must have rigged it up.

[hassmaschine]

Originally Posted by justinisbxmb

Thank you for the response, how can I remove it safely? I know I could maybe drill it out but I don't want to risk messing up the block.

Find two nuts the same size. Torque them together on the stud so they are "locked" in place. Use a wrench on the inner nut to loosen the stud. I do the same thing in reverse if I'm installing a stud and I don't have the proper tools.

The risk is fairly serious here, probably more than anywhere else on the N52. If those threads fail because of galvanic corrosion, that stud is going to rip out of the engine, your serpentine belt will slip off the tensioner and get wrapped around the crankshaft, sucking bits of the belt into the engine, potentially destroying everything.

People complain about water pumps, oil leaks, ticking.. meh. None of that stuff will destroy your engine (and likely "total" your car from a financial perspective). At worse, they are an annoyance. But let that tensioner go...

[justinisbxmb]

Originally Posted by hassmaschine

Find two nuts the same size. Torque them together on the stud so they are "locked" in place. Use a wrench on the inner nut to loosen the stud. I do the same thing in reverse if I'm installing a stud and I don't have the proper tools.

The risk is fairly serious here, probably more than anywhere else on the N52. If those threads fail because of galvanic corrosion, that stud is going to rip out of the engine, your serpentine belt will slip off the tensioner and get wrapped around the crankshaft, sucking bits of the belt into the engine, potentially destroying everything.

People complain about water pumps, oil leaks, ticking.. meh. None of that stuff will destroy your engine (and likely "total" your car from a financial perspective). At worse, they are an annoyance. But let that tensioner go...

Thanks for the advice, a few of my car friends told me to just let it go and it will be fine but I do not want to destroy my engine because I didn't fix a $5 bolt issue...I'm actually impressed that this thing has been holding up all this time I've had the car.

So what you're saying is locking two nuts together with the tensioner off? I'm lost, I was thinking about just grabbing that bolt with something like an extractor and just pulling it out. I'm sure it's a way I can get it out without drilling it out. I'm just unsure if this stud was added because the original bolt was stripped and they made a bigger thread when rethreading, it's hard for me to tell. I will have to take it apart and see if the nut will screw on to the original bolt that goes with the tensioner I have from the kit I bought. If it fits then I know it is the same thread size and will be safe extracting. I have no clue how I'm going to get it out but it will have to be done soon.

[its_dewayne]

With the tensioner off, you can use the double nut process as stated above. Once you get the nuts on the stud, tighten them together and then use a wrench on the nut closet to the engine and loosen it up. The stud should come out as if you were loosening a bolt.

[TheMidnightNarwhal]

Originally Posted by justinisbxmb

Thanks for the advice, a few of my car friends told me to just let it go and it will be fine but I do not want to destroy my engine because I didn't fix a $5 bolt issue...I'm actually impressed that this thing has been holding up all this time I've had the car.

So what you're saying is locking two nuts together with the tensioner off? I'm lost, I was thinking about just grabbing that bolt with something like an extractor and just pulling it out. I'm sure it's a way I can get it out without drilling it out. I'm just unsure if this stud was added because the original bolt was stripped and they made a bigger thread when rethreading, it's hard for me to tell. I will have to take it apart and see if the nut will screw on to the original bolt that goes with the tensioner I have from the kit I bought. If it fits then I know it is the same thread size and will be safe extracting. I have no clue how I'm going to get it out but it will have to be done soon.

Yep exactly.

As other said, I would get it off with 2 bolt method and then hopefully your original bolt fits in and they didn't use bigger thread or hole...

EDIT: Also watch out when you take it off, maybe it rusted on the inside. Hopefully you don't snap it off.

[velorider562]

I wonder if the original bolt stripped and someone installed a heli coil and red locked tighted a threaded rod in place. I would careful inspect the rod once the intensioner is removed before attempting to use the double bolt method.

[justinisbxmb]

Originally Posted by velorider562

I wonder if the original bolt stripped and someone installed a heli coil and red locked tighted a threaded rod in place. I would careful inspect the rod once the intensioner is removed before attempting to use the double bolt method.

I was also thinking this, I feel a bit weird just taking it out right now. I have a local Indy shop I might have take a closer look at it before I extract it. I don’t want to do any damage to the engine trying it myself at this point. Worse case scenario it’ll already be at the shop so if it needs to be repaired it can be done properly.

[hassmaschine]

Originally Posted by velorider562

I wonder if the original bolt stripped and someone installed a heli coil and red locked tighted a threaded rod in place. I would careful inspect the rod once the intensioner is removed before attempting to use the double bolt method.

it's possible. But the aluminum bolt heads break off easily (especially if over-torqued or re-used). It's a lot more likely that somebody did a hack job repair with a threaded rod and a nut they had laying around.

Hopefully the threads are OK. I'm not totally sure about the metallurgy between stainless steel and magnesium vs zinc plated carbon steel. If it's a heli coil, I suppose you don't have any choice but to leave it (you can't repair the threads otherwise).

But if it's just a threaded rod into the block you're best off repairing it properly. And god forbid they used loctite, although you can just heat it up to break it free (just don't catch the magnesium on fire, lol).

[justinisbxmb]

Originally Posted by hassmaschine

it's possible. But the aluminum bolt heads break off easily (especially if over-torqued or re-used). It's a lot more likely that somebody did a hack job repair with a threaded rod and a nut they had laying around.

Hopefully the threads are OK. I'm not totally sure about the metallurgy between stainless steel and magnesium vs zinc plated carbon steel. If it's a heli coil, I suppose you don't have any choice but to leave it (you can't repair the threads otherwise).

But if it's just a threaded rod into the block you're best off repairing it properly. And god forbid they used loctite, although you can just heat it up to break it free (just don't catch the magnesium on fire, lol).

Smh I hate this lol. Part of me wants to just leave it how it is. I bought the car two years ago and just now replacing the tensioner, it was fine the entire time. I will try to get my best judgement when I take the tensioner off again sometime this weekend.

[Joebie]

I think you're overthinking this.

Take off the belt, take off the nut, remove the tensioner.

Inspect the threads the rod is going into. If everything looks good, take out the rod and reinstall with the correct hardware. Removing that rod should not present a challenge whatsoever unless for some goofy reason they used loctite, then use the two nut trick. The only real way theres any issue here is if they've helicoiled AND loctited it, which if that happens to be the case you should just reinstall the new tensioner with the existing hardware i think.

Done. 10 minutes max.

Even if you find that there's a helicoil or something in there it doesnt effect the process. It's something you're going to have to live with whether that's the case or not.

I don't know if it helps but here's a picture of what a properly installed tensioner looks like in its relaxed state (no belt)

[Efthreeoh]

I'm going to jump in. Based on the one pic, it definitely looks like a stud based on the end thread-chamfer, or it was a cut bolt and the cut end was threaded into the engine block; it definitely is not the OE bolt with the head sheared off.

So you never said what material the stud holding the tensioner is made out of. Hass wrote a proper response but assumed the stud is steel, which is where the discussion of galvanic reaction came up. If the stud is indeed steel then you may want to remove it. If it is steel and has been in the block at least 2 years, unless it was treated with a good coat of anti-seize, it may be welded to the block at this point. I would think if that is the case, the edges of the threaded hole in the block would show signs of corrosion. I suggest you test the stud with a magnet and see if it is steel. Unfortunately if it is stainless steel the magnet will not attract, but stainless hardware is pretty easy to decipher from steel hardware and aluminum hardware. If it is aluminum, then use Hass' suggested method of two-nuts locked together, but if it is aluminum, then it should un-thread pretty easily since there is no tension on it.

According to the TIS the bolt is an M11x65, so it is an M11 bolt shaft size and 65MM long. The standard pitch for an M11 bolt is 1.50, fine pitch is 0.75. Looking at the pics of the on the internet, it looks to be a course thread, so I'd bet the thread pitch is 1.50. M11 is an uncommon bolt size, so finding a M11 2-inch long stud would be quite difficult and harder than just ordering a new OE tensioner bolt from one of the 50 places it is available on the internet.

So your theory is that some grease monkey jerry-rigged a different bolt size and used a stud vs. the correct OE bolt because the threads in the block were damaged/stripped. My only concern with that theory is if a larger bolt size were used, say a common 12MM or 14MM stud, the tensioner would not fit over the enlarged bolt. I could see the original repairing mechanic simply drilling out the diameter of the new tensioner he was fitting onto his stud conversion, but you said the new tensioner you used fit over the stud too, so that tells me the stud is still 11MM or it was converted to a 7/16th SAE (American thread) stud and the block was re-tapped to an SAE thread size of 14 threads per inch. I'm thinking that re-tapping the magnesium block with a 7/16-14 probably wouldn't work. Drilling it with the correct dimensional drill, which is a Letter-U drill, wouldn't leave enough material to cut a proper thread. And doing all that is way more difficult than just getting a new OE bolt. Now a crazy idea is, maybe the grease monkey was really talented and put a M11x1.50 magnesium stud in there. If that is the case, then consider yourself a lucky man and leave it alone.

So if it were my engine, I'd try to figure out what material the stud is and determine what thread it is; so you need a magnet, calipers, and metric thread pitch gauge or an SAE thread-count gauge. I like the idea of threading the nut from the stud onto the OE bolt as a check, but if it doesn't fit, then you'll be guessing, which is why you should get the thread measuring tools.

So if it turns out the stud is steel, then what do you do? I'd leave it alone. It's held for a minimum of 2 years by this point. If it is "galvanically" corroded in the engine block, I don't see that as a problem. Hass suggests that the threads will eventually fail and the stud will pull out, but I'm not sure that is true, or it would take a very long time. When dissimilar metals galvanic-corrode they simply just weld together and now you have some funky new alloy, but the parts are fused together and will not separate. For the steel stud and magnesium block to fail, the material has to go somewhere, but in this situation, where is the material going to go? It would have to be attracted to and flow to some other part (the body of the tensioner is the closest to the stud/block - you'd have seen evidence of this on the back of the old tensioner). Some dissimilar-metal galvanized joints do fail, but not in all cases, and again the galvanic reaction has to make the metal leave the area. Here the contact between the dissimilar metals is inside the engine block at the threads not exposed to the environment. So essentially the stud, if it is steel, is just now welded into the magnesium engine block. The reason BMW uses aluminum bolts on the N52 block is so the parts bolted to it can be removed at a later date for repair. The good news I see here is, if the stud is steel, you know it's not going to bend and fail like the aluminum bolt does.

My 2 cents.