Serpentine puley bolt broken, again!

maybe someone has encountered this, trying to understand what is wrong the bolt broke on one of the serpentine pulleys, while driving quite hard (shifting bit furiously from 1st to 2nd), luckily the puley, was pushed to the tensioner pulley and the serpentine belt didn't go crazy but the question is why? all the pulleys rotate freely, silently, without any play, the tensioner mechanism seems to be very strong (it takes some force to push it back) the same bolt broke 3 weeks ago, (I admit the previous one, was not oem and could have been too soft - 8,8 or something) I installed then a 10,8 stud and a nut, with loctite etc - all well, drove these 3 weeks and today after all day driving around, the same thing happened, part nr 8 in this picture? as this has repeated I am being very cautious about this, is it the tensioner that doesn't do it's job properly at some points? would appreciate some experience of others. to fix this doesn't take too long, but I don't want this to be repeated for the third time

Wow, that is very odd. Since it broke, I would think it either has significant side load on it, or there is some type of burr or sharp edge in the threaded hole causing a stress riser, which leads to its failure. Where does the bolt break? Flat with the hole where it screws in? Or somewhere else? Because it failed in a few weeks, I suppose it would show brittle fracture lines, meaning the break starts by some mechanism and the majority of it just shears off. Can you post pictures of where it failed and of the ends of the bolt showing the shear pattern? I would determine the cause of failure before just inserting a stronger bolt in place.

flat with the surface the pulley attaches to.

will post pics later.

I would definitely look for a sharp edge there.

Definitely strange. Never heard of that before or had it happen. I would order a brand new bolt and washer. You can see what you need from this diagram: http://www.autoatlanta.com/porsche-p...section=101-10 All in about $12. And since it keeps happening, I would replace the deflection roller as well for $89. Might be a bearing problem and you don't want this to happen.

http://www.renntech.org/forums/topic...rpentine-belt/ someone has experienced this, I did find another thread even here on rennlist but cannot locate it at the moment so something is breaking these bolts my suspicion is on the tensioner, maybe the tensioning strength is not sufficien, and the spring has become weaker with age couldn't be?

Same thing happened to me about 4 years ago. I replaced the pulley and bolt, and have been fine ever since. Maybe that bolt is just undersized? Perhaps an upgraded bolt is in order for those that need to be replaced? One more thing... Check the torque spec on that idler pulley bolt. My notes say that the proper spec is 17 ft-lbs, which is VERY low torque. Maybe it was overtorqued in the past?

You say the bolt breaks so that would lead me to think that the spring tensioner pulley is doing it's job and the bolt is insufficient strength. Insufficient tension I don't think would cause the bolt to break, rather over-tension would. The spring tensioner doesn't need to really do all that much, just create enough tension to keep the belt in place on the pulleys.

but how about overrews or shifts when there is a quick drop of load (while pressing clutch etc) maybe at that point the sudden drop and rewwing up can cause this, if the spring is weakened? soon I will put the car to sleep for winter then I can fabricate a thicker bolt or something, but this shouldn't happen in the first place now should it?

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Unless there is some kind of a problem with the drive system nothing should break and hard driving, up shifting and downshifting should not have anything breaking. Is the belt ok? No sharp edges? With the belt off also check all accessory drives, especially the water pump, for any play. Is the tensioner the correct part #? In fact, are all of the drive idler rollers and pulleys the correct part #'s? Are you sure the belt is routed correctly? Are you sure the belt is the right belt? If you find all are correct and there is no problem with the belt or its routing, this breakage is probably related to a substandard bolt being used, possibly being overtightened as another poster previously stated. My advice would be to replace the bolt with a factory item. The factory hardware is probably a pretty tough item, with rolled threads. A new roller with a new bearing would be a good idea too. I think DBJoe996 offered up this suggestion first and it is a good suggestion.

well the situation update, all the things are phucked up really bad, locite has done it's job very well, the stud is stuck dead and won't move both with grinding and welding nuts to it, so the end result is engine removal as there is no space for drilling and reworking the thread... so I am pissed, I also won't take any chances and order new tensioner pic taken before messing with it...

Quotes source post: 12608868

1. belt is new replaced it some 5000 miles ago 2. no 3. all the rollers and pulleys ar perfect, no play no grinding noises no squeeling nothing, silent. 4. oem porsche 5. yes 6. yes 7. yes

Since the engine rotates clockwise, my guess is the tensioner is weak and during hard deceleration, the alternator pulley keeps rotating and it pulls the belt so hard that the tensioner pulley hits the idle pulley and snapped the bolt. '99 alternator has no over-run clutched pulley.

After looking at your picture, that bolt looks like it had quite a load on it, especially if it had only been in 3 weeks and the previous bolt also failed. The zebra stripping on it on the upper side is where it failed, so right about 10:30 position. By looking at the Pelican photo above, that is almost the exact place where the side load is applied to the pulley. All of the area below the zebra stripe failed instantaneously. See more here: http://www.maintenanceworld.com/fail...al-components/ Based on that, I would closely evaluate the tensioning system and would get a new tensioner as you stated. It's probably something obvious. Good luck!

well the tensioner is ordered, can get it at 60-70usd, which is not bad, (bmw has the same design and go around 20 but ok ) but how can you diagnose this, as it seems to have great tension in it anyway, but it can get you so much pain in the ***... any simptoms or ways to diagnose this, anyway, I assume this is a good reason to check it on your cars guys, and if it is only 70 bucks and replaceable at home, then better replace it

Does the crank shaft pulley wobble with the engine running?

no haven't noticed that.

In addition to checking accessories for play, check to verify that everything is turning freely. I think there has been an example or two of the AC pulley locking up - if one of the driven pulleys is generating too much resistance, the resulting tension throughout the belt path is going to force something to give. I don't know a great way to check the AC for issues while the car is running - I suppose an IR thermometer would help identify if one of the accessories is generating a lot more friction than the others.

(Sorry for resurrecting a dead thread ) I've been helping a buddy with the same problem on his Boxster. The #8 (from the photo at the top of the thread) idler bolt broke off even with the engine block. He hired a TIG specialist who welded a number of nuts onto the bolt without success. We then took a cast of the area using epoxy putty and machined it into a fixture to hold a hardened drill guide. This tool also let us locate & drill a 1.5" hole in the firewall so we could drill straight on. We used left hand drill bits but that made no difference and we ended up drilling it out to 6.8mm (17/64) and retapping with a bottoming tap. We then noticed what had caused the bolt to fail in the first place. Here is the boss on the block after we tapped it. If you look closely, on this particular engine, the factory hole is not centered in the boss. Here is the end of the spindle that supports the pulley. The worn area shows where it was supported against the boss on the block. It's clear that a manufacturing error misaligned the bolt hole and allowed the spindle to slip off the boss. As fate would have it, the area without support is exactly where the belt is pulling. So, if your car has experienced this failure it is likely going to continue to fail unless you find a way to support that edge of the spindle - we are considering a washer to add support and a bushing to help keep the spindle from sliding sideways. For my money this is simply a bad design. The bolt is too small and the fancy hollow spindle arrangement prone to failure. Other cars use just a heavy bolt with a shoulder and that seems to work just fine.

I think if you can grind the end of the boss flat and do the same to the spindle/spacer, all will be good - you can make up what you grind off with a flat washer between the two. The caveat being you have to make sure the face of the boss is parallel to the face of the engine and the bottom of the spindle is square�