Tensioner Bolt Breaks...who's to blame?


#1

Approx two months ago I “retrieved” my old 2000 Subaru Legacy Outback (~230K miles) from its parking spot of 8 months here in the Mtns of NC. Much work needed to be done to get it driveable again. First job was head gasket, serpentine belt and timing chain. Next job was temp regulation system (thermostats, etc), O2 sensor, Fuel filter. Next was radiator and hoses. Then Power Steering pump and hoses. There were probably a couple other items in there of less consequence…it has been a long and expensive two months. Last week the tensioner bolt broke…the engine is shot. I have had the same mechanic do all of this work. He did not replace the tensioner bolt when he took it out to replace the head gasket.

Questions:

  1. What is protocol or best practice for mechanics (either in mechanics courses 101 or 102, or even more advanced) in terms of replacing or suggesting replacement of the Tensioner Bolt (TB) when replacing the head gasket on a car fitting this description?
  2. What is the likelihood that the TB would break on its own even if reinstalled correctly?
  3. What is the lifespan of a TB?
  4. What is the likelihood of someone (my former mechanic) not replacing the TB correctly…and what would that look like?
  5. What are the signs I should look for to see if it wasn’t potentially replaced correctly?
  6. How would I tell/prove if the mechanic might be at fault?
  7. If at fault, how much responsibility should be placed at his feet (in his wallet) for the new engine?

Thanks!


#2

When I replaced the timing belt on my Subaru I used the old bolts to put everything back together, including the tensioner. There was nothing in the manual about using new bolts.

The torque on the tensioner bolts is not especially high. I don’t see why they would need to be replaced as part of the job. There are bolts holding idler pulleys for the timing belt, too, and they are reused even if new pulleys are installed.

A Subaru specialist did some work on my car that required removal of the timing belt. He didn’t install new bolts when he put it back together, either.

I don’t think the mechanic is responsible for a broken bolt, assuming he installed everything correctly and torqued the bolts as specified. And even if he didn’t, you’ll never be able to prove it.

It stayed together for two months. If the mechanic hadn’t done the job right it probably wouldn’t have lasted that long.

I think you should chalk this up to bad luck and move on.


#3

To answer without reading the story; it was the people who designed the tensioner system who are at fault. There is probably a better way to build it.


#4

Did the bolt actually break or was it pulled out of its threading?

My guess is that your mechanic, instead of taking the time to look up the torque specification for your tensioner bolt so he could then use a torque wrench, simply skipped those steps to save time and instead relied on his experience with tightening bolts which in many instances can work. He messed it up. You will have a hard time proving this.

The timing belt tensioner bolt on my VW has a nut to be tightened. I have replaced at least 4 timing belts and the original tensioner bolt is holding very well. My bolt is held in a threaded hole in the cast iron block. If yours is held in threaded aluminum, then it is much more important that a torque wrench is used.


#5

The odds are very great this bolt broke due to being overtightened. A good machinist can probably eyeball it and tell you if that is the case. A more thorough method would be having it examined by a metallurgist.

Doing something about it is a lot stickier. For what it’s worth, I’ve been working with timing belts since the 70s and have never seen a pulley bolt fail metallurgically. Granted, there’s a first time for everything but the odds are it was over-grunted.


#6

First, thanks for the responses.

To answer Wha Who?: it broke in half. And, then to ok4450, if it broke in half would it not mean that there was likely play in it and therefore wobbled around and snapped?

Is there anything I can look for (signs and symptoms) in the parts/housing/etc that would let me know if it was over torqued or under torqued?


#7

A metallurgist could easily decide if the bolt failed due to fatigue or was overtightened by looking at the broken ends under a magnifier. It may have been a combination of fatigue and overtightening although fatigue is not likely as this does not seem to be a variable stress application. A bolt that failed in tension due to overtightening would likely display some necking down of the diameter at the break. He could compare the hardness of the base material of the broken bolt at a place other than at the break to compare to a new bolt to attempt to know if the original bolt was defective assuming that the new bolt was per mfr’s specifications. You could spend a lot of money to prove that the mechanic did it and when you are finally done, might approach the cost of needed repairs.

Imagine that this bolt was suspected responsible for an airplane crash. You can be sure that the reason for the bolt failure would be determined whether due to original design error, metallurgical defect including things such as heat treatment error, metallic content error, stress riser due to mishandling, hydrogen embrittlement due to defective plating process, or installation error typically overtightening or undertightening. It’s a matter of cost, not technical capability.


#8

To be honest, this whole philosophy of looking at whether the mechanic can be blamed bothers me. The vehicle is 11 years old with 230,000 miles on it. I assume this is the third or forth timing belt change?

I’m also having trouble connecting the broken bolt with the statement “the engine is shot”. Did it break while in operation and cause crashing of the pistons and valves? How much damage was actually done?

I apologize if I’m wrong, but this whole “give me a reason to blame the mechanic” on a vehicle of that age and mileage gives me pause to wonder.


#9

To Mountainbike: I do not typically look for “fault”…in this case, however, the mechanic has lied (or to be VERY charitable: mispoken) a few times about this…including that he has never touched that bolt (impossible, I understand, if the head gasket is changed), etc. I tend to give folks in any profession the benefit of the doubt, but as I’ve asked more questions I have not been favorably impressed with his responses…evasive, misleading, and defensive.

Engine is shot relates to the fact that the bolt did break in operation and the pistons and valves did crash…

The car was only driven, by the way, approx 4 or 5 times (other than by the mechanic) for very short distances since this first repair two months ago of the head gasket.

Some of my questions also come from a desire to simply better understand some of these things…both what has happened, and then to change my decisions appropriately for the future.


#10

I don’t know how the tensioner is mounted on that engine, so I cannot comment on whether it was necessary to remove its mounting bolt to replace the headgasket. It may not have been. Others here can perhaps clarify that for both our sakes.

My concern is that this timing belt, and perhaps even this tensioner, has I’m sure been changed a few times before or it would not have made it to 230,000 miles. Even if he did remove the tensioner, I’m reluctant to place blame on the last guy to touch the bolt. The bolt could easily have been overstressed previously and this last application of torque (even proper torque) left it on the verge of snapping. A metallurgist might be able to make that determination with sophisticated failure analysis, but it’s impossible to tell without such.

I’m also unclear about the damage. Normally when a belt pops there’ll be bent valvestems and perhaps some piston dings, but that doesn’t render the engine instant trash. The dings might need to be touched up to prevent stress risers and the valves replaced, but unless a valve breaks off the stem and tears up the cylinder walls the engine shouldn’t be trash.

Frankly, the other thing to consider is that the engine has 230,000 miles on it. At this point anything is possible. Long ago in BC (Before Computers) I rebuilt a carb for a friend and coworker, for free of course. Forever thereafter everytime something broke he blamed it on me. And none of the things were even carb related. That experience left me reluctant to make any assumptions without having all the facts first.