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Tensionier pulley bolt broke

My Timing belt ws replaced 2k ago. The Tensionier pulley bolt snapped and has caused some interiort engine damage. The mechanic said “they did not over tighten the bolt, it is a faulty bolt”. The tread of the steal bolt fastens to an aluminum engine block ( or what ever it’s called, the treads on the section of the bolt outside the engine block looks perfect as does the section of the bolt removed from the engine, the treads inside the engine block where the bolt screws into also look fine. Given the condition of the bolt, is it likely that the bolt was over tightened causing it to snap or is it a faulty bolt?VW 2001 Passat 2.8 engine wagon 70k miles.

Look at the ends of the broken halves. An over torqued fastener can show signs of the stress as the metal is partially rotated at the break point. A swirl pattern in the outer regions of the broken face is a good indicator of over stress. A shear failure without over-torque will look much different as the bolt simply snaps off in one direction often leaving a rough protrusion on one side.

Without some signs of corrosion, a shear failure is unlikely unless the metal was fatigued from over torquing or an incorrect fastener was used (e.g. material or hardness grade).

You might also want to measure the length of the end that was in the block and then the depth of the block hole. This will tell you if the correct bolt length was used or if the bolt had bottomed out prior to proper torque on the tensioner assembly.

Was the tensioner replaced at the same time as the timing belt? If not, it should have been.

Has the tensioner bearing seized up or is it dragging badly? A seized tensioner can cause the bolt to break.
Just offhand, it’s not likely an overtightening problem.

Thanks for your reply. Then tensioner pulley was not replaced at the time the iming belt was replaced because it had been replaced by another mechanic when the timing belt was replaced with the water pumb 30k mile prior. The current mechanic said the tensioner pulley was in great shape and therefore he did not replace it. Should he have anyway also, the prior mechanic said he did not replace the bolt when he replaced the tensioner pulley.

In brief, yes he should have replaced the tensioner along with the roller and water pump.

I also noted that the V6 timing belt kits come supplied with new bolts, there’s probably a good reason for this.

It look like your mechanic just replaced the belt, I can’t understand why anyone would do that.

Bolts can and do fail. I’ve seen crankshaft bolts that had never been removed in the past shear right off allowing the harmonic balancer to fly off detroying everything in front of the engine. Radiator, fan, fan shrowd, radiator hoses, and accessory belts. Although a rare event, it does happen.


The mechanic is planning on replacing the timing belt(again, last replaced 2k miles ago), and tensioner pulley. I don’t know what the extent of the damage to the engine is yet, what should I expect? And what kind of repair cost is indicated. i’m pretty sure a some of the valves have been bend. I have a 2.8 v6 enginein a 2001 Passat wagon.

Then your mechanic hasn’t planned very well. The cam belt replacement should also include replacing the water pump, roller, tensioner, bolts and probably the thermostat and housing - these parts are all provided in most quality cam belt kits.

I would expect some valve or piston damage that can only be determined by both head removal. As for cost, no idea since we don’t know your mechanic’s hourly rate or the extent of the engine mechanical damage.

Did you request the belt only change or was that on advisement from your mechanic ?

when the timing belt was replaced 2k miles ago it was becuase I had no heat in the car, therefor the thermostat was replaced, the water pump was replaced at 30k and the mechanic did not replace it at the time he replaced the therm. What is the cost of the cam kit? I guess I should request he replace all and puirchase a quality cam kit, does this kit come from vw?

The camshaft tensioner pulleys and bolt(s) for your engine:

Leak-down tests, on the engine cylinders, WHEN DONE (AND DONE CORRECTLY), can reveal if any valves are bent. An engine tear-down is not necessary to determine if valves are bent.