Having just had a a cam belt replaced (using full kit) by an independent garage we are now in the proverbial DoDo...4 days later the auxiliary belt tensioner pulley bearing gave up with the consequence of the auxiliary belt ripping through cam belt cover, removing said cam belt and bending valves/head fractures around followers...new block and head needed. Are there any known issues regarding precise refitting of auxiliary belt/cam belt that an independent may have missed which would have resulted in a really good engine being consigned to the scrap bin?

 

What happened to you is a known possibility when the auxiliary (serpentine) belt fails. Occasionally the broken serpentine belt derails the cam (timing) belt. This is why it's important to properly maintain the serpentine belt path/system.

If this were my car I would be suspicious, as you are, that the independent shop screwed something up. The first question I would ask is how do you know your description of the failure is accurate? Did you determine this for yourself or did the shop tell you this? Your description is the preferred cover-up scenario for a screw-up by the shop. What might they be covering up? An incorrectly-tensioned cam belt tensioner perhaps? Apart from the tensioner the cam (timing) belt system is so simple you would have to work very hard to screw something else up. I don't know if it's possible to incorrectly install the serpentine belt (one rib off maybe?)

Now, your question: "Are there any known issues regarding precise refitting of auxiliary belt/cam belt that an independent may have missed which would have resulted in a really good engine being consigned to the scrap bin?"

The answer is yes, there is, but in your case it probably would not have changed the outcome. If your car has an alternator clutch pulley, a good mechanic will check the operation of the clutch pulley whenever the serpentine belt is off and certainly during a timing belt job. If the alternator clutch pulley has failed it should be replaced. If a failed alternator clutch pulley is not replaced, the serpentine belt tensioner takes a real beating. If it's suspected that the clutch pulley has been inoperative for a long time it's a good idea to replace the serpentine belt tensioner. And the serpentine belt should always be replaced when the timing belt is replaced, unless the serpentine belt is new. It goes without saying to always use OEM or equivalent quality parts. No C.R.A.P. (Chinese Replacement Auto Parts).

[You can get a good idea of the condition of the alternator clutch pulley by looking at the serpentine belt tensioner while the engine is running. If the tensioner is constantly jumping around your clutch pulley has failed.]

So, what are the possibilities?
1. The shop is lying to you about what happened because they know they botched the job. I lean towards this scenario because I am a realist.
2. You have had a failed alternator clutch pulley for a long time (mileage on your car?). The serpentine belt tensioner finally failed. It's a coincidence that this happened four days after your cam belt job.
3. Assuming (2) is what has happened and not (1), a REALLY good shop would have called you and told you you needed to spend another 300 (?) pounds (?) to replace parts that were still (sort of) working. What would you have said?

 

Many thanks Migbro...the serpentine belt tensioner failed...all went wrong at a fortunately low speed..a quick look under bonnet was confirmed by AA man - pulley had fully disintegrated, leaving very little attached to engine! Its difficult now to exactly ascertain what might have led to the pulley failing - we had found some info about the alignment of the serpentine belt being critical - else failure of pulley will result - sounds a likely cause but now difficult to prove. Since everything was working fine (no squeaks or squeals from that end of engine before work done) we are obviously suspicious that they screwed up somewhere along the line. We were being ultra cautious with cam belt replacement as the car covers a lot a miles. The garage reckon its 'just unfortunate'...we don't agree but proving they are culpable might be difficult.

 

OK. I don't know what you mean by the alignment of the pulley being "critical." The auxiliary belt only goes on one way. No alignment is required. So the pulley disintegrated. Plastic pulley? The moral of this story is that plastic is a poor material choice for pulleys. As your car "covers a lot of miles" that plastic pulley has seen many heat cycles. With this new information I doubt the shop had much to do with the failure. I guess I'm too cynical. I suppose you could imagine the shop's rough handling of the plastic tensioner pulley overstressed it. But the real root cause is the wrong material (plastic) for the application. I'm going to check if my Volvo has any plastic pulleys and replace them. Oh yeah, and check your alternator clutch pulley when you replace your cylinder head!

Edit: here's some really good info.

 

Having spoken to a local volvo dealership...they say that they replace the serpentine belt and the tensioner as a matter of course when they do the cambelt..I posed the question as to wheather this was volvo 'written law' or just this dealership being cautious...I await a response.

 

I see where you're going but afaik there's no Volvo rule that requires that. In hindsight it looks like a good idea but I don't think you're going to get anywhere with this with the shop that did your cam belt.