So what would cause this to happen?
Years ago I had a series of rocker arms break on my Sunbeam rally car engine. It got annoying (and not finishing events was becoming costly) after the third time. I had the broken rocker looked at by a metallurgist and he believed there had been a casting flaw from which a crack had propagated. So I obtained a big box of arms, had them all inspected and picked the best. I then polished them up, before rebuilding with new shafts and replacing the whole rocker assy on the engine. I never suffered another breakage. It’s likely though that the heavy duty double valve spring system was overloading the arms compared to their design capability and maybe they’d have never broke if left in a standard engine.
This problem almost certainly does not affect yours however. Not least of all because you suffered two broken arms at the same time and in the same cylinder. I would suggest that the chances of any kind of manufacturing defect afflicting two distinct components such as to cause such a failure are vanishingly small. It would seem to me that some outside influence is far more likely to be the root cause.
The rockers would not have been disassembled from the head whilst changing the timing belt, so cannot have been dropped for example. So what could be the possible cause? Well, you’re never going to know, but maybe something was dropped onto the rockers in situ. Or, as hinted at, maybe the engine was turned over with the belt removed (maybe the van needed to be moved part way through the job and had to be pushed but had been left in gear?).
It also seems rather “circumstantial” that the repair was carried out in rather a “hurry”. However I don’t think there is sufficient evidence to make any accusations. Maybe this chap is an otherwise excellent mechanic and there is no-one else you’d like to go to, in which case you don’t want to ‘burn any bridges’. When you pay him (or next time you’re there if you’ve already paid) drop some hints into the conversation that you don’t understand how it could have happened, that maybe they’d been damaged somehow, comment on the chances (as mentioned above). See if he fesses up.
If you can find someone else, maybe be more assertive and question him and see what he comes back with. If it is anything he knows he did, he’d be daft to risk loosing a regular customer over a piffling mistake.
You may just have to suck it up and put it down to ‘chance’ though. Be prepared for that possibility.
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