Will I snap my derailleur? (too large front chainwheel) I have an old racer (1994 Specialized Epic Allez - lugged carbon) that's in pretty good shape. I brought it to an LBS that's got a pretty good reputation and that I've used before, to change the chain and look at the front crankset as it was making a weird noise.

When I come to pick it up they have changed the whole crankset, chainwheels and all. I used to have a 34-50 (or so - I can't remember the details), and now the larger wheel is definitely bigger (52?). In theory I don't mind, as I was missing a higher gear for the flats.

But there's one problem... My chain is not long enough to use the largest chainring in front while on the largest cog at the back. I know that it's not 'the done thing' to use diagonal gears like that, but I did get into that combination by mistake yesterday, and was rather shocked when everything just got stuck.

My question is the following:

Is this a dangerous combination? Will I at some time by mistake get into the 'wrong' gear and by stamping on the pedals ruin the derailleur?

Or should I simply learn never to use that combination of gears? And instead be happy that I now have an even higher gear for those nice long slightly downhill stretches?
As suggested in the comment above, I would take it back and ask them to add a few more links. It's totally negligent of your LBS to fit a chain that cannot accommodate every combination of gears, however silly. There's always a chance you can fumble a shift and end up locking the drivetrain, as you've already experienced. This is definitely dangerous - if the pedals suddenly lock up and surprise you, you could crash. Another possibility is that if you keep pedalling, the rear mech will be pulled into the spokes, trashing both mech and wheel.

There is also an outside chance that with the new, larger chainring, you have exceeded the take-up capacity of the derailleur - meaning that adding links will result in a slightly slack chain in certain other combinations of gears. This is less of a problem, but bear in mind your chain may jump off if you run the small ring with a small sprocket on a bumpy road.