However...  The mechanic should shoulder some of the responsibility.  Was he
watching the vehicle?  Knowing the penchant for cooling system problems on
wasserboxer motors, he should have been.  Is he your regular mechanic?  Have
you spent $$$ with him?  Is it in his best interest to negotiate?

Anecdote:  When I was twisting wrenches for a living a customer brought in an
early 240Z for a tune.  Adjusting the carbs requires reving the motor
occasionally.  On a 4k rpm rev, the valves floated. Bent 6 valves.
Disassembly & inspection showed 8 of 12 valve springs severely weakened.
That's what caused the float.  Good customer.  I fixed it.  For free.  My
fault?  No, not really.  Good customer relations?  You bet.  Made a customer
for life.  Net cost to me?  Not a penny.  Made it up on subsequent work.

I suppose that I could have fought the customer, but why?  Court time & costs
would have more than outweighed the cost of the repair.  Karma said to do the
right thing.

If you are on questionable terms with the mechanic, it might be time to spend
a hundred bucks and consult an attorney.  If on good terms, negotiate.

Jim