What engine? The 3.5 never cracks, full stop. 3.9 can crack, but normally
just develops loose liners. 4.0 and 4.6 crack the block behind the liners,
allowing the liners to become loose and the head gasket seal to fail
allowing pressurisation and coolant loss/overheating.
There is NO tie-in to mileage, it either happens or it doesn't. Later 4.0
and 4.6 blocks are graded for core thickness around the liners, hence why
more late 4.0's fail than 4.6's, but the early ones (pre-1999?) are a
lottery. Failure is normally brought on by overheating (P38 top-hose
failures on the early models) or by suddenly working the engine very hard
and generating a lot of internal heat/stress - such as taking a vehicle
that's only ever pottered about locally and going 400 miles at high speed
with a heavy trailer behind it. (Don't ask how I know that one...)
There is NO PERMANENT CURE other than to fit either a new short block and
pray, or a replacement block fitted with top-hat liners (by a reputable
firm, email me off-group as I don't want to be in court for slander!!!!)
which is a total, permanent fix.

Badger
B.H.Engineering
Rover V8 engine specialists
www.bhengineering.co.uk
www.roverv8engines.com