I remember reading about this some years ago. There was a picture of
the failed UK crank front journal bearing ID taken thru the front plug
hole. It was solid crud where the crud ID appeared to be about 1/4
inch diameter. That engine must have been filthy inside & operated in
some very abusive and corrosive conditions. It is interesting that it
had been overhauled once & was near TBO again. Who did the magnaflux
at the first overhaul?

That crud is usually a product of condensation mixing with
the oil. The condensation comes from blowby gases, water being one of
combustion's byproducts. Sludge results from the mixing, along with
acidic compounds formed from hydrogen present in both water and oil,
and sulfur, chlorine and nitrogen from the oil. The presence of
aluminum doesn't help any, either.
Since the front of the crank runs fairly cold due to the cooling effect
of the prop, condensation is a bigger problem inside the crank nose.
The acids pit the inside of the crank, creating stress risers, and it
eventually fails. Magnafluxing at the previous overhaul doesn't prevent
it.
Short flights don't let the oil get hot enough to boil off
the condensates formed when the engine is cold. Running up the engine
without flying it does more damage than leaving it alone.

Dan