I once rebuilt a 22re in the kitchen of a little house we were renting. The engine was really tight and just required a hoan, fresh rings and factory sized bearings. During the process I replaced the timing cover with a fresher one.  The old one had some pretty good grooves from the plastic guides falling apart.  Anyway, there is a little sleeve in the timing cover on one of the bolt holes that acts like an alignment dowel pin.  The original sleeve was stuck in my block and the fresher timing cover had the same sleeve stuck in it too. These two sleeves caused a little gap between the timing cover and the block near a water passage.  Long story short, I ran it down the road and as soon as the sealant blew out coolant gushed into the oil pan and wasted the rod bearings.

I then rebuilt the engine again, in the kitchen.

Tear it apart and take a good look at the timing cover. It's probably cracked like the guy said, worn through or something stupid.  Maybe there's two alignment sleeves where there should be one . Usually these types of catastrophes are something stupid.