The answer is something between a "ping!" and a "plonk!"
OK, so technically this isn't a "Casa" project, except that it's the first time I've done actual automotive maintenance in the new garage. It's working pretty well so far.
The story to date is that two weeks ago I went down to the post office to check my mail. (There wasn't any, except for a Smeghead [pardon, Egghead] Software flyer.) When I started to back out, there was a ping/plonk and the truck stopped running. Sigh. So I went back in and called a towtruck -- $73 to get the Dodge home: the most expensive junk mail I've ever received!
My theory is that the harmonic balancer had been busted for awhile: the break wasn't fresh -- it had old oil stains in it. In fact, what I was told was a bottom-end rod knock may have been the balancer flopping around like a stunned mackerel. I think that the balancer finally sheared the locating key that it shares with the crank sprocket, and the broken piece flew into the timing chain, causing much grief.
Incidentally, the former owner tells me that he had the timing chain replaced about 40,000 miles ago. It's very possible that the mechanic who did the work cracked the balancer either by overtorquing it or by pounding on it. It wasn't a class job by any means: the cam sprocket was of the criminally stupid nylon-toothed type so popular with the Big Three from the mid-70s to mid-80s. In any event, the timing set was of the light-duty single-row type, not suitable for service in a 4WD truck.
The new heavy-duty, two-row, all-steel timing set in place. In this shot, all that remains it to reinstall the timing cover, replacement harmonic balancer, and various belt-driven accessories.
I did drop the oil pan before attempting to start it, as there were still several gear teeth and a piece of harmonic balancer to account for... . I never did find the missing chunk of balancer: I suspect that it was broken by the mechanic that did the last timing job and reinstalled anyhow.