Biodegradable Packing Peanuts: Miracle Cure?

Swapping a 4.0L fuel-injected Jeep head onto the AMC/Jeep 4.2L block is a pretty common upgrade for this bulletproof inline-six engine. When Douglas tore down his ’88 Jeep YJ’s mill and discovered unexpectedly that he’d been running the older, narrow-decked version of the 4.2, he was undeterred by the fact that the water passages in his 4.0 head would no longer line up. Following some instructions he found online, he first packed the water passages with biodegradable foam peanuts (made with corn starch!), then sealed them up with QuikSteel metal epoxy. The idea is that the peanuts give the epoxy something to hang onto while it cures, and then dissolve when exposed to the hot coolant. So did it work? Well, like many internet recipes, there were mixed results: the head is holding up well for the time being, but it does leak a few drops of coolant out of the epoxy seals.  Douglas is going to get a spare 4.0 head in the meantime and have the water passages properly welded up and decked, then swap it into his Jeep when it’s ready.