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Apparently the 4.0 head has casting ports that over hang the 4.2 block. They're completely useless, and sealed by the mating surface on the 4.0, but on the 4.2 the block is shaped to follow the contour of the cylinders which allows the head to slightly overhang where the ports are. Causes antifreeze to leak out if you don't seal them. I've read that some 4.2 blocks thus step is unnecessary, but I'll do it either way, so I don't have any surprises when I pull the 4.2 head.
I gotcha. So why would you want something that can dissolve? (Sorry, not trying to argue or be a keyboard warrior, I'm just genuinely curious about this)
 

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Somewhere around 1989-1990 (?) the 258 block casting profile was modified for the introduction of the 4.0 leaving the outside of the block straighter. This is easy to see and can be felt when placing the newer head onto the block deck. If you can see the triangle ports with the head sitting in place, you WILL need to seal the overhanging ports.

Peanuts are the only cheap, reliable way to plug up the runners so that the JB Weld or Devcon doesn't dribble down towards the valve stem. Once dried you CANNOT get the little suckers back out, so dissolving peanuts are the trick.
Asking questions is how you learn! The packing peanuts are only used as a solid backer for something stronger, like epoxy, or jb weld. Once the epoxy is hardened you don't ideally want anything extra in the cooling system.
Ohhhhhhh, ok. It all makes sense now. Thanks guys 🤘🤘🤘
 
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