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Discussion Starter · #1 ·
Anybody with a 4.2 that's done the 4.0 head swap plug their antifreeze ports with packing peanuts? Ordered some on Amazon, they do dissolve in boiling water but left a little film, wondering if that's normal/something to worry about. Don't want to go through all this trouble to have them ruin my water pump or something. Thanks guys, this forum has already been so much help. You guys are all fantastic.
 

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These are the ones I used. They will dissolve in cold antifreeze after a while, but I would do plain water while running the motor in (30 minutes) and then drain and refill.
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In a glass jar, yes. I doubt that a film will remain inside a hot block with anti-freeze though. I haven't found any residue at all when I changed the thermostat out after a few weeks of running the first time.
 

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Discussion Starter · #7 ·
Can someone explain why this is a thing? I've done head swaps on these engines before and never even heard of this.
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.
 

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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.
 

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Discussion Starter · #10 ·
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)
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.
 

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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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