Exhaust Manifold water elbow broken off in the manifold

I have a 1996 Mercruiser 454, the elbow that the waterline attaches to on the exhaust manifold is plastic. When I was trying to replace them with metal elbows, they broke off leaving the threaded part of the fitting still threaded into the manifold. They have apparently become brittle over time and heat. My problem is how to remove them. I have tried various extraction tools, WD 40, cutting them and trying to take them out in pieces, carefully. I have a tap to clean up the threads once I get them out. Was wondering if there are any other successful ways to remove them, or solvents to help lubricate or dissolve the build up possibly on the threads from pipe dope or the saltwater.

Why have you started a new thread on the very same subject? Are you a bot?

No I am not a bot. All the responses I was getting were about the drain end of the plug, not where it actually attaches to the manifold which is where it broke off. So I was trying to be more specific.

Well, you did say it was a drain plug, so...

The plastic elbow is soft. The cast iron manifold is hard. Use heat, scrapers, taps, or whatever means that gives you the most destructive pleasure. It's best if you remove the manifold and get it on the bench where you can see what you're doing. You are free to replace the plastic fitting with a bronze or stainless one. The blue drain plug is for convenience and not a critical part of the design. Just be aware when it comes time to drain the engine

o2batsea said:

The plastic elbow is soft. The cast iron manifold is hard. Use heat, scrapers, taps, or whatever means that gives you the most destructive pleasure. It's best if you remove the manifold and get it on the bench where you can see what you're doing. You are free to replace the plastic fitting with a bronze or stainless one. The blue drain plug is for convenience and not a critical part of the design. Just be aware when it comes time to drain the engine

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I agree,
Remove manifold, hit broken plastic with propane/map gas tourch and get as much out as possible.
If you retap threads load tap with good heavy grease as this will capture much of the debris the tap cuts away.

When done blow out real well with compressed air from top down and fix as desired.

If plastic/manifold threads are standard, try to replace with brass/bronze/ stainless steel.

good luck

If the plastic elbow is that stuck.... what shape is the iron in? May be time for new exhaust.