Cracked weld in the header that opens up as it gets warmer?

I water pressure tested the header and it seemed to be ok. Could a cracked weld be closed that tight when cold to hold pressure of a garden hose on full blast?

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Heat will expand the crack. I would fix the water issue before I would even start worrying about the front cylinders firing. You can cause some sever damage and front cylinders not firing will be the least of your worries. The water is probably the root of your problems.

Also the garden hose won't keep up with the volume of the sea pump.

I think the water is putting out the spark plugs close to the crack. I guess I'll swap the header from the good motor and if that doesen't fix it I'll bet it's a cracked head.

That would suck. I just don't want to see you fill up a cylinder and then bend a connecting rod.

Good luck