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The thread is engaged properly - it fits quite snugly, that's why I can't understand the leak, when my old one is fine. I would use the old one, but it won't fit thro' the gap on the slidey thing on the new riser, and the old riser is a lot thinner or I could use the slidey thing off the old one! Confused? I am - my brain is mashed!!
By the head, I'm thinking you mean the screwy on bit that fits on my shower? If so, it is metal (on both hoses).