Why do the pre-chambers have to be new?
I think they would be real cheap at the pick your onw part type Junk Yard as the average person does not hav the tool to pull them (I have never seen any removed from engines at the Junk Yard so I assume the seldom sell any.)

I could see using new ones I was replacing all of them.

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84 300D, 82 Volvo 244Gl Diesel