Pitless adapter does this style just press against pipe ID?

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[Bsweb88]

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Inside of 4” casing I expected to have something inside of pvc to make a positive connection with rubber boot. Does this style just clamp to exterior of casing and rubber form a seal with tightening of wedge?

 

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Of course the screw just fell out. Standard size appears too loose, almost like it’s a 2.0 mm thread, would they have used metric?