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3 To 4 in Bath Fan Duct Step Reducer
This step reducer lets an older bath fan with a 3 in round outlet (common in pre-2000s and mobile-home bathrooms) connect to the standard 4 in flex duct and backdraft fittings. The 3 in end is a socket with four clamp fingers. Its bore tapers from 79 mm at the mouth to 75 mm, so it fits a range of collar sizes, and a hose clamp tightens it. A 2 mm stop lip seats the collar. The 4 in end is a 100.5 mm OD spigot with a 2 mm bead to hold the flex duct under its clamp. Overall size is 102.5 mm across by 95 mm tall, with 2–4 mm walls. Print it in PETG.
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