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Tapping a Hole - Numpty Question


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I am intending to fit a breather elbow into the side of my X/F cast alloy rocker box.   The brass elbow thread is 3/8"  and the tap thread is the same width.    Question: what size of hole should I drill in the casting?  I assume given the thinness of casting wall, 5mm, I should adopt aircraft practice and start with a small hole, increasing size in stages?


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Its a BSPT thread, hope that answer the question.  I intend tapping the casting because of the proximity of the rockers and push rods.  Also I am concerned that there is a danger of any nut in there vibrating loose

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 Many thanks all.

Elie  I am proposing to drill at the front end of the cover, directly opposite the 2nd rocker bearing block, this seems to offer most room  and easiest routing for the breather  under the heater pipe

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