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Compass Marine | all galleries >> Compass Marine "How To" Articles >> Seacock Backing Plates / Alternate Method / No Through Bolts > Drill & Tap
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Drill & Tap
09-APR-2009

Drill & Tap

Please don't cut corners on the quality of your tap and always use the proper sized drill for the tap.


This is an "F" drill for a 5/16" X 18 thread tap. You can also use a 17/64" drill with a 5/16" tap. Fiberglass or epoxy sheet is susceptible to off center tapping so be careful to not cut crooked or wobble your tap side to side as it will make the tapping bigger than you want. Practice in a scrap piece of fiberglass first if your not comfortable with this.


Once tapped through use an air gun to blow the flutes of the tap out. Do this before reversing it out of the fiberglass..

Nikon D200
1/60s f/5.0 at 48.0mm iso500 full exif

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Michael 09-May-2011 21:31
If you are using a standard sea cock (Flange & Valve are one piece) You will need to get a "F" Drill bit around 6" long as the standard "jobber" bit is only about 4" and the Drill press chuck will hit the valve preventing you from drilling your holes. (This was a problem with my 1 1/4" sea cock)