Forespar Marelon Thru-Hulls — Understanding Threadforms

Understanding Threadforms (M6)

• All Marelon thru-hulls have parallel threads: "NPT" style but not tapered. No thru-hulls, of any material, have tapered threads. The parallel form is called NPSM (American Standard Straight Mechanical Pipe Thread) or NPSC (Straight Coupling Pipe Thread) – basically the same.
• Plastic injection-moulded thread forms are made to ANSI B2.1 and SAE J476.
• NPT (American Standard Pipe Taper Thread) is the standard for industrial and household pipe fittings; its end threads are deliberately imperfect to seal as the joint tightens.
• Marelon valves, elbows and tailpipes have parallel threads matching the thru-hulls. None of these is self-sealing: use pipe dope, thread-seal compound or Teflon tape on all Marelon plumbing fittings.
• A tapered male thread will screw into a Marelon female thread, but the connection may be weak and harder to seal. A male Marelon fitting (or a male metal fitting) will not work in a female tapered thread.
• Cross-threading or mismatched threads can cause leaks and failure of the connection.
• Thread angle: British BSP 55°, American NPT 60°.
• NPT tapered male tailpipes are offered in some sizes (model "HC", numbers 910146 thru 910168, page 8 of the Marelon Plumbing Systems Catalog). BSP threads are available on some fittings.

BSP – British Standard Pipe Versions

Marelon Valves on Bronze Thru-Hulls (M4)

Replacing only the valve on a sound existing thru-hull is common practice. MF 850 and MF 849 ball valves, and "93" Series integrated valves, are installed on bronze (and stainless) thru-hulls.

• Be sure the bronze thru-hull is sound (no electrolysis or corrosion damage).
• Clean the threads and remove all old sealant.
• Do not cross-thread: bronze will "cut" threads if forced onto Marelon.
• Use adequate Teflon pipe tape or plumber's dope as thread sealant.
• Do not disturb the sealant on the thru-hull by twisting it in the hull.
• Do not over-tighten the valve: hand tight is all that is needed.
• Bond or ground the bronze thru-hull properly: it is still subject to electrolysis and corrosion with a Marelon valve on it.

Forespar cannot guarantee thread compatibility with some foreign-made bronze thru-hulls.

Series 93 NPT Parallel Threads (M22)

• Marelon NPT-parallel (NPSM) threaded flanged seacocks accept bronze or stainless steel thru-hulls.
• ABYC / U.L. approved for below the waterline.
• Multiple port valves ½" thru 2"; valve locks available on 1" thru 2".
• Marelon valves do not require bonding.