Edit: upon magnifying your image, I notice that there may be
damage to the area where the black arrow points.
the uneven wear pattern from the gasket (yellow and orange lines) indicates a
less than even seal . . . the threads could be the culprit - not the plug. your plug looks normal.
The old style plugs are normally galvanized iron or steel and are much harder than the soft aluminum drain.
I remember my father (in 1960) throwing that plug into the orange grove and replacing it
with a brass plug - because the original plug rusted.... that is when I learned to use the brass plug.
in the worse case scenario, the threads may have been stripped by cross threading.
you may need to have a plumber run his thread chaser through it. or, find a cheap 1/2" pipe tap.
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The type of vintage plug you have is very common for that style of drain.
I have two 1959 boats now that have the 1/2" NPT threaded hole.
I do not use the plug with the gasket, I use the standard 1/2" plumbing brass plug
with zero leakage. Available at any big box store for about $4 ($7 at BPS).
try this one - I know it does not have the handy finger-twist top but,
if it doesn't leak, it may be worth the wrench time.
if your plug is the 1/2" size, find a short 1/2" pipe with threaded ends and rework
the threads in your boat drain.
last option would be to load up the plug threads with teflon tape. that will only last a few trips.
if you choose to try to repair it yourself, I found a 1/2" NPT tap on the Sears website for $5.60.
$24 at Lowe's and NAPA. shop around.
Note: Pipe threads are different than your normal standard bolt threads.
Pipe threads are tapered to ensure a good leakproof seal.
so if you run the standard 1/2" bolt tap into your drain, it will not work.
last final resort would be to remove the existing drain completely and
install a flat brass threaded drain with the plug as you have now.
- or - close up the old drain and install the drain pipe with the external plug.
please let us know what you did after you fix it.
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