Transmission cooler seawater plugs

[Robert McManus]

Owned my 2009 boat for approaching 2 years. It has twin Cummins QSB5.9 engines with ZF85A transmissions. I was checking the transmission fluid coolers the other day. There are four 1/8″ NPT plugs, one on the bottom and one on the top on each end of the seawater side of the tranny cooler. Inspecting the two TOP plugs they appeared to have never been removed. When removed they appeared to be in “new” condition. When I removed the seawater inlet side bottom plug it literally almost fell out leaving almost not plug piece still in the hole with the top of the plug itself being significantly eroded/corroded away- see pic. For info this cooler has no zincs. The 1/8″ plugs could be fit with a zinc but the angle of the plug installed would limit the length of the “pencil” to about 3/8″ of an inch without touching the tube sheet which seems pretty useless. I’m planning on reinstalling new bronze plugs in the 4 holes and checking them annually. Is there a need to convert them to zincs?

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[Robert McManus]

4 plugs, 2 on each end

The bottom ones on both sea inlet and outlet had the wear. The most was on the inlet.

[Tony Athens]

The factory QSB gear cooler was not designed for zincs–Some couch engineer thought he’d be clever and used the typical 3/8″ NPT zinc location for mounting the cooler. Those 1/8 NPT plugs are for draining the cooler or for possibly hooking up a water feed to a shaft seal.. Never seen 4 per cooler, only two? Are you sure?

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