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Old 27-06-2011, 07:57   #1
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Sealant for leaky Lower Unit screw?

Hey all,

The lower unit on my 2002 15hp Mercury two stroke has seen better days. It's not dead yet, but I need some help with it.

Because of a stripped screw and mangled screw hole a couple of years back, I had to glue (with Marine-tex) a small aluminum plate over the oil vent hole and tap a new screw hole in that. It worked fine for a couple of years, but when I had the water pump replaced last month, the guys at the shop apparently over-tightened it because the threads stripped out and it leaked. I solved this problem by re-tapping the threads in the aluminum plate one size larger and getting the appropriate bolt for it. I wrapped the screw threads in teflon tape, put red RTV gasket maker under the bolt head and tightened it down. Then I put more gasket maker over the top of the bolt head just in case.

This looked and felt completely leak-proof, and for about a month it worked great. Then I noticed that the hole was starting to ooze gear oil again, this time just a tiny tiny bit. I was able to tighten the bolt another half turn, and now the leak has stopped again. Maybe the vibrations had loosened it up a bit or the gasket maker shrunk when it hardened.

Obviously I need to drain, inspect and replace the oil. But how can I avoid going through this process yet again? I am hesitant to tighten the bolt too much, lest I break the threads in the aluminum plate, but if I don't tighten it enough it will leak for sure. Is the teflon tape helping or should I use something else? Is there a better sealant to use? Thread locker perhaps? If so, which one? (please be specific)

Any advice is appreciated.


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Old 20-07-2011, 08:02   #2
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Re: Sealant for leaky Lower Unit screw?

Teflon is a lubricant. Unless I am mis-reading here, that is not helping to keep the screws from loosening. Google shop for thread lock, most likely you need the blue variant (red is like epoxy). Clean the threads and apply the thread lock. If a lockk washer works here, use one.

Threadlocker, 50 Ml - 2A269 : Grainger Industrial Supply

Never used this, but I have used Loctite:
LOCTITE Quicktape 249 Threadlocker

My gripe about glue of any type, is after it is adhered, and you remove the cover (maybe to inspect an impeller), the residue acts as contamination making it tough to create a new seal.
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Old 20-07-2011, 08:11   #3
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Re: Sealant for leaky Lower Unit screw?

Essentially glue it in place with some 5200 and let it cure for a week. 5200 will work as both a lock (roughly equivalent to blue Locktite) and a sealant. It will still be removable if you ever need to. Clean all oil off of the plug and the hole with acetone.

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