Now that fall is here, i'm winterizing a 6HP Tohatsu. The zinc totally wore away and even left the pocket with moon craters of corroded aluminum. It's not smooth anymore and useless as a zinc cavity. Questions and witty comments requested from anybody on a repair. I don't want to take it to the dealer if I can do the same thing or more creative.

1. Mill the cavity flat and put a SS helicoil in the destroyed M6 threads? I think the hole is too fragile for tapping it out to the next larger thread . 

2. Mill a flat spot elsewhere on the lower end and putting a helicoil thread there. I looked inside the lower end casting and can find a safe spot for a second zinc spot. This seems easy and I would epoxy the original cavity full.

Thanks in advance for anything.

 

Maybe helicoil makes another product as well, but my understanding of them is that you drill out the ruined threads, tap the larger hole with the appropriate tap and then screw in the helicoil so the inside of the helicoil matches the same diameter & thread pitch as the original bolt.

Unless I'm mistaken, if there's not enough meat to tap out to the next larger size thread, there won't be enough meat to install a helicoil either.

I'm hoping that I can get into some good aluminum with a larger thread or helicoil. Even if I go with Plan B. might be smart to screw into stainless threads over the next several years.

How old is this OB? Tohatsu has a 5 yr warr per the website...

They wouldn't cover something like this. A corroded zinc that damaged the housing is a zinc that was not replaced in time. Good thought, I will run it over to the motor shop to hear what they say before I start drilling and praying.

was this left in the water? 

Helicoil does make two stage kits where you can put one inside another. That was quite few years ago so would have to check if they still do.

 

They do it is called Twinsert.

Thread inserts for metals HELICOIL® | Böllhoff (boellhoff.com)

The solution turned out to be very easy. I went to a Tohatsu dealer and he said to put a pair of zincs on the skeg. They will make perfect contact after sanding off the paint and getting to area flat. They do this for hot marinas with excess stray voltage in the water. Plus, I can thru bolt to clamp both on and no worry about damaged threads. I will seal up & epoxy the original cavity.