Broken thermostat bolt

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
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Hyperbarics
Posts: 1
Joined: Tue Aug 09, 2022 2:57 pm

Broken thermostat bolt

Postby Hyperbarics » Tue Aug 09, 2022 3:39 pm

Hey all, I'm new to this forum and joined because I'm a first-time owner of a boat- and have run into a problem I could use some experienced advice with.

I'll start with that I'm a commercial diver, and have recently been getting into fossil diving in my free time. I live in one of the most Miocene fossil-rich (yes megalodon teeth) areas of the country, and after borrowing friends' boats for a few weeks, decided to find my own dive boat. Ent up finding a great deal, and brought her home. The motor is a 1992 115hp Mercury, and fires off strong. Runs well, but haven't run it much at all due to the fact that it wasn't pissing, and had a small leak at the bottom of the thermostat. So I ordered a rebuild kit, and have been doing fiberglass work in the week it took for the parts to arrive.

I have also pulled the lower unit with an impeller kit according to the model number on the outboard, but it seems to be the wrong one. Whether it's a replaced lower, or the kit is wrong, the impeller looked brand new, so I put it back together and moved on to the thermostat. All of the top bolts came out without any problems. Getting on that lower bolt tho- she wasn't gonna come. I spent days of soaking it in penetrating oil, using the torch, hitting it with the small impact driver, and leaning on it with a rachet right up until it felt like the head would spin off... At a loss I've been working on other aspects of the boat and praying.

Eventually it came to the point where I was going to turn it. It was either going to come loose, or break, but the thermostat had to be done. I put the 1/2 inch impact on it and finally broke it loose. The threads stayed in place, and the shank portion of the bolt came out, covered in some sort of epoxy or glue. My guess is that the previous owner broke the bolt, and just glued the shank back in. And I bet it worked for a long time, judging by the amount of corrosion and mud inside the thermostat, nasty.

Easy-outs aren't going to do the trick, seems like the previous owner boogered the broken bit up trying, so even with a punch, a drill bit won't bite. I figure my two options are trying to build a weld on the bolt, with a copper tube in the hole to protect the block, and weld a nut to turn, (which if fails, now you have a hole full of stainless weld), thread the block's shank portion and use a bigger bolt, or just glue the damn thing back in and deal with it in however many years it will take to fail again.

TLDR: Someone broke a thermostat bolt and glued the damn thing back in.

Has anyone dealt with a problem like this? What is the smartest way to go about fixing this? I'm almost leaning towards gluing the bolt back in, with a clamp around the motor while it cures to ensure a tight fit. That's the most forgiving and least permanent option. What sort of epoxy/glue/product would you suggest? It's just about the last thing I need before I take it out on sea-trials, so I am wide open for suggestions.

Don SSDD
Posts: 313
Joined: Fri Oct 23, 2015 6:58 am
Location: Nova Scotia

Re: Broken thermostat bolt

Postby Don SSDD » Wed Aug 10, 2022 6:08 am

I’d keep working on drilling it out. Try a dremel to try to flatten the top surface you need to drill into so you can punch it more easily to get a drill started(I’m assuming the bolt surface where the bolt is broken off is very rough and your punch or small drill won’t bite and slides off?). Whatever glue or JB Weld type product that was used to fasten the broken bolt can’t be too hard to drill and the remaining bolt bit is stainless, you should be able to drill it out enough to use an ez out?
1986 Outrage 18 with 2001 Honda 130 HP
Former Owner 1991 Guardian 19 with 1994 Evinrude V4 140HP
Former owner 1987 Montauk with 1998 Mercury 90HP
Nova Scotia