Stripped Lower Unit Drain Plug

[Fishinmagician78]

I have a 115 mercury 2-stroke outboard that has a stripped vent hole in the lower unit so the plug won't tighten. A marina told me they wouldn't touch it because it wouldn't be factory spec and if the repair didn't work they could be on the hook for a new lower unit. Boat is located in Corning so looking for somewhere reasonably close to do the repair. This is an older motor so buying a new case is out. These are the three options I think would work.

Option 1: have the hole welded and re-tapped for the plug

Option 2: tap to a bigger plug if possible

Option 3: have the plug welded in and forget about it. (This outboard has two holes on top, a vent and a level check) the stripped one is the vent and only slightly higher than the level check plug so would it really matter if I sealed it?

If any of those are feasible I would love to know what the best option is. I also have no idea where to get the repair done. Marina? Machine shop?

[Fish Hunter]

I'd be leaning in direction of a Heli-Coil repair kit.

Available thru most NAPA stores, MSC Industrial Supply, or Grainger.

[Duckman1]

Fish hunter said it right. If you know the thread in there now you might be able to tap it for the next size English or metric size. Any quality machinist you know should be able to help you. With a helicoil you are going to generate a lot more chips. When drilling out use a good vacuum to make sure chips are out. If cost is no object pull lower unit gut inside out then helicoil.

 

Good luck John

[W.W.IV.]

I'd go with Fish Hunter and the Heli-Coil first, it will come with the right size tap. You might have to drill out the hole so you don't have to force the tap and maybe brake it. there's a tang on the inside of the coil used to screw it in and then you brake it off. You don't want the tang floting around inside the houseing. You will most likely have to pull the lower unit off or if the tang falls into the houseing you might be able to flush it out by draining the oil after the coil is installed. Good luck.

I have a 115 mercury 2-stroke outboard that has a stripped vent hole in the lower unit so the plug won't tighten. A marina told me they wouldn't touch it because it wouldn't be factory spec and if the repair didn't work they could be on the hook for a new lower unit. Boat is located in Corning so looking for somewhere reasonably close to do the repair. This is an older motor so buying a new case is out. These are the three options I think would work.

Option 1: have the hole welded and re-tapped for the plug
Option 2: tap to a bigger plug if possible
Option 3: have the plug welded in and forget about it. (This outboard has two holes on top, a vent and a level check) the stripped one is the vent and only slightly higher than the level check plug so would it really matter if I sealed it?

If any of those are feasible I would love to know what the best option is. I also have no idea where to get the repair done. Marina? Machine shop?

[Fishinmagician78]

Thanks for the info guys. I've read that keenserts are slightly better than helicoils. Any thoughts? Also, is it possible to thread them deep enough that the plug and gasket still fit flush against the sealing surface? Will I have to flush the lower unit to get shavings out? Splitting the case probably isn't an option. Since the threads in the lower unit go much deeper than the length of the plug I only lost the last two or three threads. Could I just thread in a keen sert using the threads that are still there and then put the plug in? Would that be strong enough to hold?

Also, I don't know any machinists so if anybody knows any in the Corning area that would also be appreciated.

[W.W.IV.]

Thanks for the info guys. I've read that keenserts are slightly better than helicoils. Any thoughts? Also, is it possible to thread them deep enough that the plug and gasket still fit flush against the sealing surface? Will I have to flush the lower unit to get shavings out? Splitting the case probably isn't an option. Since the threads in the lower unit go much deeper than the length of the plug I only lost the last two or three threads. Could I just thread in a keen sert using the threads that are still there and then put the plug in? Would that be strong enough to hold?

Also, I don't know any machinists so if anybody knows any in the Corning area that would also be appreciated.

I don't know anything about keenserts but the coil have to be drilled a taped. If you could screw a keensert into the existing thread you will end up with a smaller diamiter opening. I think you meant to say the first to threads are gone. You can buy different lenght coils, you will still be able to seat the plug as intended. You don't have to be a tool and die man to do the job. make sure you drill straight and the tap will follow the hole. The old shop vac should suck out the chips.  You still should flush out the oil and start out fresh.

Edited January 24, 2015 by W.W.IV.

[baitrigger]

The guys are spot on. If you need help any good auto mechanic could knock this job out.

[Fishinmagician78]

Thanks guys!

Anyone know the thread dimensions of a mercury drain plug? I'm thinking 3/8-16 but don't want to get the wrong one.

[Iceman]

Just put some red Loctite on it & forget it, since you say it isn't needed.

[Fish Hunter]

I'd use a little grease on the Heli-Coil drill and tap to coagulate the chips,

The repair should be very long lasting.