Repair Threads on Lower Unit Oil Drain Hole

[wayne123]

I have an 1975 85HP Evinrude and the threads on the lover unit oil drain are stripped. I thought the way to fix it would be to use a Heli-coil. I called three repair shops and they say sometimes a Heli-coil works but there is not much material to work with and often it does not.

Has anybody ever used a Heli-coil to repair one? Any other ideas?

[Macgyver55]

In a situation like this if possible, tapping it out to a larger size may be a better option.

[wayne123]

I have been debating about that vesus using the heli-coil. Using a fine thread tap might help too so I have a better chance at getting more good threads.

The Heli-coil will allow me to use a standard drain plug and the threads will be stonger if I have enough material to install it proplerly.

[Sonicrunch]

Yeah, that material is so thin down there.

I would be tempted to have some aluminum welded in to make it thicker. Then drill and helicoil it.

Maybe JB weld could get you by for the short term? Just to give you some extra thickness to work with.

[wayne123]

I am thinking I will keep the welding in reserve. I figure worst case I could have them weld an alumimun nut on the outside of the housing. It would be a little crude but it would still get me on the water.

[Surface Tension]

If it were mine I'd tap it and use an o-ring instead of the plastic washer.

If you use a helicoil there is a good chance it will come out when you remove the screw.

[Wish-I-Were-Fishn]

Fastest repair, tap to larger size. Best repair, weld a nut on to thicken the area. Heli-coil won't work and the last thing you want is an issue with your repair failing because it will destroy your lower unit.

[Cliff Wagenbach]

I believe that Heli-coils are made form stainless steel. Stainless and aluminum are not compatible.

Re-tapping to a larger size with a rubber o-ring is probably your best and quickest option.

Cliff

[RumRiverRat]

I believe that Heli-coils are made form stainless steel. Stainless and aluminum are not compatible.

I disagree.

I am a Machinist and we Specialize in Aluminum Alloy (6061-O,6061-T4,6061-T6,6061-T651,7075) parts for the Aerospace industry.

We are a key supplier for the Air,Fuel and Hydraulic Systems for Airbus. We install Helicoils in 65% to 75% of threaded holes in these parts.

[Cliff Wagenbach]

I disagree.

I am a Machinist and we Specialize in Aluminum Alloy (6061-O,6061-T4,6061-T6,6061-T651,7075) parts for the Aerospace industry.

We are a key supplier for the Air,Fuel and Hydraulic Systems for Airbus. We install Helicoils in 65% to 75% of threaded holes in these parts.

[wayne123]

I know Heli-coils were originally made for air planes to strengthen the threads not for repairing the threads. I am not sure if Heli-coils are made of stainless steel or if stainless steel and aluminum get along.

I ended up buying both an oversize tap and the heli-coil kit. When i get to the cabin and can look at the motor I will decide which route to take.

Thanks for all the help.

[delcecchi]

People use stainless steel screws many places in aluminum boats. I bet the screws holding everyone's transducers on are stainless as are the brackets.

[hydro]

Stainless steel and aluminum do have issues with galvanic corrosion, but that is addressed by the sacrificial zinc anode on the motor. There are lots of stainless parts in contact with aluminum on the typical outboard.

[wayne123]

I ended up using the larger tap (7/16-20). It uses the same tap drill size as the 3/8-16 heli-coil. My theory was I had a better chance of getting more full threads with 20 threads per inch than 16 and I did not have to worry about the helicoil seating properly.

I did have trouble when drilling - the bit grabbed quickly and my hole is slightly off centered. I did end up with nice threads.

Next time up to the cabin I need to bring a tool to enlarge the flat spot for the plug to sit. I also need a different fitting with the 7/16-20 threads for my pump to fill it with oil. You really need to plan ahead and bring the right tools to the cabin with you or projects take multiple weekends.

[wayne123]

It ended up I could not get a satisfactory plug for the new threads because of the curvature of the lower unit. I tapped it one more time with 1/4 NPT and put a pipe plug with an internal hex head. It is working well but I am keeping my eye out for a deal on a used lower unit. I really don't like the interference threads in the aluminum housing.