Help Stripped Lower Unit Thread

[Capt No Slack]

I have a 30hp yamaha two stroke

The threads (female) in the lower unit itself for the gear oil drain screw/plug is stripper. Can not tighten the drain screw just keeps turning. Threads on the drain screw itself are perfect.

How can this be fixed with out purchasing a new lower unit casting???

I have done some internet reading about various inserts such as Heli-Coils, Acme Keylocking Threaded Inserts, Keensert etc...etc....

some claim just use a Heli-Coil and that will fix

other say to never use a Heli-Coil so Im a little confused.

looking for a solution from people who have actually fixed a similar problem.

any advice will be appreciated

[zenoahphobic]

Can't see why helicoil wouldn't work. I wish all threads in aluminium are provided with them as standard.
I don't know whether you can find the thread that fits your plug.
I would not worry having the buy a new casing, that's overkill and many options are available as you say you have done some reading.
It does not matter how the thread is restored, it is the washer that does the sealing.
You also have the option of using a larger plug if you can get a tap and just cut a slightly larger hole.
Not the help you requested, just advice that it is no big deal to use various systems available . If your really worried the hole can be welded (provided you can find a tap.
I have known people to epoxy plugs in (on motorcycles and car sumps) and redo after 12 months or more at next service!

[chummy70]

Heli coil will work just fine as mine has them in the oil drain plug. Drill tap for heli coil size u need. Heli coil will not fail unless you over torque it. I believe loctite makes a thread strip repair for stripped threads. But if mine I'd use heli coil. Hope this helps . If you was close I'd fix it for ya. Easy fix.

[rodbolt17]

having Heli-coiled more than my fair share over the years.
the toughest part is you have to shorten the coil about 3-4 threads.
simple with a good pair of side cutters.

[robert graham]

having Heli-coiled more than my fair share over the years.
the toughest part is you have to shorten the coil about 3-4 threads.
simple with a good pair of side cutters.

Just curious, but if he Helicoil wasn't shortened would the extra threads hang out inside the lower unit/gear case?....guess it might rub on a gear or other internals?...

[rodbolt17]

on some,yes the coil inserts to far in to the case,thats why I shorten them.

[Capt No Slack]

Ok......If I decide to use a Heli-Coil I have a couple of question?

Do you Loctite the Heli-Coil in place for added security?

Do you remove the Tang? Is it necessary to remove the Tang? If you do remove the tang how do you do it to prevent from falling inside the lower unit?

When you Drill and Tap the stripped hole, how do you stay straight and true to the original whole. Because if you drill and tap at the wrong angle the drain screw with new washer won't match up flush and true to create a good seal. So any tips on drilling and taping in alignment with the existing striped hole.

I've read about using bearing grease on drill and tap to catch metal shavings.

Should I flush the gear case with something to make sure I get all metal shavings out. Like diesel fuel, motor oil, mineral sprits etc..... which is best what substance should I use to flush after install.

Thanks

[rodbolt17]

anytime your drilling for a thread or thread repair concentricity is key.
no offsets or angles.
I don't lock tite them but you can.

I use small needle hose and grab the tang and work it off.

nice thing is you already have a pilot hole.

[Capt No Slack]

My drain screw is metric, 8mmx1.25pitch

Auto Parts Store had a thread repair kit with tap, coils, and insertion tool.

Kit says I need to drill the hole out with a 21/64th or (8.3MM) drill bit and then use the tap.

QUESTION: since I already have a "Pilot Hole" that receives an 8mm screw could I simply skip the drilling and proceed straight to using the tap that came in the Heli-Coil kit???? also feel like if I could skip the drill maybe less metal shavings to worry about, but I don't know????

Do I need to worry about the metal shavings from the drill/tap? Will I need to flush my gear case after, if so with what substance?


I apologize for so many question, I just can't afford to screw up.

I really appreciate all the advice guys

thank you very much

[rodbolt17]

drill it.
use grease on the drill and the tap.

can you force the tap and make it work?
most likely its cast AL.
I guess you figured out they are proud o dem dar heli-coils.

[Capt No Slack]

Rodbolt17, special thanks!

I DID NOT repair my threads today! But I did practice in a piece of scrap aluminum, just to learn and build my confidence.

With the practice run I was able pacify most of my concerns. The drilling, taping, Heli-Coil shortening, insertion, and tang removal was straight forward and fairly easy! I did realize the stripped hole must be drilled the "Heli-Coil Tap" is to big for the existing stripped hole.

The only concern I have now is the metal shavings from drilling and taping entering the gear case. Yes I will use wheel bearing grease on my drill and tap.

But Im still concerned some will linger after the process! How would you flush them out!

Im poor and this lower unit has to last me for a while I would hate to ruin bearings/gear form metal floating around in my lower unit.

thanks

[zenoahphobic]

Concentricity, lovely word sought of summarises it.
Have a good look at your damaged thread to see if it is evenly stripped. If not carefully "strip" it clean. (Deliberately turn the screw so that it turns freely).
If you have a drill that is smaller than specified then use this after reaming with the screw. Then use the right drill bit. (Three stages if you like).
To reduce shavings also from not getting in do all this with the unit mounted with the hole underneath vertical. Flush this way using a syringe into the filler hole. I use turps or kero a thin liquid, not gasoline as it may hurt the seals.
Allow to drip and turn all shafts. Then flush with correct oil.
You will need to monitor leak perhaps softer thicker washer may be required to compensate for any subsequent eccentricity/ crookedness.

[Capt No Slack]

O.K now I have a couple more questions. This is what my stripped lower unit drain hole looks like!

What is the EXTRA ALUMINUM in the hole (right side of hole) towards the leading edge of the lower unit?

IS THIS NORMAL????

I am the second owner of this motor.

I assume it is just aluminum from the torpedo/bullet like leading portion of the lower unit.

or has someone damaged this in the past removed the gears/shaft/guts and welded it up to fix, if so might would explain why my screw stripped?????

Hopefully it is normal

please help

[TownsendsFJR1300]

That looks like part of the case, INSIDE, broke away.. NOT good....

You'll need at least 1/4" of material to heli-coil into.

Do you have that all the way around the opening?

If not, that missing/broke piece, HAS TO COME OUT..(it looks like a pretty big piece missing)

Once repaired with the heli-coil, the threads will be substantially stronger than the aluminum threads.

Also, re flushing. You can use regular oil but let it all drain out (taking any debris with it). If need be, filter the oil (strain thru a coffee filter), and do it again.

[fairdeal]

that's a remarkably clear and sharp photograph - but I have no clue what I'm looking at.

While I've changed LU lube many times, I never took that close a look up & into the drain hole....

Whatever that "piece of metal" is, it looks like it has already been touched by a drill or tap