Severe galling. Stainless in Aluminum

[jeanathon]

I am looking for tips to remove these two allen headed stainless screws while leaving as much cast aluminum in place.
This is a Selden in the mast electric furler, and the electric motor is bad. There were 4 bolts holding the mechanism on the mast. I had to drill two of them, and was able to remove two with an impact drill. Unfortunately a lot of aluminum threads stayed on the screws themselves. This is not too much of a problem since there is plenty of aluminum to thread and replace with a helicoil, but hopefully these pictures show that is not the case when it comes to removing the electric motor.
I have some time as the motor had to be sipped from overseas, and so far I am squirting penetrating oil daily on them. However I won't know if the oil is actually penetrating till it is put up or shut up time. I've considered cutting into the screw shoulder so the penetrating oil has more direct access to the aluminum threads, but that obviously weakens the allen head.
Thoughts? Tips and/or tricks?

[Dennisb2]

Asumming that the motor is throw away. I would use a dremmel tool with a grinding disk to cut off the mounting tabs to open space to work on screws. I would then take it to machine shop or a friend with Bridgeport type of milling machine to bore out the screws and tap for A threaded insert.

[Shrew]

I'd keep hitting it with some penetrating oil.

acetone/antifreeze (50/50)
PBBlaster
etc

I'd whack the top with a punch and a hammer to shock it.

Use heat

I'd try tightening before I loosen.

Consider a bolt extractor, you should be able to use the existing head.

Just a few thoughts.

[skipperpete]

“Vyce grips” ….accept no substitutes…..as tight as you can get em to grip on the head of that unbrako bolt. The worst that can happen is you twist the head off the bolt….. then you get to drill out the remaining bit.

[PamplomBlue]

Soak the thing in deisel for a week.
That will loosen things up

[Wotname]

Originally Posted by Dennisb2

Asumming that the motor is throw away. I would use a dremmel tool with a grinding disk to cut off the mounting tabs to open space to work on screws. I would then take it to machine shop or a friend with Bridgeport type of milling machine to bore out the screws and tap for A threaded insert.

This technique or something very similar is the only answer that will really work here. The SS screws will have to be mechanically removed by some sorting of cutting action.

The SS screws are not galled (as in the traditional sense), rather the aluminium has corroded in a manner that forms an impervious (and expanded) salt locking the SS threads into the aluminium. I have forgotten the exact chemical process of the corrosion. No amount of penetrating fluid will help and using heat is inherently futile as the aluminium goes from solid to liquid in a heartbeat.

IMO, it is best to bite bullet early and go to the machine shop before doing further damage.

[SailingHarmonie]

Just to be clear, this is NOT "galling". Stainless threads in aluminum do not "gall". It really DOES matter what is happening here, because the approach to repair is very different.

"Galling" usually happens on a boat when stainless threads run on stainless threads under very heavy load. The chromium oxide layer is rubbed off, and the pure metal is directly in contact on the male and female threads under very high pressure, and with friction generating high local temperatures. The two pieces of metal "cold weld" together. They essentially become one piece. This happens only during assembly, and not over time. There is NO fixing this. No amount of magic mystery oil or any other technique will un-weld the parts. They need to be cut, or drilled to separate. This can be easily prevented by using a good thread lubricant.

The locking together of stainless and aluminum is a completely different thing and is due to galvanic corrosion of the aluminum that is in contact with the stainless steel. This occurs over time, and in contact with slat water. The resulting aluminum oxide is very hard, and occupies more space than the original metal, making removal extremely difficult.

I have never found the application of any oils to be very helpful in this case. If possible, repeated heating (VERY HOT) and cooling of the parts can usually break free the grip. Almost always, if you can unscrew the stainless fastener, the aluminum threads will have corroded to the point that they are no longer suitable for load bearing fasteners, so a heilcoil is going to be needed no matter what.

When you reassemble, do use TefGel or similar product. When installed this way, I have always been able to easily remove stainless fasteners in aluminum with hand tools after 15 years of salt water exposure.

[Tin Tin]

Try Loctite LB 8040 freeze and release. I was surprised that it worked so well for me. It needs quite a bit of spray, more than you think to get it working. How well it works in SS/AL I don't know. USe this stuff and a rattle gun for the bolt head.

[GordMay]

Originally Posted by Wotname

This technique or something very similar is the only answer that will really work here. The SS screws will have to be mechanically removed by some sorting of cutting action.

The SS screws are not galled (as in the traditional sense), rather the aluminium has corroded in a manner that forms an impervious (and expanded) salt locking the SS threads into the aluminium. I have forgotten the exact chemical process of the corrosion. No amount of penetrating fluid will help and using heat is inherently futile as the aluminium goes from solid to liquid in a heartbeat.

IMO, it is best to bite bullet early and go to the machine shop before doing further damage.

You’re correct, it’s not likely ‘Galling’. ”Adhesive wear” [AKA: cold welding, seizing, galling, or fretting] occurs from the process of fastening the bolt. Here the friction, and pressure, put on the bolt, when torquing, causes the threads to seize, or “cold weld”.

It almost certainly ‘Galvanic Corrosion’ [AKA bimetallic corrosion, or dissimilar metal corrosion - but NOT 'electrolysis’] occurs when two dissimilar [far apart on the galvanic series] metals are brought in contact, with a conductive electrolyte solution [or moisture], most commonly, salt water. In these conditions, these two metals will create an electric current, which will accelerate corrosion.

I’d try using a reverse twist [left handed] drill bit, just smaller than the bolt diameter, and counter-clockwise [reverse] drilling; and most of the time, the S/S bolt will come out, while drilling [like with a tapered screw extractor].

[jeanathon]

Thanks all. Especially for letting me know I've been using galling incorrectly all this time!

[jeanathon]

Originally Posted by Dennisb2

Asumming that the motor is throw away. I would use a dremmel tool with a grinding disk to cut off the mounting tabs to open space to work on screws. I would then take it to machine shop or a friend with Bridgeport type of milling machine to bore out the screws and tap for A threaded insert.

Apparently these motors are as rare as rocking horse poo. This one will be kept for possible rebuilding in the event the one replacement motor in existence, isn't.

[wholybee]

I have has good luck with the hammer type impact tools. Not an electric impact driver, but one you hit with a hammer. Same idea as was suggested above to hit it with a hammer an punch (which can also work) except that exactly as the shockwave from hitting it is going through the bolt, a turning force is applied. That often seems to be able to break the salt/corrosion bond between the metals.

But, sometimes all that does is break/twist off the bolt head. So be careful.

[Don C L]

What Gord recommends is also called an "Easy Out;" a reverse thread screw that would go into the head of the screw there after you drill it out a bit. That and a good soaking of weasel piss (1/2 acetone and 1/2 automatic transmission fluid) and heating and freezing also would be what I'd try. But I would not be optimistic I'm afraid. On a scale of 1 to 10, how badly do you need to put that (rebuilt) motor back in there as it was? Since that is off the boat, Wotname's suggestion of taking it to a machine shop is probably the best of all.

[Bowdrie]

A friend had to replace the mounting saddle of his vang, it was curved-to-fit the mast and fastened by eight 1/4-20 hex head machine screws.
Four of the screws came out, the other four broke their heads off.
Thus, with the mount removed there were four "studs" of machine screws sticking out of the mast with little chance of success of removal.
We went to plan "B".
Took a 12V battery and jumper cables, one cable went on the mast step, the other was touched to the "stud".
When the stud started to "glow" the current was stopped and vice-grips were applied, the broken machine screws then came out with little force needed.
Caution: should you try this, completely enclose the battery in a soaking wet beach towel/blanket, a procedure as this has caused battery explosions on occasion.

[thinwater]

Originally Posted by SailingHarmonie

Just to be clear, this is NOT "galling"....This can be easily prevented by using a good thread lubricant.


And by relieving pressure, if possible (don't use the nut to draw it up), and turning very slowly, with cooling breaks.


The locking together of stainless and aluminum is a completely different thing and is due to galvanic corrosion of the aluminum that is in contact with the stainless steel....

The other thing I have found very helpful, if you have more time and if the threads do not have grease or penetrating oil in them, is a weak acid, such as lactic acid (CLR). A long soak will dissolve the corrosion products. I think it actually works better in many cases, but it takes a day or so to penetrate. If it does not work, let it dry and use penetrating oil. But if you have used oil this won't work. It has to be first.