Stainless in aluminium alloy

[alanporter]

My stainless steel davit post is set into an aluminium alloy base (? Zamak). It is held in by a 3/8 inch stainless cap screw, which I cannot move. I have tried various penetrating oils, anti-rust preparations, even Tabasco sauce, all to no avail. Can anyone offer advice on how to free this screw ?

 

[PaulS]

You might try heating with a propane torch or even a soldering iron sometimes helps. The different expansion rates of the two metals may break the screw loose.

 

[jfkal]

You may have to just drill it out /forums/images/graemlins/crazy.gif /forums/images/graemlins/crazy.gif

 

[William_H]

If it is a stainless pole in a ali base then the screw may be the least of your worries and the only way to free the pole will be to destroy the base. Not a problem if it is already damaged of course other wise reass your need to remove the screw or the pole. Otherwise try heat but don't count on it. sorry olewill

 

[eyehavit]

Alan, my suggestion is this:
Aluminium has a significantly higher co-efficient of thermal expansion than stainless. As PaulS says, heating the part makes sense is worth a shot. I suggest you first heat the part to around about 150 deg. C This is enough to affect an expansion change between the seized aluminium and stainless threads. Do not attempt to unscrew while hot, but instead apply a recognised anti seize lubricant such as WD40. Even engine oil if that's all you have. The heat will make the oil flow very freely and it will flow in-between the threads. If you can, build a little dam to provide a small head of oil.
Let the part cool to more or less room temperature and then try to remove the screw.
The heating causes a differential expansion and "cracks" the corrosion bond between the dissimilar metals. The addition of oil while hot helps to wash away broken micro-particles and as the part cools, the oil will help lubricate the threads.
If that doesn't work you can try to unscrew while the part is hot, but be prepared for stripped threads.
Good luck

 

[asj1]

I have had similiar problems with stanchions and spent many hours without success. To all intents and purposes the two parts are probably now effectively welded together. Decide if this is a real problem or not - if it is you will need to replace the whole unit by unbolting it from the deck etc - and access may be an issue.

 

[Birdseye]

The problem is that the corrosion products between the stainless and the ally are both as hard as hell and take up more volume than nthe metal which has corroded. So the pressure in between is high - in my experience you havent a hope with lubricants unless the set up was loose in the first place.

I tried acids and strong alkali but heat and brute force was the only answer, and that didnt always work.

When you replace it, coat the inside of the ally fitting with duralac. Worked like a dream on my stanchion bases.

 

[alanporter]

Thanks everyone for your helpful advice. I will try the propane torch method this weekend.