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topic 604

Joining threaded aluminum parts without galling


(1996)

Q. Can anyone tell me if there is a rule of thumb dealing with the proper procedure for connecting two threaded aluminum parts without the danger of having them seize?

Joseph Anderson


A. Huh! Good luck.

I have spent hours carefully hacksawing aluminum hose fittings off of faucets, trying not to damage the threads on the faucets.

I have disconnected the electricity, then struggled to remove broken light bulbs with aluminum bases from their sockets with pliers.

I once bought a brand new garage door that had some aluminum parts in the lock mechanism, and a warning to use only brass keys, not aluminum ones. My wife didn't read that caution, had a duplicate aluminum key made, and stuck it in before the door was 2 hours old. We're still looking for the King Arthur who can pull this Excalibur from the lock.

Just a private prejudice, but I feel that there is no lube or assembly technique that makes an aluminum-to-aluminum screwed joint acceptable. But maybe I'm wrong.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1996)

A. Try anodize and dry film lubricant for aluminum. Comes in a spray can and is not baked. More can be added as needed if it is regularly used. If it is all-thread, use a stainless fitting with a grease. If it is in a very corrosive atmosphere, find another metal.

James Watts
- Navarre, Florida


A. A reader of Metal Finishing called to recommend DuPont "Krytox [linked by editor to product info at Amazon]" LVP fluorinated grease. He warned that it's over $35 for a one-ounce tube though.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


September 11, 2010

Q. Stumbled onto this great site while googling "steel to aluminum corrosion". I'm an American professor in Hong Kong (not an engineer or metallurgist, but fairly handy mechanically for a literature guy). In the process of disassembling an Italian (Tecnogas) 4-burner cooktop for conversion from natural gas to LP, mainly by changing the orifices, I discovered that one of the burners has what appears to be a cast-steel lock collar threaded onto a cast aluminum neck. Possibly the neck is an alloy of some sort. The collar, around 1 3/4" OD x 3/16" height, has 8 small radial ribs, evidently for tightening or loosening by impact.

Without removing this collar, I can't separate the sheet metal halves of the range to access the individual burners with their orifices. Naturally I first soaked the threads with WD-40, not my favorite break-free product, but all I have here. After a suitable interval, I then tried tapping the collar loose with a hammer and appropriately-sized nailset. No go. Repeated soaking and increasingly heavier whacking in both directions produced no movement. I tried heating the lock ring with a butane torch to break whatever oxidation might be present between the threads, whacking while hot, then later with more WD-40. Still wouldn't budge. If I had a Dremel I'd cut the ring and worry about replacing it with something similar later, but I don't. Now I wonder what reasonably available chemical I might use, if any, to break the corrosive bond without damaging the aluminum alloy any further. Thanks for your patience! --George

George O'Connell
- Hong Kong



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