[alfa] SS galling on exhaust systems

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi Jay,

Indeed some grades of SS gall badly, especially when high temperatures are present.  The
sort of SS used for fasteners sold in hardware stores is the "el cheapo" grade and not
really suitable for this application.  What I normally do when I have to use any SS
fasteners, and I use a lot for antenna mounting hardware, is to run a tap through the
nuts and a die along the threads to make the fit looser.

Most high volume fasteners are made using a thread rolling die which is not the best
option. SS is a material that work hardens, like copper, and the tiny surface
imperfections when viewed under a microscope show these to be like the teeth on
a file.  When you come to undo the fastener the teeth lock into each other and scrape
tiny particles of material off that then lock the nut/bolt together.  When this happens
the best option is to work the nut backwards and forwards a few degrees to abrade
the swarf away, flooding the joint with some sort of lubricant to wash the tiny metal
fragments out.

If you just try and undo them the material builds up locking the combination solid and
then the bolt normally twist off.  Another dodge is to apply some Copper-Slip paste
on the threads when assembling, this slows down the corrosion that starts the binding
process, and SS does corrode, it is only Stainless and not Rustless!

Another option, which I would use with exhaust applications, is to use brass nuts instead
of SS ones.  These never sieze as the two metals are different.  They may get a little
tight when you come to undo them but it is always possible to get them apart.  I normally
make the nuts from hex bar, again with a sloppy thread with a tap run through them several
times, and I make the nuts extra length so a socket or tube spanner can be used to get
extra purchase rather than a spanner.  When you torque them up they squeak as the nut
is tightened, this provides a little extra lubrication as  brass is an alloy of copper and
tin.  Tin is a fairly good dry lubricant, in the old days exhaust manifold nuts were
normally brass on mild steel studs.  These would always come apart, even after 20
years of heat.

I hope this helps.

John
Durban
South Africa
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