[V8] [A8] steel bolt in aluminum

[Mark Rosenkrantz]

Yes, steel bolts are used for their shear strength.  Aluminum is far too
ductile in some applications.  Ductility can cause joint failure.

You need to install a barrier coating on the threads.  If this step isn't
done, galvanic corrosion will quickly take hold and ruin parts.  Barrier
coatings such as anhydrous lanolin (which is dirt cheap), lanacote (trade
name), or tef-gel (also a trade name, good stuff).  To use Locktite, the
entire mating surface needs to be coated.

And Jack... Hello!  I've not seen you in a while!

Mark Rosenkrantz

On Sun, Dec 27, 2009 at 6:30 PM, Kent McLean <kentmclean at comcast.net> wrote:

> Rich, Jack wrote:
> > I'm replacing my front axles, and the pinch bolt holding in the upper
> > control arms was a bear to get out on the passenger side.  The drivers
> > side is worse -- the head sheared off when I tried to spin it, and it
> > cannot be hammered out at all.  I'm going to have to drill it out.
> > But I can't help but notice that the bolt is magnetic, i.e. some sort
> > of steel, while the wheel bearing housing both looks like aluminum,
> > and is non-magnetic.
> >
> > Do I have the correct bolt installed?  Is it supposed to be steel?
>
> Jack,
>
> I haven't seen much activity on the A8 list (this being the first posting
> I've
> seen in a while), so I'm taking the liberty of forwarding it to the V8 list
> (as
> I think they will have a vested interest as they move from their A8s) and
> the
> main quattro list (because there are a lot of knowledgeable people there).
>
> A quick glance at the Family Album doesn't help, as it doesn't seem to give
> the
> materials of the items. For example, if I'm reading it right, Model A8Q,
> Year
> 1998 (you didn't say), MG 4, SG 07, Illustration 407-10, shows bolt 27 as
> Part
> Number "N 101 312 03", "screw, hex. hd.", M10x105x32.
>
> So, is it plausible to have a steel bolt in an aluminum casting? My guess
> would
> be yes, as an aluminum bolt would have to be real thick. But that's just a
> guess.
>
> --
> Kent McLean
> 1990 V8 w/5-speed and other mods
> 1991 200 TQA #3, with mods
> 1999 A4 Avant, V6 Tiptronic
> gone: '91 200 TQA x2, '94 100 S Avant, '89 200 TQ "Bad Puppy"
>
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