Re: [alfa] Studs in aluminium

[bearbvd@xxxxxxxxxxxxxx (Greg Hermann)]

At 9:04 AM 11/6/04, John Fielding wrote:
>Hi All,
>
>Studs designed to screw into ali are normally a coarser pitch than normal.  If
>you look at the manifold studs the end that takes the nut is metric fine, the
>end that fits into the head is metric coarse.

Course/fine thread studs are good design practice in cast iron as well as in Al.
In fact,  they are good practice even if the anchoring material is the SAME
as the nut (normally steel). Reason for this is that a fine thread will
develop more clamping force at a given torque level that a course thread
will -meaning that the nut will turn on the stud before the stud will turn
in its thread, even if the anchored end of the stud is not 'loctited' or an
interference fit thread, or both.

Ali because it has a lower
>tensile strength than steel requires more thread area to get a good "bite".

This has more to do with the tendency of Al to 'gall' (wipe or smear when
subjected to movement under a high surface loading) than it has to do with
its strength.

>When fitting a stud make sure it is absolutely clean, both the stud and the
>tapped hole.  Apply a little Copper-Slip paste to lubricate the threads or if
>you prefer a little Loctite for extra security.  Avoid the use of stainless
>steel studs, these tend to gaul and cause the ali to pick up and clog the
>roots of the stud.  Most AR heads have been helicoiled because of stripped
>threads by over enthusiastic application of torque.

Never-Seize isn't too good an idea on the anchored end of a stud. Loctite
is good. If you are trying to get an interference fit stud into an Al
casting, heating the casting a 'bit' will be a GREAT help.

If a steel stud (even the highest strength available) is properly installed
(including sufficient depth of thread penetration, generally about 3,
perhaps four, diameters) into an Al casting. the stud will fail in tension
before it will pull the the threads out of the casting !!! Good design
practice also includes counterboring the threads a bit below the mating
surface of a part--so that the mating surface immediately adjacent to the
tapped hole will not be distorted by the tension that is (eventually)
applied to the fastener.

  Of course a good torque
>wrench should be used, regularly calibrated and kept in a box when not in use.
>Torque wrench figures assume a wet thread, a dry thread will require
>considerably more torque and this in part is the cause of stripping.
>
>Wurth sells a huge variety of replacement studs for all sorts of
>applications.
>
>When fitting manifold studs one should not fully bottom the stud into the
>head.  Screw it in by hand until the plain part meets the top of the hole then
>double nut and turn it one half turn more, if it does not screw in easily then
>it needs more cleaning.

FAR better to invest in an appropriate set of stud 'collett' tools
(available from Snap-On, for one) than to double nut for installation or
removal if you do any significant amount of this work.

 Greg
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