Agreed!

The stress applied to the stud or bolt is related to the force at the capture
surface dissipated over the thread surface area. There is a minimum number of
threads to take the applied torque before stretching the thread stock. This
minimum is related to thread pitch and diameter but is of the order of 3 or 4
threads. I typically use a value in number of threads equal to the diameter
of the bolt/stud. So 12.5 mm OD, 12.5 mm of thread. At 1.5 threads per mm,
the math works out to about 8 threads - overkill! Unless the spacer is
flexible or compliant (very bad) the thickness of intervening material
between the top of the thread capture surface and bottom of the bolt/nut face
is irrelevant to any questions of strength, or wheel retention.

Offset, bearing load, scrub radius etc are all important factors and not
impacted by stud strength discussion.

Aluminum is more than adequate for a spacer given its negligible
compressibility and, as noted has been used by Porsche at stunning stress
levels. Flexure or the wheel hub to rim probably occurs long before any issue
related to the spacer. This only applies to spacer inserts (stud goes through
wheel then spacer then into thread retainer). For wheel adaptors (stud
threads into adaptor which bolts to hub, I would only use steel because of
the annealing requirement to maintain strength of Al alloys.

As to the machining of cast or forged suspension components - I would never
do this without bead blasting and annealing equipment access.

My two centimes as a Physical Inorganic Chemist.

Frank Grunthaner