[DeTomaso] Campagnolo paint preparation

[JDeRyke at aol.com]

In a message dated 3/15/11 9:30:27 AM, garth_rodericks at yahoo.com writes:

> Because they crack! Magnesium castings, especially open-mold castings 
> like our Campy wheels, tend to be porous, or at least have very large 
> crystalline areas.  All wheels flex and bend slightly in use.  Magnesium flexes 
> too, but the flex-energy is not dissipated completely......
> 
Absolutely correct. The definition of stress relief is annealing. It can be 
done wrong as the quoted article mentions but it does not say that 
annealing causes embrittlement. Cooling hot castings too quickly causes that; SLOW 
cooling is needed. I use Lockheed aerospace heat-treat specs of 375-385F for 
3 hrs followed by OVERNIGHT cooling- still in the turned-off oven, wrapped 
in blankets.
Military fighter jets use magnesium wheels for the same reasons racecars 
do, and the military throw   mag wheels away after a certain time in use. Both 
mag castings and forgings build up stresses from rotating, cornering and 
the separate hoop-stresses from inflation, along with slow hardening of the 
alloys over time that makes it brittle. 
The only drawback to annealing (done correctly) is the paint discolors, 
forcing repainting. From 30+ years of experience in DeTomaso and Ferrari mag 
castings, once every 10 yrs or so seems to be often enough. Used mag wheels 
have cracked ON THE SHELF as stresses work at minor imperfections, so 
annealing is certainly recommended! Good luck- J Deryke