Here's an article reprinted to the DeTomaso Mail list which details the correct process for restoring a Magnesium Campy wheel. I'm in the process of doing this myself. FWIW, this is an older article, so I don't know if Larry Stock still offers this service.
Restoring Magnesium Campagnolo Wheels the Right Way
by Mike Drew and Jack DeRyke
The Pantera was delivered wearing Campagnolo wheels cast in exotic magnesium alloy. Campagnolo is a very old company in Italy, and the methods they used were the tried-and-true types right out of the dawn of the Industrial Revolution. Thus, our magnesium wheels were cast in molds hand-carved from mahogany!
Naturally, wooden molds, however hard, could only withstand a limited number of times that molten metal could be poured into them before sharp details were blurred. Eventually, these details were lost, and casting quality began to drop off as well so the mold would be scrapped. This short life of the wheel molds may in fact be the reason there are at least four separate models of Pantera wheels that are known to have been shipped at various times during the life of the DeTomaso/Ford joint project in the '70's.
In the meantime, Campagnolo as a company was going through changes. The light-alloy bicycle accessory business was spun off in the '70's, the wheel company was sold (but kept its name the first two times), and new management were brought in after being taken over by Technomagnesio (one of Campagnolo's chief rivals in the alloy-wheel business).
Finally, the magnesium wheels made late in the Pantera project were pressure-die-cast. This method is where the molten metal is literally pumped into steel dies under relatively high pressure. The metallic dies are much more expensive than the old mahogany ones, but they also last much longer, quality is higher and the higher pressure during the metal solidification results in a denser casting with fewer, smaller pore areas. Intuitively one would expect that the very late Campy wheels are stronger for these reasons.
Magnesium, the lightest structural metal known, has one desirable and several not-so-desirable attributes. On the plus side, it is only 65% as heavy as aluminum while having as much strength as the better aluminum casting alloys, meaning it can be heat-treated and welded like aluminum.
On the debit side, magnesium is a 'reactive' metal: when magnesium contacts plain water or even moisture in the air, a chemical reaction occurs that results in the outgassing of hydrogen gas from the water molecule; the leftover oxygen combines with magnesium to produce the familiar white mag-oxide powder. If left exposed, Campy wheels will literally dissolve into powder!
At 650 degrees Centigrade, magnesium burns in air at such high temperatures that most substances in contact with it melt or burn as well. Large chunks are difficult to catch on fire but grindings or shavings should be treated like gunpowder-and definitely kept dry! Fine mag metal powder will react hydrogen gas off so quickly, the gas spontaneously lights off, catching the rest of the metal on fire. Naturally, pouring water on a mag fire will result in a hydrogen explosion (remember the Hindenburg?) and even more fire! Sand or dry-chemical extinguishers are the only hope for stopping a magnesium fire. In fact, it may be best to simply stand back and let it burn itself out....
Magnesium alloys have the property of age-hardening-that is, a mag casting will get harder and progressively more brittle as it gets older, regardless of its use or storage. Magnesium expands some 2-1/2 times as much as aluminum when heated. This means that for best results, the casting must be physically restrained-bolted down-before heating or it will literally 'crawl away' from the welding torch!
And due to its very light density, magnesium castings, especially open-mold castings, 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 100%. Instead, stresses build up inside the castings, eventually concentrating at an imperfection such as a casting pore or a crystal grain. Such an area will eventually separate and connect with another pore or weak spot. A few more cycles of this and you have a crack growing in the casting, with the accumulated stresses concentrating at the point-ends of the crack. This process works exactly like a micro pry-bar!
Detecting cracks in mag castings cannot be done by conventional magneflux: magnesium is totally non-magnetic. So a fluorescent dye called Zyglow is dissolved in a very thin solvent like petroleum ether. The casting is dipped in the solvent, left for a few moments, then wiped off. A spray cleaner is used to further clean the casting. Any cracks or deep pores will retain some of the dye, however. Illuminating the casting with near-ultraviolet light will show up any retained dye.
Porous, rough castings like our wheels require someone talented in the art of interpreting florescent patterns that show up. The layman could look at a perfectly good cast-mag wheel glowing in numerous areas and be afraid to mount a tire on it! X-ray casting checks can also be done but are even more expensive, require more highly trained operators to evaluate the film records and are sometimes inconclusive, requiring a dye-check to be sure!
Cracked magnesium castings should only be welded after stop-drilling both ends of a detected crack. This is a technique whereby the crack is outlined and a small hole drilled thru the casting just in front of each end of the crack, not in the crack itself. This is so when welding or grinding heat is applied, any growth of the crack will expand into the drill-hole rather than continuing to tear the base metal apart at the front of the crack. Then, the entire cracked area is V-ground away to prevent the crack from progressing sideways, and to provide clean metal to weld on. The V-groove is also necessary since vertical cracks in thicker sections don't weld very well.
Remember the cautions regarding the dangers of accumulations of grinding or drilling chips from magnesium repairs! Magnesium welding is best done only by an expert, with lots of experience and understanding of the nuances of magnesium welding. The incautious can very easily start a fire that will literally consume everything in its reach-including your house, garage and Pantera!
And surprisingly enough, the act of welding or grinding a casting induces even more stresses, so the repaired casting must be stress-annealed. This literally bakes out the accumulated stresses or any additional ones from the repairs and results in a like-new wheel that has many more years of safe driving left.
Fortunately, the annealing temperature (recommended by aerospace users of magnesium) is only 350-375 degrees Fahrenheit. The piece is put into an oven, heated to 350-375 degrees, held for an hour or so, then very slowly cooled to room temperature-the slower the better! Ideally, the heavily insulated oven is simply turned off and left closed until the next morning. If the oven cannot be turned off, wrap the hot casting in thick blankets and leave it undisturbed for at least several hours. Note-this will discolor any silver finishes painted onto the wheels. Do the decorative painting last!
After the wheel is cast or weld-repaired, if it is to be painted it must first be protected from moisture in the air by painting it with a zinc chromate solution. The zinc chromate reacts with the magnesium to produce a barrier layer that tends to be self-healing to small scratches-the zinc chromate literally spreads across the scratch, again protecting the underlying metal; not as well as a full-thickness coating, but at least there is some protection! Zinc chromate is widely used in the aviation industry, and should be available at better paint stores, as well as at any municipal airport repair facility.
Bare wheels that are simply painted with conventional primers and paint may trap moisture between the wheel and paint, resulting in corrosion under the paint, which eventually shows up in the form of bubbles in the paint.
In recent years, it has become very fashionable to powdercoat (or powderpaint) wheels rather than painting them. The resultant finish can be superior to paint, but only if the wheel is correctly prepared. Some people simply strip the old paint and then apply a coating of powderpaint, without actually repairing any damage to the surface, or checking the structural integrity of the wheel. When powderpainted in this fashion, often the wheel will out-gas during the painting process, leading to bubbles in the finish.
Larry Stock of the Pantera Parts Connection found himself with a collection of Campy wheels from various sources, some of dubious ancestry (including Mike Drew's old wheels!) Even though modern 17" wheels and tires are all the rage right now, he has found there are a select group of individuals who are looking for the best possible original factory wheels, and he set about taking these cast-offs and bringing them up to better-than-new specification.
The wheels were first carefully bead-blasted to remove all the old paint and underlying zinc chromate. The blasting process also removed any oxidation which might have accumulated. Afterwards, the wheels were annealed at 375 degrees in a large oven, which was allowed to slowly cool overnight.
>From there, the wheels were hauled to a sophisticated testing facility used by NASA and Lockheed Aerospace. The wheels were dipped in a liquid penetrant (Zyglow), then rinsed off and evaluated for possible cracks and imperfections. Any such imperfections were clearly marked, and one wheel was condemned and ultimately discarded.
Once the faults were identified, the wheels were then transported to a NASA welder who carefully welded up damaged areas of the wheels. While none of the wheels exhibited any significant cracks, several had large hunks missing from the lip of the rim. These were caused by the fitment of conventional wheel weights. The metal in the wheel weights reacts with the magnesium, turning it to powder. Tire-shop monkeys who traditionally remove old weights by hitting them with a hammer as often as not remove the lip of the wheel as well! For these reasons, whenever possible stick-on wheel weights should be used instead of clamp-on weights.
The welder went out of his way to put excess material back into the wheels, so now they needed to be brought back into spec. But before any machining would take place, they were returned to the oven and annealed again, to restore whatever strength might have been compromised by the application of high heat in only one area of the wheel.
Larry then took them to his fully-equipped machine shop. A rear axle/brake disc/stud assembly was inserted into a large lathe, and the wheels were bolted to the axle. Then sophisticated cutting tools were used to carefully remove the excess material and restore the original contours of the wheel. The average wheel required two full hours of machining in this fashion.
Of course, machining introduces its own heat factors, so back into the oven they went! After annealing, the powderpainters sprayed on silver powderpaint, which had been carefully color- and texture-matched with an original, mint-condition factory painted wheel. Back into the oven to bake the silver paint on, and anneal the wheels again! Finally, a protective clear-coat was powderpainted atop the silver, then the wheels went back into the oven for the final time.
The resulting wheels are absolutely flawless, exquisitely beautiful, and literally much better and stronger than new. The entire process was extremely labor-intensive and took almost six weeks to accomplish. Larry now has several full sets of early-style (Pre-L) wheels and a few L-model wheels in stock, which he'll sell for $325 each with the exchange of your old wheels. Alternately, he can have your existing wheels repaired for $325 each.