Sticky #6: Magnesium Wheel Reconditioning

Hi. Glad to see there is a Pantera forum. Forums are most excellent.

I am writing to ask for some advice.

I recently purchased a good used set of 15 X 7 Pantera wheels to use on the track with my '65 Mustang. I'd like to clean them up.

Is it ok to sand blast them or should I use different media?

Thanks in advance...

Steve
Original Post
Steve,
Try a search on Campi wheels. There are all kind of articles on cleaning, stress releiving and finishing the wheels.

I think sandblasting is too agressive for Mag wheels. Media blast or manual cleaning will be best. Search around lot of good suff. Use the find key
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
http://www.poca.com/detomaso/2000-06/1205.html
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.
Thanks for the great information, guys. I ended up bead blasting most of one of the wheels yesterday at the Military base auto hobby shop (@2.50/hour it's a steal!). I've added a picture of before and after. I believe I'm going to anneal them in our oven (Wife willing) and simply spray them with Gibbsbrand penetrant. Thanks for the info - I would have never known to anneal them!

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One more thought regarding the use of 7" Campy's on your Mustang...

You almost certainly will need longer wheel studs on your Mustang to use the Campys. The Campy wheels are thicker than the Mustang's steel wheels, and there won't be a sufficient number of threads for the lug nut to grab on to in order to safely secure the wheel to the car.
Bob,
There are a few threads that might help you with this, here's a couple that covers the steps in some detail (and may scare you into shipping them to a vendor to do it for you - unless you're comfortable welding magnesium, and your significant other doesn't mind you using the oven to do some baking.

Campi Wheel Reconditioning
Here's a step-by-step description of one member's experience (in his case there was no welding) ...
Wheel Refinishing
That's too disturbing of a picture to look at in that restoration thread.

If you are going to refinish the original wheels, usually they just need to be repainted on the exterior.

That's what I'd recommend also. Why blast the wheels down to metal if it isn't necessary?

Just clean them up and respray them. You can buy the paint already mixed from Hall ready to spray.

I'm sure you can easily get it from the other vendors also?

What I got from Hall was acrylic enamel. That's a very durable paint. Kinda thick but definitely good for the porosity of the magnesium if there is any exposed.

I have seen more then one original set of Campis stripped down and polished and in my opinion were nicer polished then the current set of aluminum repros (which are really nice themselves).

Campagnolo wheels are famous for their quality. Besides the accuracy of the original patterns and molds, it is also due to the quality of the magnesium that they used. It is far better then what I see in the original American Racing Torque Thrust D wheels that were used on the Trans Am Mustangs and Cougars, and better then the original GT40 and Cobra Halibrand castings. Those have the reputation for being brittle after all the years.

Granted no one ever thought of problems 40 or 50 years down the road. Same as with wiring in the harnesses?

There were stories of the Halibrands in the '60s of the wheels not holding air because they were more porous cool and the air molecules would escape through the magnesium. I have never heard of anyone stating that of the Campis, even after some cars being neglected for all of this time in some cases.

The problem is magnesium and oxygen don't like each other at all. Best leave them painted and the original paint and primer on them as much as possible. That's the safest bet, and original. You did say original right?
Yes, Doug. Owner is FIRST and only owner. Car has 19,000 and was nicely stored indoors.Even the inside of the original carb is almost completely clean. Amazing, considering that there was no INTENT to "store" it, it just happened that way. The wheels are original to the car.
Snaponbob,

I purchased a set of 10" Campys a couple years ago that were straight but cosmetically poor. I followed the restoration precedure linked by #5754, including plastic bead blasting (about $50/rim), annealing and painting with Eastwood Argent Silver. In addition, I filled the curb rash with Lab Metal. The results are rims that appear new.

Below is a picture of the final result. The decals and center caps are offered by several suppliers.

I agree that if your rims are cosmetically acceptable to you, it may be best to keep the original paint intact. But otherwise, it is not difficult to restore them yourself.

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PanteraDoug,

You are correct. The rear tires are BFG Radial T/A P295/50R-15, and the fronts are P215/60R-15, on 10" and 8" rims. These gave me the classic look I wanted; I even put the white letters facing out.

The pictures in my picture gallery are not from Westchester, but were taken between blizzards here in the frozen tundra of Wisconsin.

-Matt
quote:
Originally posted by BigBlockFan:
Snaponbob,

I purchased a set of 10" Campys a couple years ago that were straight but cosmetically poor. I followed the restoration precedure linked by #5754, including plastic bead blasting (about $50/rim), annealing and painting with Eastwood Argent Silver. In addition, I filled the curb rash with Lab Metal. The results are rims that appear new.

Below is a picture of the final result. The decals and center caps are offered by several suppliers.

I agree that if your rims are cosmetically acceptable to you, it may be best to keep the original paint intact. But otherwise, it is not difficult to restore them yourself.


So, plastic blast, and paint. Any primer? BTW, have not heard of Lab Metal. Google time, I guess!!

edit - just saw what it is. OMFG !!!!!! Where has THAT been all my life??
For magnesium you will want to use a zinc-chromate primer. Unfortunately, zinc-chromate has been identified as some sort of health risk, so I don't believe it is sold here anymore. I ordered my primer from Aircraft Spruce; they recommended zinc-phosphate as a substitute, which I used.

I painted my rim set a couple years ago and have not had any problems with paint adhesion or the rims holding air.

Be sure to follow the painting temperature and humidity recommendations for the paint, though. I sprayed the front and rear rims using Eastwood paint from the same batch, the only thing being different between the front and rear rims was the temperature, humidity, etc. The front rim paint is noticeably softer than the rear paint. I think it was due to painting the fronts at too low a temperature.

Finally, when using the kitchen oven to anneal the rims, you may want to do it while your wife is out of town. A smell that I thought was quite mild and harmless, my wife equated to ricen poisening. What made her go especially wild was that we have a cute parakeet residing in our living room, who she believed was in mortal danger. I ended up making a makeshift plastic drape to direct the out-gases to the outside vent hood. Problem solved.
quote:
Originally posted by BigBlockFan:
For magnesium you will want to use a zinc-chromate primer. Unfortunately, zinc-chromate has been identified as some sort of health risk, so I don't believe it is sold here anymore. I ordered my primer from Aircraft Spruce; they recommended zinc-phosphate as a substitute, which I used.

I painted my rim set a couple years ago and have not had any problems with paint adhesion or the rims holding air.

Be sure to follow the painting temperature and humidity recommendations for the paint, though. I sprayed the front and rear rims using Eastwood paint from the same batch, the only thing being different between the front and rear rims was the temperature, humidity, etc. The front rim paint is noticeably softer than the rear paint. I think it was due to painting the fronts at too low a temperature.

Finally, when using the kitchen oven to anneal the rims, you may want to do it while your wife is out of town. A smell that I thought was quite mild and harmless, my wife equated to ricen poisening. What made her go especially wild was that we have a cute parakeet residing in our living room, who she believed was in mortal danger. I ended up making a makeshift plastic drape to direct the out-gases to the outside vent hood. Problem solved.


I wouldn't go that route. My wife can shoot the rectum out of a mouse at night at 1,000 yards. I'm much easier to hit.
And FWIW, I tried annealing only one wheel in the kitchen oven when the wife was out of town for a weekend - set off all smoke alarms in the house and created an awful stench in the house. I have since picked up a free oven from a friend who did a kitchen remodel and use it in the backyard to anneal my wheels.
quote:
Originally posted by garth66:
quote:
Question why is it important to cook the rims? I am hoping to glass bead blast them in my blast cabinet, prime and paint them, and call it good. Some I have spoken with say to anneal them, and others say there is no need.

Read this thready for more info:
http://pantera.infopop.cc/eve/...660097094#1660097094


I have read that one quite a few times. I see that annealing the rim is needed after repairs, but I still understand the need if simply media blasting and repainting. Not being argumentative, I'm just missing something.
Several of the articles mentioned that magnesium may work-harden after many years of use, and annealing will normalize this stress. The rims being over 40 years old, I didn't take any chances. Annealing them isn't much more difficult than baking a pizza, so I did it; although I can't say if it made a difference.

Cast magnesium builds up stresses while driving, and those stresses do not entirely come back out by themselves. They may also age-harden and harder usually means more brittle.  Stories abound of magnesium wheels without tires cracking during rack storage! The U.S Military developed an annealing process to stress-relieve such wheels on high performance fighter planes. Powder-coating if done properly will also stress-relieve magnesium wheels. The keys are to get the entire wheel up to 275F and keep it there for a while, followed by extremely slow cooling- overnight is best. This temp will cause the Argent Silver paint to turn tan, requiring repainting. And because zinc chromate primers are difficult to find today, aggressive blasting is not needed nor recommended. The silver paint is decorative; the chromate primer is vital!

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