Seeking Advice Regarding a Leaking Magnesium Wheel

Hi all, I am looking for some advice.

I have an original set of Campagnolo Magnesium wheels on my Pantera. One of the wheels has a slow leak which I had fixed last summer by removing the tyre and then having it re-seated. Even though I thought this has solved the issue, the problem has now returned. I suspect the leak is caused by a poor seal due to corrosion.

i am going to remove the tyre once again and clean up the inside of the rim, does anyone have any further experience or advice which may help?

Many thanks, Jon

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When I first bought my original Pantera, I had porosity at the bead (we believed).  We had an inner tube installed.  It seemed to work fine.  I will also say that it was that same rim that later cracked about 1/3 of the way around.  I'm not aware of any reason why the inner tube would have caused the crack, though.

It may be time to have the whole set serviced by someone familiar with magnesium wheels. There are other members on this forum who could explain much better than myself but essentially there are metallurgical processes (such as annealing, cold tanking etc...) which need to be performed in the proper order to ensure the structural integrity of the wheels. Then comes the sealer so moisture cannot creep into the metals causing porosity or most concerning- cracks.  Of course paying the bill for that job will not be fun...

 

Be careful with fix it in a can .It can really mess up wheel Balance with gel, liquid rolling around.  We sell quit a few tires a year and see this issue often.  Ussually it is corrosion at the bead or around the valve stem seal  - Where it seals in the rim.  

Locate the leak yourself with a spray bottle and diluted dish soap (2 or 3 capfull in a quart) . Fill fhe tire properly and spray  down the whole tire and rim-  leave it sit for a few minutes. ( Not long enough to dry out keep it wet). If it is a porus rim or bead leak/valve stem seal you will see a fist full of bubbles.  Now you will know what to address. 

We use a special low speed die grinder with a  wire wheel encased in plastic (so the wires don't fling out) and buff the corrosion off.  After you buff the rim you must seal it up , we use a rubber type cement and paint the bead and valve stem area where it contacts the rim. (Mag rims  may need to be sealed with clear before rubber sealer and skip the rim buffing if there is no corrosion)  Make sure you clean the tire bead with a wire brush and solvent . All of this can take a bit if time.  Many tire guys don't have or take the time to locate detail clean and prep a rim. This could take quite a bit of time.  A entry level  tire buster can't spend that much time on a leak.  We deal with corrodeing aluminum rims and flaking chrome quite a bit.  Magnesium rims are a rarity now a days but done properly I cant see how it could hurt to save a rim for street use.  Opinions may vary. 

  JerrySr

#2636

Magnesium, far different than aluminum, builds up stresses from being used as a wheel; it flexes in use and eventually becomes brittle. The stresses cause a micro-pore in the casting to become a crack. TIG-welding, followed by annealing is the only sure way to fix a cracked or pitted mag' wheel, and it takes an expert in TIG to grind out all traces of the crack/pit before welding.  Otherwise, since mag' also expands about 2X that of aluminum, the crack will continue to unzip itself in front of the weld bead and you can chase a tiny crack halfway around the wheel. Note the huge casting is safe to work on but the fine grindings from crack removal are extremely flammable!

a TIG-welded wheel also needs to be firmly strapped down with 6 or more c-clamps; my welder used to say 'Campy wheels will crawl off the welding bench if not held down'! A loose wheel can heat-distort into a 'potato chip' as well. For your expert welders' use, Campy street wheels (including the shown Gr-5) are cast of AZ-91 alloy while Campy Gr-4 real-racing wheels are cast of ZE-41alloy. Use the appropriate welding rod.

As a preventative measure on undamaged Campys, I suggest annealing every wheel at home about every 10 years; it's easy and can be done in a kitchen oven. All you need is at least 375 degrees F for 3 or more hours, followed by VERY slow cooling. I take the hot wheel out after 3-4 hrs, wrap it in a wool (not synthetic) blanket, put the still-hot wheel back inside, turn off the oven and leave alone till its stone-cold the next morning. There will be an odor.

This technique comes from aircraft repair shops working on military fighters that also use magnesium wheels, and from Lockheed Space Systems. It removes all residual stresses from age or weld repairs on mag', but causes the protective silver paint to turn tan, so it will then need to be repainted. Do NOT strip the old primer- it is difficult to replace today and is self-healing to a point. Modern water based primers just aren't as good.

All this was detailed in a POCA Newsletter article, Feb 2018, available in the Club Archives for download, including (for Brits) a recommended shop in Bishopthorpe, England that does this work. Good luck- J DeRyke

Bosswrench  not wanting to high jack the thread but clarify  Any ( even light) corrosion can not be buffed and sealed? ( street use only)  I have not de mounted my tires yet after sitting  in storage years with no leaks but there is a couple spots from wheel weights in the bead area. If so there may be another set of Stock Campys soon listed on Ebay to finance Polished Aluminum LOL

I have read the  threads on annealing and repairing stress cracks ( great info and a lot to take in) 

 

 

As long as you recognize that corrosion converts metal to powder so the wheel will be slightly less strong in that area. The primer and protective silver paint has also failed or there wouldn't be corrosion. The weakest spot in our wheels is the inner bead where the tire seats, and it's worst on the inner bead of the more heavily loaded rear wheels. Water inside a rim usually comes from a poorly maintained air compressor that concentrates water as it compresses moist air.

Sure- you can thoroughly wire-brush the white mag-oxide away, spot-prime with zinc chromate (if you can find it) and spot-paint with Argent Silver metallic paint, which should match Campagnolo's stock paint. Be lavish with the primer & paint- the more layers of protection, the better. Just keep an eye on the area in the future and don't stress the rim too much; high speeds on potholed roads, or autocross competition might best be avoided if the affected area is large.

 

Thank you Bosswrench,  Great info. I will definitely closely inspect and treat any of those areas as a preventative/preservation step. If any are too bad I'll turn it into a wall art LOL . Good to know "All or Nuthin"  isn't the only option for all situations.

Annealing is on my to do list. I love the look of the Campys ( almost because they are not machined perfect)

Enough hijacking the thread. Hope this helps others

JerrySr

 

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