Do's and Don'ts of Magnesium

by Alan Buehner

Originally printed in the 2000 issue #9 of Still….Keeping Track

Magnesium is a strange material that was used in many parts of the Penton motorcycles. It was used primarily in the wheel hubs and motor cases. When heated, this material expands. I got to see this firsthand when I took a KTM motor case in to have a crack welded. Yes, magnesium parts can be welded, however, this should be done by an expert who has the right equipment and magnesium welding rod. I put on a pair of welding goggles and watched as he proceeded to weld up the crack. As soon as the arc touched the metal it lit up with a bluish-green glow that immediately heated up the metal and caused the crack to open up (like a miniature grand canyon). The welding rod was used to fill the crack in and another amazing thing was that unlike steel which stays burning hot after being heated up, the magnesium immediately cools down and can be handled almost right away. The main point that I am trying to make here is that when magnesium is heated up, it expands. When trying to remove and install bearings and seals from magnesium parts, heat the magnesium up first with a propane torch and the bearings and seals will slide in or out very easy. Do not force bearings in or out of "cold" cases. This will cause wearing on the surfaces and will result eventually in a loose fit where they will spin or fall out.

NOTE: for safety, do not use torches or open flames around flammable materials in your work area.

Do not use excessive heat on magnesium. If magnesium gets too hot, it will literally bum itself up (self destruct). If you take magnesium parts to someone to have welded, make sure that you let them know that it is magnesium and make sure that they have the experience and equipment to weld it.

Do not use Armor-All cleaner on alloy and magnesium (e.g. wheel hubs and rims). This will cause immediate oxidation of the metal and if left on for a period of time will eat away at the metal. Read the labels on any products before using them on magnesium and aluminum. Experiment by using any chemicals on a scrap part first to see what happens.

Do keep the magnesium parts painted. This will keep them from corroding. Corrosion of magnesium is evident from the formation of a white powder around the affected area. This results in pitting which eats away into the metal.

When disassembling or reassembling motor cases, do not force them. Check to make sure that all of the screws holding the cases together have been removed and make sure that an internal part has not moved out of alignment and is preventing the cases from fitting together. Sachs and KIM motors are finely engineered and are designed to fit together precisely.

Do not use gasket cement in reassembling motor case halves. Doing so will not only make it difficult to separate the cases but will result in the destruction of the facings of the cases when the gaskets must be scraped off.

Do use a thin film of grease on the case halves and gasket surfaces to provide a tight seal. This allows the cases to be pulled apart easily. Soaking the gaskets in warm water makes them flexible and allows them to stick to the greased surfaces and stay aligned over the holes during assembly. Torque the case screws to proper specifications (5-8 ft lbs.).

A common problem with the KTM motors when they have been left setting around for years is corrosion in the crank area. One tip that I received a few years ago was to use 3BOND clear coat epoxy for coating inside crank areas.

I asked our Penton experts, Kip Kern of Indiana and Bobby Lucas of Texas for their ideas on how to fix a corroded crank area and they sent me the following suggestions:

Kip Kern

To clean magnesium parts I suggest the following:

Lightly glass bead blast parts at 60 psi with 80/100 grit glass bead (be sure that you wear eye protection and a respirator or dust mask).

Carefully inspect parts after blasting for cracks and weak spots. Pay particular attention to:

  1. Case screw hole areas (for cracks)
  2. Kick starter stop bolt area (splits around hole)
  3. Bearing/ Crank flange areas (cracks)
  4. Front/ rear hubs (splitting along seams from spokes being too tight)
  5. Ignition cover, bottom inside area (holes where moisture collected)

Wipe internal case areas down with a light film of heavy grease and rub it in well to seal the magnesium. I use acid etching primer on outside of cases followed by paint to seal outer cases. Remember, keep cases sealed internally and externally from the elements to prevent corrosion! Moisture is the enemy! Dissimilar metals can also cause corrosion.

To remove bearings: heat the cases slowly in a small electric oven at 300 degrees. and then press/ pull the bearings out using proper tools, pullers, or a press. Do not get in a hurry as you can damage the case material.

CAUTION: Remember to always have a fire extinguisher available or a fire plan ready when working with magnesium. Only allow trained, certified personnel to weld this stuff!!

  • Do not use Silicone on cases
  • Do not use Locktite on case screws upon assembly
  • Be sure to clean between the motor mounts and the frame (remove paint) for proper electrical grounding - I use electrical joint compound/ grease on the motor/ frame contact areas to ensure that I have a positive ground for the ignition.

Bobby Lucas

Complete mechanical removal of corrosion should be practical insofar as practicable. Such mechanical cleaning should be limited to the use of stiff, hog-bristle brushes and similar non-metallic cleaning tools, particularly if treatment is to be performed in a home shop environment.

Any entrapment of steel particles from steel wire brushes or steel tools or contamination of treated surfaces by dirty abrasives can cause more trouble than the initial corrosive attack. Corroded magnesium may generally be treated as follows:

  1. Clean and strip the paint from the area to be treated.
  2. Using a stiff, hog-bristle brush, break loose and remove as much of the corrosion products as practicable.
  3. Treat the corroded area liberally with a chromic acid solution, to which has been added sulfuric acid and work into pits and crevices by brushing (with a non-metallic brush) the area while still wet with chromic acid.

WARNING: Chromic acid and sulfuric acid are hazardous chemicals. Be sure to read the labels and follow the instructions on the containers for proper use before buying and using. Make sure that you wear eye protection and suitable clothing and gloves. Make sure that the area you will be working in has adequate ventilation.

  1. Allow the chromic acid to remain in place for 5 to 20 minutes before wiping away the excess with a clean damp cloth. Do not allow the excess solution to dry and remain on the surface. Paint lifting will be caused by such deposits.
  2. As soon as the surfaces are dry, restore the original protective paint. Use at least two coats of Zinc chromate paint. When the metal's surface temperatures are above 250 degrees F, an extra coat of paint primer (minimum of three) should be applied ( crank area).

PJI offers a KTM engine paint that so far has held up on engines I've restored for the past few years. Remember that the inside of your engine is also painted. Pay attention to the inside of your ignition cover and always vent your magneto. Remove your ignition cover after you wash your bike and run up your motor for a good air drying.