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Refinishing of S65 magnesium valve covers is becoming more and more popular, and is a needed option given the deteriorating state of the original 10+ year old parts in our cars. It often takes owners by surprise to hear that a valve cover gasket job isn't going to fix their leak problem. The issue that complicates such a seemingly simple practice is the magnesium material these covers are made of. Refinishing magnesium can be difficult in itself, but its long term prognosis can be less than ideal when certain aspects of the process are not considered. For this reason, we consider that not all refinished covers are created equal.




The problem:
Some BMW engines, such as the S65, were made with Magnesium covers for a variety of reasons amid BMW's Efficient Dynamics program, but these present challenges a decade after their manufacture. Magnesium is one of the lowest metals in the galvanic scale, acting as an anode to its surroundings. This means the covers breakdown and corrode rapidly compared to materials such as aluminum. This creates problems, even when simply attempting to replace gaskets on an original cover. Removing an original gasket almost always peels the coating away with it, exposing bare magnesium. New gaskets cannot seal against a pitted or corroded surface, or a bare magnesium surface so th results are persistent leaks, even after a gasket job.



By now, all owners are likely going to observe the eyesore in their engine bay caused by failing valve covers. Leaks on the ground, grease in the engine bay, smoke and odor from the oil dripping onto the headers and burning off. The official BMW solution is to replace the valve covers at great cost. Many owners have tried to get around this by using excessive sealants, or painting the covers without understanding the root cause. Some entrepreneurs have caught on to the market need and attempted to offer painted covers as a side hustle, but never really addressed some of the critical concerns in the process. As a result, we see poor results in only a few thousand miles. The owners of these "refinished" covers below believed their covers were leak free until we showed them they were indeed leaking:



Misunderstandings:
S65 valve covers carry some specific misunderstandings about their function and design when discussing leak repairs or refinishing:

Oh, your valve cover must be warped...

This is a hugely popular misconception that rarely if ever occurs in reality. These magnesium castings are not prone to warpage in the same way that a component that sees high temperatures might be. Magnesium castings are inherently resistant to warpage. For academic discussion- if they did somehow warp slightly, the hardware would secure the cover adequately as these covers are not particularly rigid. These covers simply do not experience sealing issues from warping.

You just need to resurface the valve cover with a new gasket...

Many believe that resurfacing the base of the cover, in the same way you might resurface a cylinder head or a gasket flange, will level the sealing surface and resolve sealing issues. In reality, the groove of the cover perimeter is the primary surface that the gasket seals against and cannot be "resurfaced" in the traditional sense (against a sanding block). The gasket is crushed into the groove. Therefore a good seal requires that the groove valley be completely smooth. Rubber profile gaskets won't seal against rough surfaces.

Just sand and paint the cover.

A popular suggestion, sanding and painting is often recommended as a remedy but usually doesn't take into consideration the complexity of the part requiring complete coating removal, nor the fact that many paints are permeable to a degree, allowing oxygen to reach the magnesium substrate which permits corrosion to continue. All of the original coating must be removed inside and out. Media Blasting is ideal for this, although not just any media type should be used with magnesium. The coating must also be a high solid type that seals the entire cover.

Don't paint the inside of the cover.

This is a detail that requires clarification. We know that seals require a smooth surface to function. Therefore, coating inside and out where seal surfaces are covered in a wrinkle finish is unacceptable. Again, it must be understood that gaskets and o-rings WILL NOT give a lasting seal against bare magnesium. It must be sealed from oxygen or it will corrode quickly and the seal will fail. Therefore, completely coating a cover inside and out is desirable and recommended, but must be done correctly with a smooth coating after adequate surface preparation. Our covers get coated in a special primer, then only the external (non sealing) surfaces receive the final finish coating.




The restoration process:
Ideally, a restored magnesium component would be "pickled" prior to finishing, however that is impractical and cost prohibitive for small batches. Media stripping of the part immediately prior to coating is adequate to ensure good adhesion. Ultimately, the longest lasting solution to this issue is a brand new casting via purchase of a new OEM cover from BMW. Through years of trial and error and hundreds of refinished covers, we have determined what processes work and what doesn't. Here is an overview of how we refinish a used valve cover:

1. -Disassemble and clean. Each cover is completely disassembled and washed in our CUDA cleaners with a heated aqueous solution before then being soaked in a degreasing 140*f ultrasonic bath which helps remove oil traces from any "pores".
   
2. -Grinding. Our least favorite yet most important step. Each cover must have all pitting, corrosion, and surface defects removed prior to blasting and coating. Pitting and bubbles are removed, sealing grooves routed out, and all corners and surfaces are smoothed mechanically with various tools. Degradation severity is how we determine whether each cover is a viable core for refinish. Some are so badly pitted and corroded that the time required to remove the defects is greater than the restored value of the cover. We reject about 1 out of 4 covers received as cores. This process alone takes us roughly one full week to work through a batch of cores. Still, some covers are better than others. The finest ones are allocated for gloss or smooth finishes, the average covers destined for wrinkle finishes which hide more defects.
   
3. -Blasting/Etching. Each cover is media blasted to remove remaining coatings, create a consistent surface finish, and to prepare the surface for proper adhesion.
   
4. -Outgassing. The covers are baked to outgas any remaining traces of oil which may result in coating failure, or at minimum, flaws in the finish. Due to the porosity of cast magnesium, perfection here is not expected but the extra care and attention here pays off in the finish quality.
   
5. -Coating: The initial sealing is performed via a heat activated polymer primer (powdercoat) which is applied inside and out to completely seal the metal from the environment. This is where some will argue against coating inside the cover. It is important to note that the process is different when refinishing compared to a new casting. The goal is to create an encapsulated substrate which cannot be done if only coating portions of it. Additionally, the primer is chemically unique and the cured surface very smooth which is ideal for the gasket seal. It is NOT recommended to apply the final finishes to the inside of the cover. The final Powdercoat finish is applied to the cured primer but the sealing surfaces are painstakingly masked off to preserve the smooth surface.
   
6. -Reassembly: Each cover is evaluated by the coater and by us before reassembling the internal baffles and additional parts and seals.

Installation in vehicle:
Anytime a refinished cover is installed, we always recommend attention to detail and some additional considerations. First, cleanliness is key. No grease or oil can be present on a sealing surface. Remember always that a gasket sealing surface must be smooth. Prep any rough surface. As extra insurance in resealing refurbished covers, we usually recommend a slim bead of sealant in the gasket groove. O-ringed sealing surfaces should be smooth and glossy. The perimeter crush bolts are another topic. The rubber crush seals are not sold independently by BMW. We find that they are quite reusable as they rarely are found hard or brittle. However(!), when removed, the coating usually comes off with the grommet. The rubber grommets must have the remnants of the old coating removed from the rubber for it to be reused. Installer discretion is required and ultimately the seal longevity is determined by how attentive the installer is during assembly.





Bonus Material:
As visible as the covers are, we like to add a little attention to detail to the upper timing covers by blasting and vapor honing them for as-new appearance. These are installed on every job we do in our shop, but also offered on a core basis when we ship out covers. It's also the ideal time to upgrade the failure prone plastic VANOS spring cover caps with our Mporium CNC spring cups.





From start to finish, it takes around 2-3 weeks for us and the coater to prep and refinish a batch of 20-30 covers. We've done these in many colors, including some fund unique coatings like hammered finishes. It's a lot of fun as it offers a certain degree of personalization.