Magnesium. A wonderful metal for parts where weight savings are sought. Unfortunately, over time magnesium parts fail from existence.
Our example fan housing and partner fan for this study are out of a 1980 Porsche 911 SC Targa with an '85 3.2 liter engine.
Genuinely coming unglued, this housing remains functional (albeit now dragging its partner fan down with it) thanks in large part to its surrounding retaining strap. Because there's no guarantee of this housing remaining together for seconds or years, a housing run in this condition is risky. Consider what occurs when a housing collapses at high rpm---messy is an understatement.
Once structurally weakened, fan housings tend to go out-of-round given the pull applied by the fan belt.
When a housing has been pulled out-of-round enough, its partner fan will begin making contact with it.
Over time, frictional heat has caused the edges of this fan's blades to disintegrate. SPECIAL NOTE: Magnesium can burn ferociously. Allowing contact between a magnesium housing and fan as occurred here is bad news. Should a magnesium part ignite, the fire can cause serious damage to the engine.
As heat accumulates, gases within the metal expanded... blistering the fan's face side... and erupting on the inside...
Early model 911 magnesium cooling fan housings should be inspected on some regular basis. Intervals depend on miles driven. Once biannually is an average interval. For a thorough inspection, remove the housing from the engine.
Given any degree of failure, a fractured housing should be replaced---without hesitation. For a driver's Porsche, we do not recommend replacing a failed unit with an old, used magnesium part that may not yet show signs of fatigue/fracturing. Any such used magnesium can be moments from failure. Where restoration is concerned, alternate thinking applies.
New, aluminum housings are (as this blog is written) available and are a good substitute---aluminum being significantly more durable than magnesium. For a street driven Porsche, the minimal added weight of an aluminum fan housing is insignificant compared to the same magnesium part. For racing, and where weight reduction throughout a car is critically observed, alternate thinking applies.
TIP: An audible signal that a fan housing has failed is provided when the fan makes contact with the housing. With the engine at idle, this contact sounds like a high pitched ringing bell. Once this "alarm" sounds, it's advisable NOT to run the engine or drive the car---there's simply no telling what will happen or when! The owner/driver of this case study housing clearly delayed attending the housing's failure. A significant risk was taken in this case but fortunately the owner dogged a bullet---one of significant caliber. A RISK BEST AVOIDED.
SMART MOVE: Change a failing fan housing before it makes contact with its partner fan and ONLY the housing needs replacement. Allow contact between housing and fan to languish... then BOTH the housing AND fan need replacing.
New aluminum fan housing and fan ready for installation...
The replacement OEM Porsche fan here has been polished by the owner (who knows this finish will deteriorate over time if not maintained. While polishing, casting seam residuals were removed between each fin. Because there's no way of knowing exactly how much metal was removed between fins, it's best under such circumstances to consider the fan out of balance by default. This fan was brought back into balance before installing.)
New housing with alternator just installed (as part of this Porsche '85 3.2 engine's comprehensive service.)
Prior to installing the new fan, an application of antiseize goes on the alternator shaft. This will allow easy removal of this fan anytime in the future.
Before & after photos of the case study's Porsche 911 engine. Casually looking at the old fan housing here does not reveal its status. Only by removing it from the engine is its structural compromise seen. In the alternative, the sound of this engine at idle immediately indicates fan and housing making contact---a distinct signal that the magnesium housing has failed and is out-of-round.
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