Winter Projects 2019/20 - pt 6 - Magnesium Engine Parts, Camshaft Timing Gear Housing Repairs.

 The last installment of Gretchen's Blog showed how the stripped aluminum threads in the air flow meter were repaired. This time we will discuss the magnesium engine parts and the repairs that they needed.

The Cam Cover, Intake Manifold, and Cam Timing Gear Housing - all the silver-painted engine parts seen in this photograph - are made out of magnesium, the least stable metal in the Galvanic Series.

The cam cover, cam timing gear cover, and intake manifold are all made out of magnesium on the Porsche 944S. These same parts also appeared on a few other Porsches as well. The cam cover is actually a 928 part, The 944S2 shares the cam cover and cam timing gear housing, and the 968 uses the cam timing gear housing only. Magnesium is a metal that has many advantages, and a few disadvantages, when used for car parts. Today it's used quite extensively in the automotive industry (and several other industries as well), so it seems that Porsche were once again ahead of their time. An early adopter of automotive magnesium casting technology in production, Porsche used this metal for engine and transaxle cases during the 60's and 70's in the 911, and many Porsche race engines and transmissions used magnesium as well, although those are not large-scale production examples. Volkswagen also used magnesium for many years in their 4 cylinder boxer engine cases. In the 50's and 60's magnesium was a popular material to make racing wheels from because of it's light weight (this of course is where the term "mag wheel" comes from).

Why Magnesium?

At this point I'd like to make it clear that no one uses pure magnesium in manufacturing. When I say "magnesium" in this post, I'm referring to one of several magnesium alloys, which normally contain pure magnesium plus a few percent of aluminum, zinc, and other metals such as silicon, manganese, copper, and zirconium, depending on the specific alloy. Magnesium in its pure state is the lowest, or least noble metal in the Galvanic Series, meaning that it corrodes easily when placed in contact with almost any other metal in the presence of an electrolyte - most commonly, water and some amount of salt. When magnesium is alloyed with the other metals mentioned above, it has an "acceptable" level of corrosion resistance, especially when properly passivated prior to finishing. Passivation is usually a chemical conversion dip or bath using hexavalent (nasty stuff) or trivalent (not as bad) chromium solutions.

Think about some of the advantages that would have made this metal attractive to the Porsche engineers for use in their manufacturing environment:

• Low weight - magnesium parts weigh 2/3 the weight of the equivalent aluminum part; this is good for performance, fuel economy, and emissions.
• Good strength - internal engine components and other highly stressed parts aren't made from it, but it's great for covers and lower stress components.
• Can be die cast or sand cast fairly easily - reportedly it's a much simpler process than aluminum die casting.
  
• Magnesium can be easily machined - this translates into longer tool life, which offsets the higher initial cost of the metal (compared to aluminum and steel).
• Acceptable corrosion resistance when properly alloyed and passivated - meaning that under normal use it can be expected to last the anticipated lifetime of a vehicle.
  

It's not too difficult to understand why Porsche decided to take advantage of this material from time to time. However, some properties of this metal are usually seen as negatives for both manufacturer and consumer alike:

• Higher material cost - however as pointed out above, ease of manufacturing usually offsets this.
• Highly flammable (under certain conditions) - The flammable nature of magnesium often freaks people out. A raging magnesium fire is extremely difficult to extinguish, and can do an unbelievable amount of damage. But in reality this is mainly a manufacturing concern, as it's only small particles - the chips and filings created during machining - not the comparatively large castings themselves, that are flammable. If proper procedures and precautions are observed during machining operations, the risk is quite low.
• Difficult to finish - Magnesium is very unforgiving when it comes to finishing. Passivation, or conversion, is the key to making a coating adhere properly to the magnesium surface. There are various chemical conversion processes, mainly invented by the Dow chemical company, such as Dow 7, Dow 17, and Dow 19. Many of these involve controlled hazardous chemicals and can pose health risks to the applicators. However, there are now a few safer DIY treatments available.
• Galvanic Corrosion - This one goes hand in hand with the point above. Galvanic corrosion will occur underneath coatings applied onto un-passivated magnesium. The most significant disadvantage for us, the end-user, is mag's susceptibility to galvanic corrosion in the presence of an electrolyte (moisture and salt). Alloyed or not, magnesium will be the first thing to go when it's exposed to water and salt (read winter driving, coastal locations, even moisture in the air contributes), because of its rock-bottom place in the galvanic series.
  

The Galvanic Series. Pick any two metals to join or place together. The one that is more anodic will experience galvanic (electro-chemical) corrosion in the presence of an electrolyte. The rate of corrosion is greater the further apart they are on the list.  So for example, if tin and copper are placed together and soaked with salt water (an electrolyte), the tin will corrode a little. If tin and titanium were combined instead, tin would corrode very quickly. You can also see from the chart that sometimes, it's never a good idea to use stainless steel fasteners or copper anti-seize with magnesium or aluminum parts*, but zinc plated bolts and anti-seize are ideal. Chart courtesy of Corrosionpedia.com .

*A side note - Using stainless fasteners with...

Before I give any wrong impressions about the use of stainless steel fasteners on engines, you have to understand that the whole galvanic corrosion issue has another aspect to it, which is that the severity of the galvanic reaction/corrosion is also quite dependent on the relative surface areas of the two dissimilar metals.

To paraphrase Wikipedia a bit, "When two or more dissimilar metals come into contact in the presence of an electrolyte, one metal (the more reactive of the two in the chart above) acts as anode, and the other (the less reactive of the two) as cathode. There will be a slight voltage between the two metals, and this voltage will be the driving force for an accelerated attack on the anode metal, which will begin to dissolve into the electrolyte. This leads to the metal at the anode corroding more quickly than it otherwise would while corrosion at the cathode will be inhibited."

However, and this is the point of putting you through that last paragraph, it depends heavily on the relative surface areas of the two dissimilar metals! If the cathode is very small compared to the anode, for instance when you have an aluminum or magnesium engine block (anode) with a stainless steel bolt (cathode) screwed into it, then little or no corrosion is going to take place. This is why you typically don't see issues with stainless exhaust studs in aluminum heads, or steel bolts in a magnesium intake (like the 944S, unless it's winter-driven or located near the ocean). I'll be going to some lengths here to make sure that I pair the safest combination of fasteners, etc with Gretchen's magnesium parts; I'm interested in absolute best corrosion resistance, and if it seems like overkill, so be it.

Another disadvantage that concerns threaded mag parts is low ductility. Magnesium doesn't take kindly to having threaded fasteners screwed in and out of it multiple times, the threads are sort of brittle. When there are conditions present for galvanic corrosion to occur, even at the minimal level that you get with a large anodic part and relatively small cathodic fasteners, there's a greater issue. Drilled and tapped holes are prime places for corrosion to start and thrive. All the ingredients are there: potential for trapped moisture, dissimilar metals are present, and if the fastener is removed and replaced more than once or twice, physical thread damage occurs. Add to that the inevitable over-torqued bolt and occasional cross-threading and you have a damaged thread to repair. This is the main reason why specially coated Heli-coil screw thread inserts were used for the thread repairs.

The cam cover looked a lot prettier when I brought Gretchen home. This picture was taken by the previous owner before I bought her. If you zoom in and look closely though, you'll see blistered paint all over the cover. I had to get to that corrosion and halt it; these things are way too expensive, and hard to find, to just ignore the problem.

Corrosion Under The Paint

March 2, 2020 - At some time in the past, Gretchen's magnesium parts had been spray-bombed silver.

The cam cover had been sanded down a little bit (you could see sanding marks and some of the remaining original finish on the cam cover), and the paint applied right there in the engine bay, which unfortunately left overspray on some of the other parts. But it must have looked pretty good afterwards. Some black plastic lettering had been glued into the recessed PORSCHE letters on the cam cover and they really made it stand out.

Inevitably though, the bare magnesium started to corrode, especially underneath the lettering. When I purchased the car in 2019 the paint was blistering and flaking off in many areas. Rather than let this corrosion continue, I had a go at soda blasting the cam cover with the intention to passivate and then clear coat it, as the cover was off the car to inspect the timing chain tensioner and pads anyway.

Soda Blasting worked really well to remove paint but didn't have much effect on corrosion or surface finish.

The soda did a great job of stripping both the new and original layers of paint, but the surface had become pitted and corroded in a few spots, and the corrosion was left untouched by the gentle action of the soda. I had to remove that corrosion down to clean metal for the passivation process, and I wanted a lightly textured, burnished surface finish. It became obvious that soda blasting wasn't aggressive enough for this. I would have to try walnut shell blasting, or maybe glass bead blasting if the walnut didn't achieve the surface I wanted. I wasn't set up for either of those types of blasting media, and I really wanted to get the car running, so I decided to postpone the magnesium refinishing until I could do it properly (there will be a future post about this). I cleaned the cover off and re-installed it onto the engine. Krown T-40 corrosion inhibitor, applied every few weeks, keeps the bare metal from corroding any further.

Gretchen's cam cover as it looks today, January of 2021, the less than ideal result of my aborted attempt at soda blasting and refinishing. This is only temporary though, one day soon it will be beautiful again!!

What about Powder Coating?

I've seen quite a few pictures on the internet recently of freshly powder-coated magnesium parts on 944S, S2, 968, and 928 cars. I have read conflicting information about this. Some say that powder coating is one of the worst possible ways to finish magnesium, that it will corrode very quickly if you powder coat it. There are also plenty of people, many of them 911 owners, who claim in the forums that they have had it done and it has held up great for several years at least. Their pictures look great (although to me there's nothing as nice looking as an unmolested factory original engine bay), and I'm sure those people were really proud of their results. This company, located in England, specializes in powder coating magnesium motorcycle wheels, and they certainly appear to know what they are doing. I think the whole question of whether it's OK to powder coat mag just boils down to finding a company that knows exactly what it takes to make it work and will do the job right.

 

Stripped Threads

Gretchen's intake manifold and both halves of the cam timing gear housing had stripped bolt holes. The camshaft cover has a few small threaded holes for the plug wire holders, which looked fine at the time, so I didn't do anything with them, but the screws that go in these holes are stainless, so to provide the best protection to those threads, I'll take care of them as well.

These special inserts by Heli-Coil are specially coated to eliminate galvanic corrosion between the magnesium base metal and the stainless steel insert. Bolts to be threaded into these STI's are yellow zinc plated steel with zinc anti-seize.

If you've read the previous post, you'll know that Heli-Coils are the preferred screw thread insert for me. But they're made from stainless steel wire, and if you check the Galvanic Series chart above you'll see that the various grades of stainless are nowhere near magnesium, and that could possibly cause corrosion issues *(discussed in paragraph above) if you put them in contact with each other. There is a special type of Heli-Coil, which in the McMaster-Carr catalog is called a "Lubricated Stainless Steel Helical Insert for Dissimilar Metals". These have a special coating on them that isolates the stainless wire from the material you're installing it in. They're kind of expensive, but I think they are the right product for the repair. In addition to these coated Heli-Coils, new yellow zinc fasteners were used in the repaired holes. Zinc is right next to magnesium in the galvanic series, making zinc-plated fasteners the best choice for these parts.

The cam timing gear is adjustable on the 16 valve engine. When you want to remove it, you have to lock the gear in place by installing three M5 x 20 bolts into the holes normally occupied by the ignition rotor mounting screws. This will ensure that the timing gear doesn't move in relation to the camshaft (thereby upsetting the cam timing) when the gear assembly is removed

The front half of the cam timing gear housing is held on by three M6 bolts threaded into the rear half. Two of those holes were stripped, as well as the two threaded holes for attaching the rear half of the plastic belt guard. To remove the rear cover, the timing gear had to be removed, followed by the three M6 bolts that mount it to the head.

A Standard puller was used with M5 threaded rod and jam nuts to make a custom puller setup for the timing gear. The M5 rods replace the M5 x 20 bolts installed previously to lock the cam timing. By carefully swapping the bolts for the threaded rod one by one, the cam timing is held in adjustment.

The center camshaft bolt came out without any problems, thanks to a VIM tools XZN socket and a 24 inch breaker bar. My 2-arm gear puller wouldn't fit behind the cam gear because the rear housing was in the way. To get the gear to come off the camshaft, I modified a standard steering wheel puller using some 10.9 grade M5 threaded rod and some M5 nuts and washers. Once cut to size, the three pieces of rod were swapped one at a time with the M5 x 20 bolts that had been installed earlier to lock the adjustable timing gear assembly, and then the puller was assembled onto the three rods with more M5 hardware.

There was just enough room for the drive screw in front of the puller

The cam gear came off easily, but I was surprised to see a large, black, fluffy deposit of badly oxidized magnesium just below the camshaft. The corrosion had caused the surrounding metal to swell and had actually been rubbing on the Hall Sensor trigger wheel, located right up against the back of the cam gear. One good discovery was that the exhaust cam oil seal was in perfect condition, so it was left alone.

With the cam gear off, you can see a large area of fluffy black magnesium oxide right below the camshaft. The magnesium had corroded badly in this one spot and actually ate right through to the other side. The Hall Sensor can be seen to the left of the camshaft. It is responsible for sending cam position information to the DME.

The majority of the cam gear housing was in great shape, other than some surface oxidization on the outer surfaces from being painted incorrectly, and the stripped threads. On closer examination, I found there were also several smaller areas of corrosion. One of the lower holes for the front cam gear cover bolts was so corroded internally that the boss had been split open.

A close view of the area damaged by corrosion, after removal and cleaning. The rest of the area is pristine, and a nice example of the original color and finish. This magnesium would certainly have had some sort of passivation applied prior to painting.

All four stripped holes can be seen here. The tall bosses are for the front cover mounting bolts. Internal pressure caused by corrosion build up has caused the left boss to swell and split open. You can see the beginnings of a split in the right one as well. The rear timing belt cover is bolted to the two threaded holes at the outer edges.

The back side of the cam gear housing is in pretty good shape. The machined mounting surface shows only very slight surface corrosion, and there is only mild corrosion beneath the original finish shown at the bottom right. The worst is again that area right below the opening for the camshaft, almost in the center of this picture, where the corrosion has eaten through from the other side.

The only real way to repair the corroded sections is by TIG welding. Most people are unaware that the TIG process was actually developed for magnesium welding in the mid to late 1930's. The correct procedure would be to mill away the corroded material , build it up with magnesium weld filler rod, and then machine it back to its original dimensions. As of yet I have no mill and no TIG welder, and I'm not aware of any machine shop around here that I would trust to do the work, so I decided the best thing to do was clean up the corrosion with a die grinder, coat the bare magnesium with corrosion inhibitor, and continue with the Heli-Coil thread repairs. A TIG welder, a mill, and a lathe have been planned for the shop for some time anyway. If I happened to come across a cam gear housing in good condition I could buy it, but in the meantime the corrosion would be slowed down enough to avoid any serious problems until then. Once I'm equipped and skilled enough to do these repairs you'll see a post on that.

I used a regular rotary burr and die grinder to remove the corrosion down to bare metal, then I applied a coat of Krown T-40 to keep corrosion in check.

The back side of the hole. before grinding

The cam gear (rear) housing after grinding away the corrosion. Heli-Coils were installed in all of the threaded holes, and all bare surfaces were coated with Krown T-40. The Hall Sensor was installed in its place and the housing was re-installed on the head.

The stripped out holes were drilled out to the correct M6 STI tap size. The process was exactly the same as for the air flow meter in the previous post. Because of the unexpected corrosion damage I had to deal with, I didn't think to take pictures of the Heli-Coil process for the can gear cover. Sorry! But the only difference is that coated inserts were used here.

The rear cover re-installed on the engine. You can see the repaired threads clearly in the left cover mounting boss at the bottom of the photograph. The threads in the right one are recessed for a locator roll pin that fits in here along with the bolt. The exhaust cam oil seal wasn't leaking and looked great, so it was not replaced. Krown T40 corrosion inhibitor has been applied to all exposed magnesium surfaces. It's an excellent product that really works.

Due to the splitting of the front cover mounting bosses, the STI thread repairs are not as strong as they would have been otherwise. The bolts could not be torqued past 3 Newton-Meters before the inserts began to twist in the compromised base metal. To ensure these bolts don't loosen and fall out into the timing belt path, the rear timing belt cover mounting bolts have been marked with red marks and photographed so I have a visual reference to check against for any movement. Safety wire will be added later to ensure they don't vibrate loose and fall into the timing belt path. The two timing gear cover bolts are not much of a problem because they sit up close under the outer timing belt cover. If they were to loosen, they would be captive in their holes.

The red marks on these bolts are for a visual reference - they can be checked from time to time against this photograph to see if they are loosening or not, since they can only be tightened to about 3 Nm. Normally, these would be tightened to about 6 Nm. Safety wire would be a better failsafe and will be added in the near future. If these bolts were to work their way out, it's possible they could destroy the timing belt or seize the water pump pulley; either one is fatal for the engine.

Next Time, I will detail the repairs to the threaded holes in the magnesium intake manifold. I was much more diligent about photographing all the steps, and there are some interesting views showing how I secured the rather large manifold to my small drill press table, and also some techniques to ensure threads are cut parallel to the hole. Thanks for visiting!