Decaying magnesium

[4sd911]

I have my 2.7 engine apart and the magnesium case at the cylinder base is badly pitted, with loose flakes (see photo's). Is this common? What cause's this? Should I be concerned? Time for a different case? This tear down is because of high oil pressure. I think the relief and safety valves are sticking as the bores are also badly pitted.



This engine was part 10 years ago for a rebuild, I've logged 37,000 miles since that time. 993 head studs and time certs were installed at that time, they seem to have worked well as upon removal of the heads all head bolts were good and tight.

[Don Settergren]

Moisture? Have you ever noticed foamy oil? How about white residue on the oil cap? Below is what my 964 pump looked like after flood waters entered the case. I researched 964 pumps on Pelican and they seem to be made of magnesium. My aluminum case and the rest of the engine were fine - just the oil pump was horribly rotten. Hopefully I am totally off-base here!




I have more pics if you are into this sort of gore.

[4sd911]

I never noticed the oil itself being foamy, there is definitely foam (moisture) in the oil tank in winter months.

[Don Settergren]

Oil is less dense than water, so water damage should be at the bottom of the case under the oil. Your damage is higher up... Maybe it is something else, or maybe the moisture clings to the walls to the case. It looks superficial to me. Its been a quiet weekend on Pelican. Hopefully someone on the board has seen this before and chimes in on Tuesday.

[Eagledriver]

It's probably due to moisture between the cylinders and the case. To try to prevent it you should make sure to drive long enough to fully warm up the engine each time you drive (as much as you can).

I wouldn't worry about it though. The mating surface looks good and the corrosion doesn't look too deep.

-Andy

[304065]

I would mechanically debride the loose flakes with a nonmetallic brush and hit the entire area with a magnesium conversion coating like that made by Henkel. It's expensive and toxic but a good idea to stop the corrosion in its tracks. Of course once the jugs are on the thing will be full of oil and it wont be a problem, but you can't be too careful, particularly given the investment you have in head studs.

[4sd911]

Here is another picture of the worst spot. Also the oil galleys to the main bearings are flaky, see picture of bearings they are quite pitted. This will need to go to a machine shop to clean the oil galleys. If the case is properly heated can the studs be salvaged. I couldn't get a good picture of the oil galley.

[CliffBrown]

Originally Posted by 4sd911

Here is another picture of the worst spot.

In this photo it looks like some serious corrosion on the outside of the case. Has this motor been in an area near the Ocean (salt water).

Not a factor by itself but when added to the internal appearance it looks like this case did have some moisture in the case for an extended time period.

I used to rebuild magnesium case aircraft motors from aircraft based near the Ocean and that type of damage was normally seen as the result of prolonged exposure to warm moist (humid) air, especially those with long periods of inactivity.

[matt demaria]

The main bearings are pitted from fatigue. This is from the bearing shell flexing. Not a corrosion issue. Flywheel bearing looks as if the bearing bore is out of round. It looks as if the bearing is pinching the crank in several spots. Measure the bore with dial bore gauge.

[4sd911]

No not near ocean. Only moisture that I see is the usual foam in the tank during winter, and same in my 912. I've used castrol 20/50w since the rebuild but changed to swepco last year.

[lucittm]

The pitting looks more like galvanic corrosion. This is caused by dissimilar metals coming in contact with each other through a conductive medium like water vapor or liquid water.

Very common in marine applications of aluminum where it comes into contact with seawater. The fix is to use a sacrificial anode of a less noble metal such as zinc to react with the other metals thus saving the precious aluminum.

Not sure how it would work with a piece of magnesium. But I would make sure there was no water in my engine, ensure the case is grounded, and smear some grease in the the area where the pits are. Once started, it never really stops. In the aviation world this is a big problem with condensation and magnesium. You case would be in the trash bin if it were an airplane engine. I would run it for another 100,000 miles and not worry about it.

Good Luck,
Mark