Mahle piston coating faliure
 

Hi
A friend of mine bought new Mahle pistons for an alusil block (in 2012). After some delays and a lot of electrical gremlins (first fire up early 2014) the engine finally ran descent late this year, but consumed a lot of oil and pushed out oil in the catch tank.

After dissemble for investigation the pistons was found in this condition. Chunks of the coating was found on the oil pickup strainer.

Anyone seen anything like this before?

And btw, what is this coating? Dosen't look like the old plating for alusil.

http://i1037.photobucket.com/albums/...psjpdkctze.jpg

http://i1037.photobucket.com/albums/...ps2wcg7hp2.jpg

Alusil pistons are Tin plated those pistons are Phosphate coated and poorly at that.

regards

Is that what Mahle use as coating now?
I heard that the old tin-coating isn't used anymore due to environmental reasons, if that is true.

Alusil requires a harder surface because the barrel is soft and the dissimilarites offset one another to help longevity I do not know if Mahle even does Alusil anymore. I only mention it because the Pistons for Alusil have a different color more raw Aluminum in appearanace. the Piston shown in your post looks like a 4032 silicon with Phosphate coating and it doesn't even look right.
regards

I'm not sure you can use new Pistons with used Alusil cylinders. From what I understand, you can reuse the original rings and Pistons if they are still good and you keep track of where they came from in the engine but you have to get the cylinders nikasil plated in order to run new Pistons. I'm not sure if you can even get the correct rings for Alusil.

Others will confirm. I believe that when the Alusil P&Cs are broken in, it's the cylinders that wear to the rings because it's the softer material. Traditionally it would be the other way around.

They can be reused if they are treated in the way kolbenschmitts procedure describes, as these were.
But the issue here is the useless coating that peels off in chunks!
So the question is has Mahle fallen of the waggon and started producing crap?

they have a lot more problems than they used to

regards

Alusil is tricky.

The cylinder is made from a Hyper-Eutectic Aluminium alloy which is around 16% Silicon.

When the cylinder solidifies there is an underlying skeleton of needle like silicon formed in essentially Aluminium Matrix.

It can be likened to an old fashioned version of a metal matrix composite but it doesn't provide huge strength and isn't sold as an MMC.

The surface of the Aluminium is etched away using an electrochemical process and this leaves a slightly proud surface of Silicon which is relatively hard and wear resistant.

The surface morphology should also allow the retention of a reasonably good oil film.

I think the first application of this technology may have been the Chevy Vega followed by the Porsche 928.

It is now used commonly by BMW following the failures they suffered with Nikasil blocks caused by high sulphur fuels.

Traditionally Pistons for use in Alusil were Iron Plated to prevent galling between the aluminium of the piston and the underlying matrix of the bore.

The Tin plated layer was added as a boundary lubricant to aid break in.

I am not sure which other piston coatings work but many aftermarket pistons are simply coated in XYLAN which is PTFE and is simple and cheap but not long lived.

The green coatings found are normally XYLAN and in moderate quantity costs are around $10 per piston. Black XYLAN is also commonly used.

Moly Phosphate conversion coatings are superior but I don't know if they are compatible with Alusil but would like to know.

I have heared two similar stories with this pistons and I wanted to ask you:
1.What oil did you use when this happen?
2. What type of petrol did you use?

Not sure, but I think it was a break-in oil from Millers and 5% ethanol mix at 98 RON octane, we have 5% ethanol in all gas here.

This engine was later rehoned, nicasil plated and piston coating removed. Piston to cylinder clearance to Mahle specifications, which was almost double compared too stock. After a short breakin the bores were scratched again!!

The owner and engine builder (not the same) have no idea whats going on. Best guess, Mahle doesn't know how to make pistons anymore...

When the pistons are scuffed like that, why is it surprising there's damage to the coating ?

Its not. But at the time it was unclear If it was down to too tight clearense or something else. In some spots it looked like it was just flaking off.

Was the bores scratched under the piston ring travel area? I think the coating is the problem. I'am looking into this matter since I already purchased a set for my engine rebuild and I also see that tow more guys (with 944 turbo) had similar problems with this pistons.
This is what I find in another forum regarding this piston issue.
http://forums.pelicanparts.com/uploa...1500314257.jpg

Not sure. But since he removed the coating and the issue still is there, we don't think its the coating, its a clearance issue. Either the pistons expand more than they should or the bore distorts when heated up.
Knowledge about the 951 seems very limited among engine builders.