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I've heard a lot of rumors about the Mercedes-Benz M112/M113 and the Porsche M96 blocks not being repairable.

By different people, I've heard

• the sleeves both are and aren't coated with Nikasil.
• the sleeves both can and can't be recoated.
• the sleeves both can and can't be honed.
• the sleeves both can and can't be removed.
• the blocks both can and can't be re-sleeved.
• Mercedes-Benz and Porsche both use the exact same materials for their sleeves and pistons.

Caveat: I am only referring to the original engine that was shipped with a new car. A lot of Boxsters (986/987) and 996/997 series 911's have had their engines replaced. Regardless as to whether the dealer replaced an engine under warranty or not, anything is possible on a replacement engine.

The first rumor that can be dispelled is that the sleeves are coated with Nikasil. Nikasil is a silicon carbide coating. A 'coating' is a layer that's applied over a surface. Nikasil is a true coating. The cylinder surfaces on the Mercedes-Benz M112/M113 and Porsche M96 are not coated with anything - they are prepped. There is a difference and I'll get to that later.

The second rumor is easily answered. The sleeves can be coated. Anything can be coated. The sleeves can't be recoated, as in replacing the original factory coating, because the sleeves were not originally coated.

The sleeves can be honed. You can hone anything. They may not seal properly, but they can be honed. Knowing how they're made, I wouldn't hone them. I know a lot of machine shops that won't hone an OEM M96 or M112/M113 cylinder. Some people claim they can. I'm not sure most of those who claim they can, know how the sleeves are manufactured. More on that later.

The sleeves can be removed on a M96 engine. A number of companies specialize in removing and installing sleeves in an M96. I suppose it's possible to perform the same procedure in an M112/M113, but I'm not sure why anyone would want to. The Mercedes-Benz M112/M113 press release says 'Mercedes-Benz was the first automaker to use innovative cast-in silicon-aluminum cylinder sleeves with a low friction surface'. It also says the sleeves are cast first, then, rather than being pressed into the block, the block is pressure cast around the sleeves. The outer 20% of the sleeve melts, bonding with the block. Aside from stating the sleeves are 'an integral cast-in part of the block', the release goes on to say 'this sleeve technology is also designed to provide exceptional block stiffness while minimizing weight'.

Based on this, we know the sleeves are structural members of the M112/113 assembly. This casting process also explains why the sleeves are not available as a separate part. By not offering the sleeves as a separate part, it sounds as if Mercedes-Benz didn't intend for them to be removed. That doesn't mean they can't be removed, just that it doesn't sound like Mercedes-Benz intended that it be part of any service procedure. This may make more sense if I explain how a Nikasil sleeve is made, and how the Mercedes-Benz and the Porsche sleeves are made.

To Nikasil coat a sleeve, an aluminum sleeve is dipped in an electrolytic bath of nickel, silicon and other metals for about an hour. The electrolytic reaction causes a silicon-nickle coating to adhere to the aluminum surfaces. When finished, the aluminum sleeve has a Nikasil coating. This makes it pretty obvious that M112/M113 sleeves are not Nikasil. The Mercedes-Benz press release said the sleeves were 'silicon aluminum'. It didn't state that they were nickel and silicon coated aluminum.

Mahle is the supplier of Mercedes-Benz sleeves. It's well known that Mahle produces Alusil sleeves. Alusil is a high-silicon content aluminum alloy and has been in use for almost 100 years. To make an Alusil sleeve, a high-silicon content aluminum alloy sleeve is dipped in an acidic bath. The acidic bath etches away the aluminum on the outer surface, exposing a very hard, very long-wearing, silicon surface. The finished product is still a silicon aluminum alloy.

Porsche's M96 sleeves are also silicon aluminum alloy. It's well known that the process for making Porsche's M96 sleeves is called Lokasil. Lokasil (localized silicon) is a faster, less costly process than Alusil. Kolbenschimdt Pierburg (KSPG) makes the sleeves for Porsche. Lokasil is a bore liner made of silicon fibers. When the bore liner is inserted into the block mold, the fibers burn out, leaving a high-content silicon surface in the bores.

Basically, both the M96 and the M112/M113 use sleeves that are made of silicon aluminum alloy, and they both have silicon wear surfaces. In both cases, the wear surface isn't a coating, but a manufacturing process. I've used the term 'wear surface', but the phrase doesn't accurately describe the sleeve. Typically, the silicon doesn't wear out. Foreign objects (such as a broken ring, broken valve, etc.) can score a sleeve, but the sleeves don't usually 'wear'. This is opposed to Nikasil, which is known to wear quite rapidly if exposed to high sulfur fuel.

There are numerous engineering shortcomings in the M96. These deficiencies can require sleeve replacement during a rebuild. However, the sleeves are usually damaged as part of the infamous D chunk failure, or they crack because of case expansion. Although worthless, the ultra-hard cylinder surface usually doesn't show signs of wear. Because the Mercedes-Benz M112/M113 doesn't suffer any of the engineering problems that the plague the Porsche M96 and result in sleeve damage, I'm not sure why anyone would sleeve an M112/M113. That doesn't mean a good reason to sleeve an M112 or M113 doesn't exist, it means "I" don't know of one.

Regarding the last rumor, both engine designs use silicon aluminum sleeves. Both engines also use pressure cast aluminum pistons that are coated with an iron particle reinforced resin. The silicon content in an Alusil sleeve typically differs from the content in a Lokasil sleeve, but one could say they use the same materials.

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© 2008 Marcus Blair Fitzhugh