[James Brown]

Hear is a post from Wayne in 2002. As always, great info:

I fell like I have posted this at least 3 times on this BBS in the past month, but here it is again:

Nikasil vs Alusil Cylinders - In 1973 Porsche introduced a new type of cylinder used on the 911 2.7 Carrera RS. Engineered by German manufacturer Mahle, Nikasil cylinders are manufactured out of a dense, aluminum alloy that is centrifugally cast in a mold. The cylinder bore is then electroplated with a very thin layer of nickel-silicon carbide. Originally designed and used on the venerable 1971 917, these cylinders provide several unique advantages over the older-style ones. The primary advantage is that the micro-thin layer is extremely durable, and allows for thinner cylinder wall thickness as a result. As a result, the piston bores can be enlarged without changing the original cylinder head stud bolt pattern. In addition, the reduced friction along the cylinder walls combined with the surface properties of the nickel-silicon coating creates a tighter seal against the cylinder. The result is a slight increase in overall horsepower, due to the increased efficiency. These Nikasil cylinders are the most durable of any of the production cylinders and are highly sought after for engine rebuilds. These cylinders can be retrofitted to the earlier cars, however, you will need to install the updated piston squirters in to your early case (1970 and earlier) if it doesn’t already have them. The piston squirters lower the piston crown temperatures so that you can run the close clearances used by Nikasil or Alusil pistons and cylinders. It’s also important to note that Mahle makes Nikasil pistons and cylinders for 2.2L and 2.4L engines.

In 1974, Porsche introduced the Alusil cylinders, manufactured by Kolbenschmidt. The Alusil cylinders were primarily used as a less-expensive alternative to the Nikasil cylinders. These cylinders are manufactured out of a special 390 eutectic aluminum silicon alloy, and are used with a special iron-plated, ferrocoat piston. Like the Nikasil cylinders, they are plated with a special coating on inside bore. This coating is electrically etched to leave a microscopic layer of silicon particles exposed on the cylinder wall. The iron-plated piston and the silicon-plated cylinder walls operated together to create a durable dual-surface. In addition, the Alusil cylinders have the same thin-wall construction of the Nikasil cylinders, meaning that they too can maintain the same head-stud spacing pattern.

So what are the main differences between all of the available pistons and cylinders? The early biral cylinders can be honed and reused just like other cast-iron cylinders on non-Porsche cars. Starting in 1974, Porsche mixed and matched the Alusil and Nikasil sets, so it’s really the luck of the draw as to which set you have in your car. For the most part, Porsche used mostly Alusil in the 2.7L and 3.0L engines because of the reduced cost of production. The Alusil cylinders, unfortunately, cannot be honed. The honing process destroys the etching layer, and renders them useless. In fact, a general rule of thumb is that the Alusil cylinders are a one-time-use product, and should not be used again if the engine is rebuilt.

Alusil cylinders cannot be reliably reringed. This indeed is a common misconception in some Porsche circles. There are no replacement rings available that are specifically designed for the Alusil cylinders. Of course, throwing out your current pistons and cylinders can lead to the large expense of new ones, so a lot of people reuse them anyway. In some cases, the new set of rings seat fine, and they indeed can be reused successfully. However, you cannot hone these cylinders, nor predict whether the rings will indeed seat properly. The correct action to take is to purchase new pistons and cylinders, otherwise you may be tearing down your engine again in less than a mere 1000 miles. These Alusil cylinders typically have a ‘KS’ Kolbenschmidt logo cast into their base, although for a time, Mahle also made Alusil cylinders. The coating on the Alusil cylinders is non magnetic, so you should be able to tell the difference with a simple refrigerator magnet.

If you are tearing apart your engine for reasons other than worn out rings or valve guides, then you might opt to reuse your Alusil cylinders. This would be the case, for example, if you were tearing down your engine to replace broken or pulled head studs. If this is the case, I would recommend that you take your pistons and cylinders off of the engine, put them high up on a shelf, and don’t touch them until you are ready to reassemble. Don’t pull the pistons out of the cylinders and don’t dislodge or disturb the rings. Of course, you are taking a risk here that you will have worn rings in the near future. However, if your engine is a 3.0L, with 100K on the odometer, then there is a good chance that you can get 100,000 additional miles or more out of your set of rings, cylinders and pistons. I don’t necessarily recommend playing the odds like this, but if you’re rebuilding a good running engine with excellent leak-down numbers for the purpose of replacing head studs or some other non-wear problem, then it might be a good bet.

The Nikasil cylinders can indeed be honed and reused. They typically have a ‘MAHLE’ stamp on the lower side of the cylinder. The nickel-carbide surface needs to be lightly honed with a special silica impregnated tool, or what is commonly known as a grape or flex hone. The surface properties are too hard for normal tool steel honing machines. The Nikasil coating will be ever so slightly magnetic when you place a magnet next to it. The honing process is performed using special tools – either a specific tool designed to hone the cylinders, or a grape hone. Either way, you should only have an expert familiar with the Nikasil cylinders perform the honing process.

-Wayne
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Wayne R. Dempsey, CEO, Pelican Parts Inc., and Author of:
101 Projects for Your BMW 3-Series • 101 Projects for Your Porsche 911 • How to Rebuild & Modify Porsche 911 Engines
Coming Eventually:
101 Projects for Your Porsche Boxster & Cayman (2010) • 101 Projects for Your MINI Cooper (2010) • 101 Projects for Your Porsche 996 / 997 (2010)