Interesting read regardingly Alusil and replacing piston rings, for any of you so inclined.
Originally posted by http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/603194-re-ring-alusil-bores.html
Quote:
Originally Posted by wild man
Really? I always thought that the idea behind stoning - I mean honing - WAS to put some scratches in the cylinder walls! Maybe I am too old school, but I was always taught that the rings will never seat properly in the cylinders of a well-worn engine if it is not honed out. Can't say that I ever needed to hone the cylinders of a porsche engine though (but I have done other makes).
Maybe you are saying that because the cylinder walls are aluminum. But I am confounded by the fact that they are siliconized, which I am told makes them even harder than steel. So that would (should, anyways) make them HARDER to scratch than steel, if I am correctly adhering to the logic involved.
Where have I gone astray? (let's stay on-topic here, please)
For old cast iron cylinders, that is exactly the point, as long as you are running standard rings. The fine honing lines serve to retain oil on the cylinders. Other types of rings, such as moly IIRC, require a much finer finish.
NEVER EVER EVER USE STONES ON ALUSIL!!! (<-important point
)
I should make that my signature.
Alusil, unlike Nikasil, is a homogenous casting (this is very often misunderstood. Nikasil is a very thin coating. The entire block on these motors is Alusil alloy. You'll notice how much quicker razor blades dull on it when scraping gaskets off). Silicon is dissolved in the molten aluminum to a concentration that is greater than the solubility of silicon in solid aluminum. Once the block is cast, it is carefully cooled to cause some of the silicon to crystalize out of the aluminum, leaving a very even distribution of very small silicon crystals suspended in the aluminum. A little bit of copper and magnesium is also present in the alloy.
The reason honing stones are so bad is that they lack the mechanical integrity and hardness to cleanly shear the silicon particles like diamond stones can do, instead tearing whole chunks of silicon out of the surface of the cylinder. A properly honed cylinder retains the distribution of silicon crystals at the surface. Once the cylinders are honed, the aluminum is lapped away from between the silicon crystals with the AN-30 silicum paste to leave a surface analogous to the stone honed cast iron surface. The AN-30 paste is not hard enough to affect the silicon, so what you end up with is a surface that is very, very durable that also has microscopic spaces to retain oil. This is also why the lapping process is relatively self limiting. The rings essentially run on silicon, which is much harder wearing than iron.
A freshly prepared Alusil cylinder has nice sharp edges on the silicon crystals, which give the surface a very fine micro-rough feeling much like a chalkboard. This initial roughness is absolutely crucial in breaking in new rings. Once the cylinders are run in, the particles smooth out a bit and transition to the very smooth shiny surface you see with good used cylinders. The spaces between the silicon crystals also fill up with ring and oil material over time. This slippery, durable surface is great for low friction, long lasting engines, but will not effectively break in new rings, especially if the cylinder geometry is not perfect such as with high mileage motors, meaning more wear needed on areas of the rings to attain a complete seal.
What the scotchbrite does is disrupt the surface just enough to clean out most of the intercrystal deposits, remove old worn crystals, and expose some new ones without removing much material and without tearing out crystals from the surface. In moderation, it leaves a very favorable surface to lap with AN-30 in order to produce much the same rough surface as regularly honed cylinders. This seems to be a very effective technique for cylinders that still have very good geometry, which Alusil is pretty good at retaining due to its inherent durability. Scotchbrite honing will not correct any bad cylinder geometry, so measuring is highly recommended! It is also important to use green scotchbrite pads. Red pads are too coarse, and blue no scratch pads, well...kind of defeat the purpose. I believe rings are much more of a wear item compared to the cylinders in an Alusil motor versus a cast iron ring and cylinder configuration. I have seen several people have good used 944 cylinders measure up very well after 150K+ miles. Again, used cylinders, especially on high mileage motors, simply lack the surface favorable for break in wear on new rings. Think of it like trying to sand with worn out sandpaper. You may get enough break in to not smoke, but how will your leakdown test out compared to a fully seated set of rings? The risk is not worth the trouble of quick and dirty re-ringing, IMHO. Do it right, either DIY or profesionally, or leave it alone.
I believe my other secret ingredient is the Deves rings. They are a special non chromed Swedish cast iron alloy that seems to break in easily without being too agressive to the cylinders both initially and in the long term. This may also increase the odds in favor of fully breaking in to less that perfectly round cylinders. How that translates as far as longetivity compared to OEM rings remains to be seen, but after 19K miles, I still have gobs of power, and oil consumption went to absolutely zero after switching to 0W-30 Castrol Syntec european formula (true group IV) as a winter oil.
Well, there's my take on Alusil from what I learned from my rebuild and all the research I did when trying to determine how I was going to re-ring my motor.
Originally Posted by wild man
Really? I always thought that the idea behind stoning - I mean honing - WAS to put some scratches in the cylinder walls! Maybe I am too old school, but I was always taught that the rings will never seat properly in the cylinders of a well-worn engine if it is not honed out. Can't say that I ever needed to hone the cylinders of a porsche engine though (but I have done other makes).
Maybe you are saying that because the cylinder walls are aluminum. But I am confounded by the fact that they are siliconized, which I am told makes them even harder than steel. So that would (should, anyways) make them HARDER to scratch than steel, if I am correctly adhering to the logic involved.
Where have I gone astray? (let's stay on-topic here, please)
For old cast iron cylinders, that is exactly the point, as long as you are running standard rings. The fine honing lines serve to retain oil on the cylinders. Other types of rings, such as moly IIRC, require a much finer finish.
NEVER EVER EVER USE STONES ON ALUSIL!!! (<-important point
) I should make that my signature.
Alusil, unlike Nikasil, is a homogenous casting (this is very often misunderstood. Nikasil is a very thin coating. The entire block on these motors is Alusil alloy. You'll notice how much quicker razor blades dull on it when scraping gaskets off). Silicon is dissolved in the molten aluminum to a concentration that is greater than the solubility of silicon in solid aluminum. Once the block is cast, it is carefully cooled to cause some of the silicon to crystalize out of the aluminum, leaving a very even distribution of very small silicon crystals suspended in the aluminum. A little bit of copper and magnesium is also present in the alloy.
The reason honing stones are so bad is that they lack the mechanical integrity and hardness to cleanly shear the silicon particles like diamond stones can do, instead tearing whole chunks of silicon out of the surface of the cylinder. A properly honed cylinder retains the distribution of silicon crystals at the surface. Once the cylinders are honed, the aluminum is lapped away from between the silicon crystals with the AN-30 silicum paste to leave a surface analogous to the stone honed cast iron surface. The AN-30 paste is not hard enough to affect the silicon, so what you end up with is a surface that is very, very durable that also has microscopic spaces to retain oil. This is also why the lapping process is relatively self limiting. The rings essentially run on silicon, which is much harder wearing than iron.
A freshly prepared Alusil cylinder has nice sharp edges on the silicon crystals, which give the surface a very fine micro-rough feeling much like a chalkboard. This initial roughness is absolutely crucial in breaking in new rings. Once the cylinders are run in, the particles smooth out a bit and transition to the very smooth shiny surface you see with good used cylinders. The spaces between the silicon crystals also fill up with ring and oil material over time. This slippery, durable surface is great for low friction, long lasting engines, but will not effectively break in new rings, especially if the cylinder geometry is not perfect such as with high mileage motors, meaning more wear needed on areas of the rings to attain a complete seal.
What the scotchbrite does is disrupt the surface just enough to clean out most of the intercrystal deposits, remove old worn crystals, and expose some new ones without removing much material and without tearing out crystals from the surface. In moderation, it leaves a very favorable surface to lap with AN-30 in order to produce much the same rough surface as regularly honed cylinders. This seems to be a very effective technique for cylinders that still have very good geometry, which Alusil is pretty good at retaining due to its inherent durability. Scotchbrite honing will not correct any bad cylinder geometry, so measuring is highly recommended! It is also important to use green scotchbrite pads. Red pads are too coarse, and blue no scratch pads, well...kind of defeat the purpose.
I believe rings are much more of a wear item compared to the cylinders in an Alusil motor versus a cast iron ring and cylinder configuration. I have seen several people have good used 944 cylinders measure up very well after 150K+ miles. Again, used cylinders, especially on high mileage motors, simply lack the surface favorable for break in wear on new rings. Think of it like trying to sand with worn out sandpaper. You may get enough break in to not smoke, but how will your leakdown test out compared to a fully seated set of rings? The risk is not worth the trouble of quick and dirty re-ringing, IMHO. Do it right, either DIY or profesionally, or leave it alone.I believe my other secret ingredient is the Deves rings. They are a special non chromed Swedish cast iron alloy that seems to break in easily without being too agressive to the cylinders both initially and in the long term. This may also increase the odds in favor of fully breaking in to less that perfectly round cylinders. How that translates as far as longetivity compared to OEM rings remains to be seen, but after 19K miles, I still have gobs of power, and oil consumption went to absolutely zero after switching to 0W-30 Castrol Syntec european formula (true group IV) as a winter oil.
Well, there's my take on Alusil from what I learned from my rebuild and all the research I did when trying to determine how I was going to re-ring my motor.