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Start with #6. You can usually get an idea what to expect by looking at the plug -- if there is any significant scoring, the plug will usually show unusual fouling. If #6 is okay, check #4 next, then #5, if my experience is any guide.
My Service advisor called me today to say that they had to "tear down the motor" which I already knew cause he told me this before, which I found very odd.I stated that I already knew this and he said that it was a bigger job then originally conveyed. I was like you said I would need a short block down and a swap of everything over when we talked over a month ago. It was just very odd. Waiting on fidelity.....
I can't believe that "repair" insurance actually works! You'll have to tell me what your premium is and if you've had the coverage since the car purchase.
Anyway, it looks like that piston and bore suffered from lack of lubrication or seizing. What a pain; Paeco in Alabama offers ceramic heat-shielding on the piston crown, and knurling on the skirt for oil retention. Since it looks like the piston is expanding faster than the bore, I would look into ceramic coating the crown.
Also, what's interesting is that above and around the top compression ring, there is no scoring, this would lead me to believe that that area is rebated specifically to accommodate for expansion. It would be interesting to mic that top area and the skirt to confirm that. If that's the case, this sounds like a chronic issue that is simply subject to the manufacturing tolerances of each piston/bore combination; and some people get unlucky with a piston that's a few 0.0001"s over and a bore that's a few 0.0001"s under; given that, you should insist on blue-printing the pistons and bores for analysis and your records.
Also, what's interesting is that above and around the top compression ring, there is no scoring, this would lead me to believe that that area is rebated specifically to accommodate for expansion. It would be interesting to mic that top area and the skirt to confirm that. If that's the case, this sounds like a chronic issue that is simply subject to the manufacturing tolerances of each piston/bore combination; and some people get unlucky with a piston that's a few 0.0001"s over and a bore that's a few 0.0001"s under; given that, you should insist on blue-printing the pistons and bores for analysis and your records.
I could be wrong, but most likely its from where the wrist pin is located and the geometry of the thrust motion. T
I could be wrong, but most likely its from where the wrist pin is located and the geometry of the thrust motion. T
No way; every engine would then have the same problem. You might say wrist-pin seizure, but even that would be a total stretch. You should have all the bores and pistons mic'd on all dimensions, on the old block, do a statistical analysis of deviation from the mean, then do the same to the new block, and use that information to judge the new block's bore and piston combinations. If this problem is not caused by thermal expansion, then it would have to be lubrication; in which case, doing what I suggest would not help. And you have to make these measurements with real micrometers, not mechanic's calipers.
Last edited by lou-in-nj; 11-09-2018 at 09:54 AM.
Reason: Add'l. Info
Although this has been discussed at great length, I'll summarize for those just hopping on this thread.
The Lokasil and Alusil process both run uncoated aluminum bores. Using a special process, the harder silicon particles are exposed at the surface. The pistons receive the ferrostan coating which is an iron skirt coat. This is the wear surface, not the bore as in conventional engines like those with iron blocks or that have nikasil bores.
Piston skirts are cammed - there are many profiles, and the appropriate one for the block and piston alloys that take into account clearances and expansion rates. Clearances are too tight in my opinion for those factory engines with forged pistons. We don't see scoring as much on engines with factory cast pistons.
Furthermore, a pin offset is used on the pistons to make the engine run quieter, however the offset goes in the wrong direction on bank 2 - that's why bank 2 always scores first. In later engines, Porsche started using bank specific offset pistons.
Long story short, when the skirt coat fails, you get aluminum on aluminum contact and scoring occurs. You have similar materials in contact resulting in galling.
The piston skirt coating can wear through normal operation or can fail from insufficient clearance, over-fuelling due to bad injectors, or skirt coating falling off due to poor adhesion (which we have seen chunks in the sumps of engines during inspection - you can easily identify it because it's magnetic and looks ribbed).
If you have damage to the piston on the lands between the crown and top ring or between top and 2nd, you have another issue going on unrelated to cylinder bore scoring caused by some other misconfiguration or failure.
That's very interesting Charles; I would never have imagined the the choice of metals for pistons and bores would be reversed. I'd like to understand the engineering purpose for this, thermal conductivity? Weight savings? If this ever happened to my engine I would look into sleeving the bores, even if it meant either machining the stock pistons, or custom machining piston blanks.
In post 177 I see the piston skirt appears to have oil retention grooves, but this may be the weak point of the iron coating.
As far as the offset wrist-pin, I now understand it's latitudinal, but it's hard to believe the offset isn't in the same direction on both banks, and it seems to me that this would make TDC for each bank at different crank angles, and would lead to a bit of a lope at best, or an engine that sounds more like a Harley.
That's very interesting Charles; I would never have imagined the the choice of metals for pistons and bores would be reversed. I'd like to understand the engineering purpose for this, thermal conductivity? Weight savings? If this ever happened to my engine I would look into sleeving the bores, even if it meant either machining the stock pistons, or custom machining piston blanks.
In post 177 I see the piston skirt appears to have oil retention grooves, but this may be the weak point of the iron coating.
As far as the offset wrist-pin, I now understand it's latitudinal, but it's hard to believe the offset isn't in the same direction on both banks, and it seems to me that this would make TDC for each bank at different crank angles, and would lead to a bit of a lope at best, or an engine that sounds more like a Harley.
This is nothing new. Alusil was developed in the 70s and there are several variations on the process including the Lokasil process used by most European manufacturers. Most everyone has of GM's debacle of the Vega, which was the first vehicle to use Alusil blocks.
We've been making pistons and cylinder sets for almost 20 years and we're still learning and applying what knowledge we gain through building test engines and inspecting used components that have been houred out or at time of rebuild. The learning never stops.
Offsets make the pistons quieter, but won't give an engine an audible lope.
Back in the day, people would purposefully install pistons with the offset going the wrong way as it would make more power, but at the sacrifice of longevity. In the case of the M96 engine, like with aircooled predecessors before it, the factory chose to only have a single piston with offset incorrect for a one bank rather than bank specific pistons for cost savings.
Most modern aftermarket piston manufacturers do not incorporate piston offsets in modern piston designs unless required by the application. We do not use offsets in any of our M96 pistons, whether from JE, CP, or Mahle Motorsports.
09-19-2018, 12:47 PM
