Combating piston swell with oil

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There's speculation on some of the Audi sites about cylinder wall scoring on the 4.2L V8. No real pattern has been established, as both well-maintained vehicles and those with unknown maintenance (used cars) have developed the problem.

Borescopes and tear-downs have indeed confirmed scored cylinder walls. It typically begins with extremely high oil consumption (1qt per 500 miles or worse) and ends with a motor replacement. It's an all-aluminum motor.

One leading theory is that piston swell is the culprit. Quite literally, the space between the wall and rings is fluctuating as the engine turns and the piston is being forced into the cylinder wall.

I understand that changing oils will never fix a manufacturing problem, but is it possible to slow this process with a thicker oil? Are there any additives that could assist? The motor is spec'd for VW 502/505, which is ACEA A3/B3 oils (HTHS > 3.5) that are heavy Xw30 and light Xw40. Typical sump size is 9L/9.6qt.

I am pushing those affected to reply with the car's maintenance history and driving habits, namely the oil type and change interval. Not much data as of yet.
 
I would think that provided the thicker oil doesn't reduce the amount of oil getting to the cylinder walls, the heavier oil film would show some improvement in terms of scuffing wear.
 
If you have a scuff that runs vertical the length of the cyl, any oil left in the scuff would likely be pushed up with the piston rings and most likely burned. Plus there would have to be less compression, and more blowby in the crankcase.If the pistons got tighter in the bore you would think any wear would be uniform in the cylinder. And a loose fitting piston would be able to shift slightly in the cylinder and rub on the wall.You have to think in long term use that with the different expansion rates of the different metals used in a motors, they would have solved this issue along time ago or just used different materials to start with.
 
I should mention that it's an Alusil block.

"From what I've read about the engine, it is solid Alusil. Alusil is made by adding about 18% silicon to the aluminum alloy before casting such that silicon particles form throughout. The silicon particles allow oil to get trapped in between, so the rings sort of float on a layer of oil-impregnated silicon."
 
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Originally Posted By: rslifkin
I would think that provided the thicker oil doesn't reduce the amount of oil getting to the cylinder walls, the heavier oil film would show some improvement in terms of scuffing wear.



That's where my head is at too. I would think the polarity of esters might help it cling better to rings and the walls.
 
Yes, we all know that the Vega engine was known for tearing up the bores. However, the same vintage Mercedes aluminum block engines, using the same materials, lasted 500K miles and above.

The Vega engine had cooling problems and needed to be rebored after an overheat. If the fairly rigorous resurfacing procedure for the cylinders was not followed, the engine would seize shortly thereafter. The Mercedes engine manual (116.96/117.96) included instructions for two steps of honing with specific grits, stroke lengths, RPMS, durations, and amount of material removed, followed by a session with special felt pads and a honing paste, again run for specific speed, duration, and stroke length. It also included an explicit warning that if the procedure was not followed in its entirety that the engine would shortly seize.

You can imagine just how many mechanics would follow that type of procedure for reboring/resurfacing a Vega engine.
 
I would think that a thin ester based oil would help. They've found that thinner oils create thicker oil films around pistons and an ester based oil would cling nicely and also aid in heat transfer. I would be tempted to try Red Line 0w-20 or 5w-20.
 
I wonder if a UCL would help at all? Certainly a Band-Aid for a poor engine design. I wonder why they don't sleeve those engines with a steel sleeve? Or do they?
 
Originally Posted By: chevrofreak
I would think that a thin ester based oil would help. They've found that thinner oils create thicker oil films around pistons and an ester based oil would cling nicely and also aid in heat transfer. I would be tempted to try Red Line 0w-20 or 5w-20.


Problem is an oil that thin will not even come close to the required HTHS of 3.5+.
 
Originally Posted By: demarpaint
I wonder if a UCL would help at all? Certainly a Band-Aid for a poor engine design. I wonder why they don't sleeve those engines with a steel sleeve? Or do they?


Upper cylinder lubricant?
 
The Audi Alusil blocks actually have cylinder liner preforms of silicon that are cast in, and the cylinder bores are machined into the parent material. This differs from the Vega, Mercedes, and Porsche 928 blocks of 30-40 years ago that used the Reynolds 390 casting alloy that contained 17% silicon. They both must be final honed using felt lapping pads to selectively remove aluminum and leave the silicon particles proud of the surface. The advantage of Alusil is that there is a higher quantity of silicon for wear resistance.

It was characteristic of these aluminum-bore engines that the piston to cylinder clearance could be run tighter than on iron-bore engines because the differential expansion between aluminum pistons in aluminum bores was less than aluminum pistons in iron bores.

When you say that the bores are scuffing, what is actually causing the scuff? Is it the rings, or the pistons? This is the first question that is asked by cylinder kit engineers. If the rings are causing the scuff, it would mean that there is insufficient end gap, or that the rings are locking in the grooves, perhaps due to carbon buildup. If the pistons are causing the scuff, it would mean that the wall clearance is too small. Pistons are normally designed so that they guide on the high point of the skirt and the second ring land. By looking at the wear patterns on the ring lands and skirt, a piston applications engineer would be able to tell where more clearance is needed. These kinds of adjustments are normally made in increments of thousandths of an inch.
 
Not sure, to be honest. We just hear/see horror stories of gouged cylinder walls.

The fact that this hits motors with almost any mileage (though rarely under 40K) makes me think it is not really an "old age" thing.

Would a sick motor show higher silicon wear in the UOA?
 
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BTW, here's the specific of the motor:

Block: homogeneous monobloc low-pressure gravity die casting hypereutectic 'Alusil' aluminium-silicon alloy (AlSi17Cu4Mg) with a closed-deck design, mechanically stripped hard silicon crystal integral liners, honed under simulated mechanical stress; five main bearings; die-forged steel crankshaft

Head: cast aluminium alloy, five valves per cylinder, 40 valves total; lightweight low-friction roller cam followers with automatic hydraulic valve clearance compensation, 2x roller chain-driven double overhead camshafts (2xDOHC - two overhead camshafts per cylinder bank - 'quad cam')


It has technically been around since 1991 but was not the Alusil and only had 32V, with much lower power output.
 
I did dig up this info too:


Not really it is caused by piston swell. its just that it can effect the skirts more then other parts. Our pistons already have Side wall coating.
Other audi aluminum motors are not having the same issues such as we are. When you look at the piston damage on many scoring motors. You can see damage from the piston and its not limited to the skirt

You posted a picture of piston swelling with the scored cylinder walls, but the scoring goes all the way to the top of the cylinder and stops about an inch from the bottom, that means it's less likely to be swelling and probably caused by a to thight of a ring end gap, which usually happens on a high performance motor from not letting the motor come up to running temp before taking off, the rings end up expanding faster than the cylinder and couse the scoring. So those of you who had engine work done, how meany of you start your car and take-of in the morning without warming up.
 
Originally Posted By: dparm
Originally Posted By: demarpaint
I wonder if a UCL would help at all? Certainly a Band-Aid for a poor engine design. I wonder why they don't sleeve those engines with a steel sleeve? Or do they?


Upper cylinder lubricant?


Yes added to the fuel [I don't want to confuse anyone], something to think about, but if it is a poor design nothing is going to help.
 
Originally Posted By: dparm
Originally Posted By: chevrofreak
I would think that a thin ester based oil would help. They've found that thinner oils create thicker oil films around pistons and an ester based oil would cling nicely and also aid in heat transfer. I would be tempted to try Red Line 0w-20 or 5w-20.


Problem is an oil that thin will not even come close to the required HTHS of 3.5+.


Who is to say that they are correct in requiring such a high HTHS? Perhaps that is what is causing problems?
 
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