bazhart
Barcelona
- Joined
- 20 May 2009
- Messages
- 1,343
It seems that most are understanding this rather odd phenomenon now of the effect of fitting a thrid radiator (see topics about low temperature thermostat and bore scoring etc) but some still think it is the piston clearance that is influential.
So to try and clear that up - it is not the clearance between the piston and the bore that causes bore scoring. Bore scoring only occurs on one side of the piston (the thrust side) - it is therefore a function of the thrust load. If it was down to clearances it would affect both sides.
According to the manufacturer - the size of the silicon particles is anywhere between 20 and 70 microns (Lokasil 1) and 30 and 120 (Lokasil 2) . In Imperial units (where a thou is one thousandth of an inch) this is between say 1 thou and 2.75 thou (Lokasil 1) and 1 thou and 4.7 thou (Lokasil 2).
I cannot find out when Likasil 2 superseded Lokasil 1 (or where they were used) but it is reasonable to assume later on and hence I am guessing Lokasil 2 was used around M97 engine change but whichever was used - the silicon particle sizes are significant.
Now pistons have a taper towards the top but this is largely to compensate for the expansion at the top (piston crown) under load. Top to bottom is around 4 thou (centre therefore 2 thou taper cold).
With clearances between piston and cylinder bore around 1.5 to say 2.5 thou - when you take into account the fact that this is shared both sides of the piston and that even under thrust load there is an oil film on the non thrust side that reduces the clearance on the thrust side - it is easy to see that a large particle of Silicon is little different in size to the remaining clearance left between the piston and the cylinder bore.
When the cylinder is manufactured in Alusil the silicon particles are mixed with the molten aluminium and are well bonded in place. With Lokasil they are first held in a sort of suspension with a binding filler that is only a small proportion of the material and then when the block is cast the molten aluminium under high pressure flows into the porous areas to bind on to the silicon. We think those particles are less well bonded in Lokasil than Alusil.
When machined and honed the particles that are bonded to the matrix but cut by the machining operation may be say 3/4 embedded in the matrix or only say 1/4 embedded - and this means some are encapsulated by the matrix (like a dovetail retention flowing round the particle) but others just rely on the bonding strength to prevent them falling out.
Once a particle falls out it is initially entrapped between the piston and the cylinder bore - rubs up and down for a while and eventually probably falls out of the bottom.
While it is rubbing against the piston - if the load is light then the oil film is thicker (because there is less force squeezing the oil film out) and the clearance between the piston and cylinder bore provides a space for the particle to flow between. If the load is high and/or the thrust load is high then the combination of thinner oil and higher loads will increase the rubbing effect on the piston face before the particle escapes.
Previously with Alusil (or with the first M96 engines) the pistons had a thin plated coating of zinc, hard iron and a flash of copper. This is much harder than the plastic coatings that replaced it in the later M96 engines and the 997 range and unlike plastic does not soften with typical piston temperatures (around 300 deg at the crown) - so the Alusil ferrous coated pistons and the early M96 engine pistons had a piston coating that resisted the wear from any detached silicon particles much better (and for much longer) than the later plastic piston coating (and don't have a bore scoring problem).
The plated ferrous coating also was bonded better to the adjacent coating whereas once a plastic coating has been cut into it tends to flake off a larger area. We have also experienced plastic piston coatings that have bubbled away from the bonding to the piston face but have not yet been cut into - where it is easy to imagine that the first particle that cuts into it will tear off a larger area.
Coatings appear either worn thin, bubbled or with pieces missing. Then once a lose silicon particle is embedded into the soft piston face it rubs against other silicon particles in the cylinder bore (yet to become lose) and soon sets up a sequence of particle degradation that escalates into a large score in classic catastrophe theory sequences.
The cylinder design suits replacement with a Nikasil alloy top hat cylinder that transforms the engine into a closed deck design and where no piston coatings are necessary and there is no resultant particle degradation. In our experience - steel or iron dry liners have numerous technical issues with clearances and differential expansion and contraction - while fitting alloy Nikasil liners without a top hat that supports the liner - results in the free open deck liner moving around under load and loosening its fit into the crankcase.
The best solution is a top hat wet alloy Nikasil liner (more properly referred to as a new cylinder) that converts the engine to a close deck design (as used in GT3's and Turbos) and has no problems with any fit into the cases (being a wet liner) and no differential expansion issues. This is what we have provided and developed for over a decade with outstanding results.
Meanwhile for standard engines still running with Lokasil bores - the only way to delay the bore scoring problem is to avoid high loads at low engine speeds and keep the oil film as relatively cool as possible (within the operating range) so it is running with a good oil viscosity.
The design with the thermostat on the engine entry makes the maintenance of consistent coolant temperatures around the cylinder thrust face difficult.
Bank two (where all the scoring occurs) has the thrust face higher in the coolant flow than bank 1 and so the temperatures in the critical areas are going to be higher.
Under normal driving it is still possible for a lose silicon particle to degrade the piston coating but anything to maintain a lower coolant temperature inside this part of the engine is therefore beneficial.
A LTT maintains a satisfactory coolant temperature in the critical part of the engine in almost all driving conditions except in extreme ambient temperatures driving hard and usually following other cars closely.
A third radiator lowers the temperature in those critical parts of the engine in extreme circumstances but raises it in most other normal conditions and for most of the driving hours.
A thermostat controlling a third radiator enables it to be useful in all ambient and driving conditions and is therefore beneficial.
The main problem with an uncontrolled third radiator is in cold weather conditions.
If you don't want to fit a third radiator control device - you could achieve an improvement by blanking off the third radiator in winter with a blanking shield.
Nothing will fix piston coating wear that has already taken place but these changes could extend life before repairs are needed.
Baz
So to try and clear that up - it is not the clearance between the piston and the bore that causes bore scoring. Bore scoring only occurs on one side of the piston (the thrust side) - it is therefore a function of the thrust load. If it was down to clearances it would affect both sides.
According to the manufacturer - the size of the silicon particles is anywhere between 20 and 70 microns (Lokasil 1) and 30 and 120 (Lokasil 2) . In Imperial units (where a thou is one thousandth of an inch) this is between say 1 thou and 2.75 thou (Lokasil 1) and 1 thou and 4.7 thou (Lokasil 2).
I cannot find out when Likasil 2 superseded Lokasil 1 (or where they were used) but it is reasonable to assume later on and hence I am guessing Lokasil 2 was used around M97 engine change but whichever was used - the silicon particle sizes are significant.
Now pistons have a taper towards the top but this is largely to compensate for the expansion at the top (piston crown) under load. Top to bottom is around 4 thou (centre therefore 2 thou taper cold).
With clearances between piston and cylinder bore around 1.5 to say 2.5 thou - when you take into account the fact that this is shared both sides of the piston and that even under thrust load there is an oil film on the non thrust side that reduces the clearance on the thrust side - it is easy to see that a large particle of Silicon is little different in size to the remaining clearance left between the piston and the cylinder bore.
When the cylinder is manufactured in Alusil the silicon particles are mixed with the molten aluminium and are well bonded in place. With Lokasil they are first held in a sort of suspension with a binding filler that is only a small proportion of the material and then when the block is cast the molten aluminium under high pressure flows into the porous areas to bind on to the silicon. We think those particles are less well bonded in Lokasil than Alusil.
When machined and honed the particles that are bonded to the matrix but cut by the machining operation may be say 3/4 embedded in the matrix or only say 1/4 embedded - and this means some are encapsulated by the matrix (like a dovetail retention flowing round the particle) but others just rely on the bonding strength to prevent them falling out.
Once a particle falls out it is initially entrapped between the piston and the cylinder bore - rubs up and down for a while and eventually probably falls out of the bottom.
While it is rubbing against the piston - if the load is light then the oil film is thicker (because there is less force squeezing the oil film out) and the clearance between the piston and cylinder bore provides a space for the particle to flow between. If the load is high and/or the thrust load is high then the combination of thinner oil and higher loads will increase the rubbing effect on the piston face before the particle escapes.
Previously with Alusil (or with the first M96 engines) the pistons had a thin plated coating of zinc, hard iron and a flash of copper. This is much harder than the plastic coatings that replaced it in the later M96 engines and the 997 range and unlike plastic does not soften with typical piston temperatures (around 300 deg at the crown) - so the Alusil ferrous coated pistons and the early M96 engine pistons had a piston coating that resisted the wear from any detached silicon particles much better (and for much longer) than the later plastic piston coating (and don't have a bore scoring problem).
The plated ferrous coating also was bonded better to the adjacent coating whereas once a plastic coating has been cut into it tends to flake off a larger area. We have also experienced plastic piston coatings that have bubbled away from the bonding to the piston face but have not yet been cut into - where it is easy to imagine that the first particle that cuts into it will tear off a larger area.
Coatings appear either worn thin, bubbled or with pieces missing. Then once a lose silicon particle is embedded into the soft piston face it rubs against other silicon particles in the cylinder bore (yet to become lose) and soon sets up a sequence of particle degradation that escalates into a large score in classic catastrophe theory sequences.
The cylinder design suits replacement with a Nikasil alloy top hat cylinder that transforms the engine into a closed deck design and where no piston coatings are necessary and there is no resultant particle degradation. In our experience - steel or iron dry liners have numerous technical issues with clearances and differential expansion and contraction - while fitting alloy Nikasil liners without a top hat that supports the liner - results in the free open deck liner moving around under load and loosening its fit into the crankcase.
The best solution is a top hat wet alloy Nikasil liner (more properly referred to as a new cylinder) that converts the engine to a close deck design (as used in GT3's and Turbos) and has no problems with any fit into the cases (being a wet liner) and no differential expansion issues. This is what we have provided and developed for over a decade with outstanding results.
Meanwhile for standard engines still running with Lokasil bores - the only way to delay the bore scoring problem is to avoid high loads at low engine speeds and keep the oil film as relatively cool as possible (within the operating range) so it is running with a good oil viscosity.
The design with the thermostat on the engine entry makes the maintenance of consistent coolant temperatures around the cylinder thrust face difficult.
Bank two (where all the scoring occurs) has the thrust face higher in the coolant flow than bank 1 and so the temperatures in the critical areas are going to be higher.
Under normal driving it is still possible for a lose silicon particle to degrade the piston coating but anything to maintain a lower coolant temperature inside this part of the engine is therefore beneficial.
A LTT maintains a satisfactory coolant temperature in the critical part of the engine in almost all driving conditions except in extreme ambient temperatures driving hard and usually following other cars closely.
A third radiator lowers the temperature in those critical parts of the engine in extreme circumstances but raises it in most other normal conditions and for most of the driving hours.
A thermostat controlling a third radiator enables it to be useful in all ambient and driving conditions and is therefore beneficial.
The main problem with an uncontrolled third radiator is in cold weather conditions.
If you don't want to fit a third radiator control device - you could achieve an improvement by blanking off the third radiator in winter with a blanking shield.
Nothing will fix piston coating wear that has already taken place but these changes could extend life before repairs are needed.
Baz
