Bore scoring- so what actually happens?

Keep hearing about bore scoring... whilst an IMS failure is an obvious thing, what happens if the bores are scored? Read a Hartech (I think) comment that all gen 1 3.6, 3.8 M96/7 engines will have scoring to a greater of lesser extent at some point (apologies and feel free to discredit if I've misquoted). Yet by no means all have needed to have rebuilds. So what actually happens... Do they need more oil? Does the engine suddenly go pop one day? Does it lose power gradually?

This is a very complex subject - so let me try and be brief about explaining bore scoring. ALUSIL crankcases (924S, 944 & 968) were aluminium with silicon (small hard particles) mixed in at a higher proportion than could be absorbed in a solution - so some formed hard nodules that were very evenly distributed and securely stuck within the matrix creating a hard bore surface. But the silicon was throughout the casting making it expensive to machine but very strong and stable - usually lasting well over 250K. The pistons had a hard thin ferrous coating that resisted wear from the silicon particles standing slightly proud in the in the cylinder bore surface - very well. If they ever failed it was almost always the rear cylinder 3 or 4 that went first (usually because the head gasket had rotted and short circuited the coolant so cylinders 3 & 4 ran hotter) because the higher temperatures made the oil film between the piston and bore thinner allowing the piston to impinge on the bore harder and any lose silicon particles that became detached from the bore to rub harder against the ferrous coating and eventually wear through it (and this has been tested and proven over many years with a magnet in scored piston areas). LOKASIL was described as being much the same as Alusil but was actually quite different having slightly larger silicon particles held in a porous - pre-formed cylinder tube with molten aluminium forced through the porosity during high pressure casting in metal tools - to entrap the silicon as it solidified. The resulting crankcase only has silicon near the cylinder bore (so was cheaper to manufacture) but the cylinder tube was weaker than Alusil (and gradually distorted oval) while the silicon particles were not only bigger but also less secure and became detached at a higher rate or lower mileage. The cylinder block used in Alusil has 100% of the coolant passing into it before the cylinder heads (helping the oil film to stay thiker) whereas the M96 and M97 engines had only around 10 to 12% of the coolant passing into the cylinder block - and so in some situations could allow the cylinder block to run hotter and the oil viscosity (and the resulting film thickness) to be thinner. The first Lokasil engines (Boxster2.5 up to most 3.4 996's) also had the same hard ferrous coated pistons (previously used in Alusil made by the same manufacturers of the Lokasil crankcases) that despite the potentially higher cylinder temperatures resisted the higher silicon degradation rates better. Towards the last 3.4 996's the piston supplier and specification was changed to softer plastic coated pistons (similar to the aftermarket ones that were generally not quite as long lasting as the original ferrous coated ones in the better cooled Alusil examples). The combination of less coolant volume/flow, bigger more powerful engines (that followed), probably larger silicon particle sizes (in Lokasil 2) and plastic coated pistons reduced the lifespan further and added scored bored to the probability of eventual distortion cracks later on. It also seems likely that the distribution of silicon in Lokasil is not as even as the silicon undisolved in Alusil and not as strongly secured - while the tube created in Lokasil is less strong than the same sized tube in Alusil - migrating oval in the thrust direction, increasing piston blow by and surface temperatures, reducing oil film viscosity and cracking or "D" chunking. Cars driven less aggressively (with lower torque at lower revs) will run cooler at the piston surface and keep the oil film thicker and apply less load to disturb the silicon particles - so will last longer although the variation in silicon distribution in Lokasil - may add a random risk to reduce failure predictability. It all means that even if a car has not suffered bore scoring yet - it could still do so in the future (sooner than that expected with "older" models) - although some could well last a long time if the material composition in all 6 Lokasil cylinder tubes in that engine was above average, the driving styles applied were suitable and some additional preventative measures were applied (like a LTT and avoiding high torque at lower revs particularly after a pause that creates heat soak). Gen 2 engines corrected many of those Gen 1 problems by reverting to Alusil, closing the deck (to stop distortion) and running more coolant through the cylinder - but the pistons still have the less durable plastic coating and so although they may last longer than Gen 1 cylinders - they would be unlikely to last as long as the original Alusil engines with ferrous coated pistons. Meanwhile the engines made with Alloy Nikasil bores (all air cooled engines for decades and water cooled GT3 and Turbo engines) have closed deck (and/or supported cylinders), more coolant (or air) flow and no bore/piston problems (and could run with almost any piston coating as there is no silicon particle degradation with Nikasil and a naturally oil retaining surface) - all just the same as our own Nikasil alloy cylinder replacements. Older 996' with ferrous coated pistons suffer from cylinder cracks and "D" chunking. Newer cars with plastic coated pistons will score and when they do the cylinder clearance increases so although the blow bye also increase (and oil consumption) they will usually keep running until the piston tilts at TDC and taps on the cylinder head. Drivers never getting their engines hot nor driving with high torque will usually delay scoring for longer. You can see pictures of scored pistons on our new video ( www.hartech.org ) or our buyers guide section 4 an 5 www.hartech.org . Baz Baz

TL;DR: Plastic piston coating comes away, scratches bores. Engine starts using oil, get sooty tailpipes. You might hear a ticking as the piston skirt hits the bore. Eventually down on compression therefore less power, fail MOT emissions.

Good concise way to cover it wasz but not strictly accurate. You are right that In a few examples plastic coatings do bubble away from the piston face from the inside but it is the silicon particles in he cylinder wall Lokasil that come away from the bore and then scratch or puncture the plastic coating - usually just wearing it off. The plastic is too soft to damage the Lokasil cylinder bore - it is the silicon itself stuck now on the soft piston face or trapped between it and the cylinder wall that impinges on the cylinder bore pulling more particles lose and starting a sequence that ends up as a score. Knocking noises start when the piston face diameter is smaller than it used to be and allows it to tilt as it goes over TDC and the edge of the top of the piston taps the cylinder head (and the evidence can be seen on the top of the piston and the head where it wears the carbon away). The higher the piston face thrust load the thinner the oil film separating the piston from the cylinder wall. The hotter the piston and cylinder wall the thinner the oil viscosity and the smaller that gap is. If the piston coating is harder and the lose particle smaller than the oil film thickness it may work its ay out to be trapped by the oil filter but if the particle is large and the film thin that is when the damage is greater. Don't forget that all that time the cylinders are gradually going oval - being insufficiently supported to prevent "creep" under load. It would be good wasz if you could put that into just a few words (knowing there is still more to this problem than I have written about makes my contributions seem long but they are also actually a snapshot and leave out other contributory factors of what I could write. Baz

Ha! Thats a challenge. How about: This begs two questions: a) why do the silicon particles come away from the bore in the first place? Natural wear? b) why don't ferrous coated pistons (early 3.4) trap this silicon - is it because the silicon does not get trapped in the harder piston coating? As you will know its very difficult to summarise complex concepts impart the key points to non technical minds, who won't bother to read or won't absorb lengthy texts. To help make points more succinctly, I found "The Elements of Style" by Strunk and White pages 18-24 useful.

Well done go to the top of the class! With Alusil the hard silicon particles are mixed in with the aluminium in a greater proportion than the aluminium can hold in a solution so a smaller proportion grow into small hard particles inside the matrix and are therefore very well bonded into place as they form within it. With Lokasil silicon particles are suspended in a porous binder material so when the molten aluminium is sent into the metal moulds under extreme pressure it flows around them as it melts the binder and sort of flows around the particles rather than that they are formed within it and as a result they are in our experience less securely bonded in the first place. Plastic softens with temperatures (piston crowns around 300 degrees C) but an electroplated hard ferrous coating does not and anyway is around twice as hard as plastic when the plastic is cold and much better boned to the piston surface. The particles therefore take a lot longer to wear it away or impinge on it. The maximum size of the silicon particles in Alusil are smaller than in Lokasil were they may be bigger than the gap between the piston and the bore. Nikasil is completely different and the Nickel and silicon are so well bonded together they form almost a homogeneous thin circular film of hard surface with particularly clever oil retention properties well bonded to the aluminium cylinder and can run with un-coated pistons because there are no lose particles to deal with - but is is more expensive to manufacture. Nikasil was used by Porsche (and many others including ourselves) for years without issues. Alusil worked with ferrous coated pistons. Lokasil doesn't last as well wit plastic coated pistons. The jury is out on Alusil with plastic coated pistons or the new type of ferrous coated pistons that are legal to produce - but will probably be a great improvement (Gen 2). Baz.

That's a hive of information and very informative. So if the jury is out on the gen 2 ferrous coated pistons, when do we expect the jury to return with a verdict? Surely quite a few Gen 2 cars have had time to clock up sufficient mileage to enable problems to manifest themselves by now. Or could it be that the gen 2 engines are just so much better that problems aren't being reported and no news is good news?

You are right that Gen 2 engines are very much better and will eliminate a lot of potential problems in the Gen 1 versions. If the ferrous coating was the same as in the very early 944 engines I would have no worries. During our research we had pistons coated in several different materials including a new type of ferrous coating and ran them all in Lokasil. They are still under test but on the last strip showed signs of scoring although not damaging the bores. We have reliable information that a few Gen 2 have scored bores - so although Alusil we regard as better than Lokasil - I naturally have a question mark over that newer ferrous piston coating (and have reports that some Gen 2 had plastic coatings - but cannot substantiate that yet. When the original bore scoring issue emerged it similarly took some time to reveal itself and much longer to understand the causes. I would be surprised if it became a big problem in Gen 2 engines but would like feedback if anyone has such failure - to help see if it is worth investigating a solution early or not. Baz

Great thread, lots of info. Barry do you have any wise words for owners with engines that have suffered bore score due to silicon particle dislodge with regards to other cylinders within that same engine? My take is that if a cylinder block has failed in this manner then it would be very wise to line bore and sleeve all cylinders within at least that bank if not the entire engine... It's surely a false economy to line 1 or 2 cylinders in a bank of 3 as the entire engine requires dissassembly and you made mention of bad batches of Lokasil.. Is the same engine not quite likely to suffer the same issues in non lined cylinders or is the effect of going to a LTT and a set of new pistons (non plastic coated for example) sufficient to rule out the same failure again? Does going to a closed deck prevent the particle dislodge issues? :thumb:

My understanding is that its only a couple of cylinders have poorer cooling than others, it is these that get the bore score because oil film thickness is contributory (i.e. the oil film is thinner when hotter and so tolerance is increased, and cannot contain the particles in solution to wash them away). The water jacket mods and LTT address this somewhat. The other cylinders are just about cooled OK in normal use so should be fine.....hence some people choose not to do them.

The flip side being if you got an engine casing from a better batch of Lokasil perhaps there wouldn't be failure at all??

Thanks Baz, that is an excellent summary of the 'science bit', and interesting to see the differences over the years. From a layman's perspective, I'm still not entirely clear; is it fair to say effects will be... - sooty/oily tailpipe - excessive oil consumption (is there a 'normal' figure to base this on?) - knocking noise at idle - gradual degradation in power as cylinder compression is lost Is fitting a Low Temp Thermostat a big/expensive job?

Fitting an LTT is an easy and cheap job and one that should be done on all 3.6 - 3.8, 996's and 997's.

Hi - Sorry to resurrect such an old thread but I want to check if the information/knowledge has changed in the last decade or so, and also that I have understood it correctly. I have a 55K 997.1 3.8S with a slightly sooty left exhaust tip and an annoying ticking. I haven't bore scoped it but it is almost certainly bore score, if fact the local Porsche specialist said he could save me the cost of the proper check by confirming it from what he could see an hear. I am not too fussed as I planned to upgrade to a (Hartech) 4.1 when I bought it. What I want to know is what will happen if I drive it while I'm saving for the new engine and what I can do to minimise damage during this period. If it is just sooty/oily tailpipe, excessive oil consumption, knocking noise at idle, gradual degradation in power as cylinder compression is lost then I will just change the oil and filter every few thousand miles and enjoy it while I save. If it is going to explode dramatically destroying all buildings in a 5 mile radius (as some seem to suggest) then I might be more cautious.

The damage is already done. Continuing driving it may make the scoring worse but with regards to the rest of the engine you should be fine. Just make sure your fluids are always at the correct level.