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Joined: 19 Sep 2018
Posts: 144
Location: Buckinghamshire

PostPosted: Fri Jan 11, 2019 9:18 pm    Post subject: Borescoring - Please settle an argument Reply with quote

I was having a discussion with a mate of mine about borescoring and I said that all bores have marks on them to a certain degree. He was trying to convince me that they should be as smooth as a baby’s bottom and anything other than that is impending doom..

Please settle this..

PS we were talking specifically about my 997 gen 1
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PostPosted: Fri Jan 11, 2019 9:34 pm    Post subject: Reply with quote

I'm no expert but my understanding is that some light streaking marks on the bores are normal. It's the skill of the technician to understand at what point they constitute scoring. Many stories on here of people getting their bores inspected and getting a thumbs down from people who aren't experienced enough to understand what normal marking constitutes of.
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alex yates
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PostPosted: Fri Jan 11, 2019 10:24 pm    Post subject: Reply with quote

Ask you mate if he has everlasting underpants.
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2006 Porsche 997 Carrera 2S

PostPosted: Fri Jan 11, 2019 10:34 pm    Post subject: Reply with quote

All cylinder bores, unless they are brand new and haven't been poked with a piston will have scoring to some degree, microscopic maybe but there will be some, even my Hartech bores Very Happy

Normal wear marking and bore scoring are very different but could easily be confused if you don't know what you are looking at!
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PostPosted: Fri Jan 11, 2019 10:36 pm    Post subject: Reply with quote

Thank you gentleman -1 pint to me.
997.1 C2S Cab Tip.
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PostPosted: Fri Jan 11, 2019 10:52 pm    Post subject: Reply with quote

show him the pictures and explaination that Baz posted all about borescore.
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Phil 997
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PostPosted: Sat Jan 12, 2019 12:02 am    Post subject: Reply with quote

theres a difference between borescuff which are very light marks and happen to all engines and the dreaded borescore as we know it on the 996 and 997.1 which are deeper. this is why a borescope should be done by someone who's experienced and can tell the difference. Thumb
911 Owners,Some Invest In The Future,
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PostPosted: Sat Jan 12, 2019 2:27 pm    Post subject: Reply with quote

alex yates wrote:
Ask you mate if he has everlasting underpants.

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PostPosted: Sat Jan 12, 2019 4:04 pm    Post subject: Reply with quote

Milo, I have no chance of SETTLING the argument though if you have registered and care to look me up in the members lounge bar I have expressed a few thoughts in page 3 of my post there..
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PostPosted: Sun Jan 13, 2019 1:27 pm    Post subject: Reply with quote

This is a difficult one because there is a huge difference between rub marks and scoring - except when looking at them on a camera.

If you have seen an old car with windscreen wiper marks across the front windscreen you will usually find no depth to them but they will show up less or more pronounced in different light. Similarly, the top of a ceramic cooker can be very difficult to make look smear or mark free even though there is no depth to the surface marks (I know to my cost as cleaning ours has somehow become "my job" in our household!).

Scoring is when something has dug into the surface and removed material leaving a depth that can be felt. Materials like Alusil and Lokasil have to be honed and this can cut back the surface of a piece of silicon leaving it tapered smaller where is sits in the matrix. Even oil and heat can gradually loosen it so age and higher mileage results in a greater deterioration rate and like "pot holes" once a depth has been created more particles often get knocked out and elongate it.

When honed some pieces are only held in by the bonding they had when formed and gradually work lose and fall out. Sometimes they disappear in the oil film between the piston and bore but other times they scratch up and down the bore and release more particles in a vicious circle that creates a long and relatively deep score.

Softer piston coatings are more likely to hold a lose piece of silicon longer to do more damage so earlier cylinders running with pistons with harder coatings last longer than later ones with softer coatings.

An Alusil or Lokasil bore is not as "shiny" as a Nikasil bore which reflects rubbing marks and makes them look like scoring but they usually have no depth to them and are not a problem. Furthermore Nikasil does not have particles coming lose out of a matrix but forms a homogeneous surface which is also oleophilic - probably the best bore surface you can get but likely to cause more confusion due to also looking worse than it is with a camera and lighting.

In any engine - during initial running in - in the first few minutes or miles - the surface of rings touch the minutely rough virgin bore surface and where they touch a little more than in other places can polish up marks in Nikasil. In other bore materials they usually remove that material instead but because they are not as reflective it does not look as much of a problem using a boroscope.

Different ring materials and surface treatments influence that running in period and lifespan. The older air cooled Porsche cylinders were mainly Nikasil and the softer rings that were fitted (especially to the 3.2) would wear down to nothing over say 100K losing bottom end power in the process but when the revs were over 3,500 there is not enough time for the gas to escape past the rings and full power could be felt as a sudden thrust that was not felt if the engine was rebuilt with new rings.

When choosing rings you can use softer rings faces that wear more quickly but polish bores less - or harder ones that can polish some even wide marks initially but then last almost indefinitely while giving good compression throughout the rev range bedding in even more gradually over a huge mileage.

So although Nikasil is recognised as a superb bore surface it is probably the most difficult to interpret bore marks unless you have the experience of stripping engines down (for some other reason like a winter race car engine rebuild) with visual bore marks only to find they had no depth or detriment at all.

Another issues however is when we fit liners for a set of crankcases for it to be rebuilt elsewhere and the builders failed to strip the inlet manifold right down to individual parts, clean then and rebuild them (to save then time and money). In that case of then small pieces of dust/debris (especially if the original failure was say a lose valve seat or normal bore scoring) can flow into the cylinders, sit on the ring tops and cause still minute marking but with perhaps a very small depth, usually not having cut into the Nikasil but perhaps squeezing it minutely away from its original position and squashing it into the aluminium beneath.

The Nikasil is so hard, strong and well bonded that sometimes we have even rebuilt racing engines with failed big ends and broken con-rods (especially when the owners tried to make them last 2 season) where our original Nikasil cylinders can be saved despite having actual pieces of metal in the upper cylinder during the failure.

Even an exhaust cat that breaks up can suck debris backwards into the cylinder on the valve overlap when decelerating in gear with the throttle closed as it creates a huge cylinder vacuum and back flow.

If unsure always get an expert in Nikasil to interpret pictures as they are unlikely to be a problem anyway.

I think the best advice is - "if it isn't broken don't try and fix it".

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PostPosted: Mon Jan 14, 2019 8:56 am    Post subject: Reply with quote

Baz's last sentence is the best advice of all imho.

I think too many people worry themselves about having a perfect 911 and spend vast sums on cars that would be ok for 000's of miles.
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PostPosted: Mon Jan 14, 2019 10:16 am    Post subject: Reply with quote

Thanks Montygraphics, generally the right way to go about ownership with the following proviso - some engines are "broke" although they are still running! (if broke means not to specification) and in these circumstances leaving it too long until it eventually stops can then cost more to repair - wasting money unnecessarily.

"Broke" I interpret not only has stopped and knackered - but also when the cylinders are scored, the piston coating has delaminated or worn off, the crankshaft shells have worn through the white metal and are running on the steel or bronze backing and starting to wear the crankshaft journals - but the car is still running etc.

Most of our repairs are after a failure that stops the engine but you will have read that a lot of engines were replaced around 35 to 50K while others around or over 100K. From seeing inside hundreds of these engines and measuring components we get a good idea of the average wear rates at the extreme ends of that variable.

Obviously engines driven harder more often or poorly maintained or running too hot or with thin oil will wear out sooner and there is unfortunately a random element in the quality and fit of some components that makes advice about expectations difficult - but despite this we know that someone wanting to keep their car for many years and many more miles that has already covered over 80K (say) needs to look out for signs of imminent failure to avoid that extra cost and for those that can afford it - a pre-emptive rebuild may turn out better financially overall - for which the added performance of our oversized engines may swing the balance.

This whole subject has a lot to do with the very simple "load/area" equation.

When new there are minute bumps in mating running surfaces - so any load is concentrated over a smaller area and load/area = penetration increases. After minute bumps are worn off - the load is shared over a larger area making the unit load less and the lifespan longer.

This is true of pistons, rings and crankshaft journals (not an exhaustive list) which is why the running in advice is not to "overload the engine" but let it rev freely under less load until those areas have "run in" better to accept and spread the loads over a wider area which the lubrication and surface finishes can cope with better.

The problem of Lokasil and Alusil is similar because any loose piece of silicon (from the bore matrix) sits between the piston and the cylinder in a thin oil film and when the engine is under load it is all concentrated on that small piece of grit rather than the whole of the piston and cylinder surface. With all the driving load on the piece of grit - if the piston surface is soft it can bed into it and then run up and down the bore causing more bits to free up and creating a bore score.

Alusil has a very much slower silicon particle release rate because the particles grow within the molten matrix whereas they are bonded into the Lokasil matrix - but both can eventually release bits.

The original hard iron piston coating used in the 944/968 Alusil bores resisted the eventual particle release well and they generally lasted well over 250K. The new Gen 2 (9A1) Alusil engines have a harder metallic coating than the earlier plastic coatings and also last well (perhaps not quite as well as the earlier harder coating - time will tell).

Nikasil does not release particles so can run without any piston coating (although a thin running in piston coating helps smooth the process and shorten running in time).

New rings also "run in" and when they do minute micron level particles of bore surface are rubbed away until the rings (under combustion pressure) spread the loads over a wider area of the ring - which then resists further wear - leaving the surface showing the original honing marks and some areas of polished lines. Unfortunately in Nikasil this process alters the reflectivity of the surface revealing patches that show where the running in has affected the smoothing of the surface and often looks similar to scoring to the unfamiliar but cannot be measured because it has worn off so little of the surface.

Our Nikasil bores are honed to a piston clearance and roundness many times more round and accurate than those we have measured even from New Lokasil bores, as a result the Lokasil has greater "running in" wear but the surface does not reveal this in reflectivity as much until actual bore scoring becomes evident (when it can still be difficult to see the difference unless the scoring is obvious). Subsequent wear is minimal in a Nikasil bore.

So - in conclusion - yes - sub say 80K, if it is running OK and shows no obvious signs of impending failure - with care and sensible driving it might last many tens of thousands miles more. Driven with a light throttle the engine may well still last for huge mileages. It is all a question of the owners driving style.

Buying that type of car to pose in, enjoy aesthetically or enjoy driving like a sports car when - over that mileage we know if it is driven like you stole it - there is a good chance wear is getting close to a failure limit. Even so with good maintenance and oil and modest driving the engines could still last a very long time. However if you enjoy the performance potential and can afford an earlier rebuild and/or fancy more performance - I do think there is a case in some circumstances to consider a rebuild sooner.

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PostPosted: Mon Jan 14, 2019 6:22 pm    Post subject: Reply with quote

From my own experience you have to consider other symptoms and not just vague visuals of the cylinder...if there was marginal marks but the car runs perfect and gives no other signs then nothing to worry about....the alarm bells for me was the oil pressure jumping up and down like a yo yo, drinking oil faster than the fuel and smoking heavily on one bank....the vague borescope was only a further indication....if I had encountered no other signs I would have driven it on, e;joyed it and would have had no reason to investigate further.
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PostPosted: Mon Jan 14, 2019 10:56 pm    Post subject: Reply with quote

I accept all the points made. I am in the treat it very carefully brigade and I hope my 997 stays running as well as it does for a long time.
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PostPosted: Tue Jan 15, 2019 7:37 am    Post subject: Reply with quote

Thanks for the great info. Sorry to go slightly of topic but it is related. I have a question for you regarding the best oil to use on a Gen 2 9A1 with 80k on the clock. The engine has always consumed oil at a rate of about 1ltr/2000miles. It has always run on mobil 0w 40. Would you use a thicker oil or stay with the same viscosity. There are no signs of smoke although the exhaust tips do get a bit dirty. Thanks in advance
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PostPosted: Tue Jan 15, 2019 10:19 am    Post subject: Reply with quote

Unfortunately if I start recommending particular oils it seems to open up a can of worms - so I am reluctant to do so - and I am not an expert in tribology only an observer and measurer of sizes of engine parts after covering various mileages - of which we have yet seen few Gen 2 9A1 engines to apply that to (which is a good sign of longevity).

However - generally - the Gen 2 engine is better made and although the surface area of the crankshaft shells is still much reduced compared to the older 944/968 models (in common with most manufacturers) - the clearances of the crankshaft shells are tighter than some previous engines - running with a better oil pump and oil pressure control system (raising oil pressures as the loads increase) - and my evidence to date supports my expectation that crankshaft shells will last longer than in the previous models so may well not benefit form a change in oil specification.

However over a long period of time the aluminium that sits at the cylinder bore surface (supporting small nodules of hard silicon) does gradually wash minute amounts away as a result of the oil continually washing against it and this leaves more of the hard silicon particles slightly more exposed than before and more becoming loose. Although the amount is incredibly small the difference may account for the shading you see in older 944/968 models at the main thrust area (where higher mileage ones can eventually seize especially with the 944 S2 which has tighter piston proflies) and so I think a step up to one grade thicker of whatever oil you prefer to use, after say 80K - might well prolong life even more.

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