Bore Scoring - which models?

Bore Scoring - which models?

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Discussion

Candellara

Original Poster:

1,310 posts

138 months

Saturday 19th January 2013
quotequote all
After reading alot of posts about this subject and a few conversations with Porsche dealers it would appear that bore scoring is more prevalent in only some models?

Is this true? I am of the opinion that the most common engine affected is in the standard 997 3.8 engine?

3.6 engines dont suffer as much as the 3.8 and GT3 and Turbo cars are generally unaffected?

premio

1,020 posts

120 months

Saturday 19th January 2013
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My 2.9 gen 2 cayman on an 09 plate had it, all 6 cylinders after 6k miles. No idea what the actual engine is called though. The engine was written off and replaced.

I've heard of cayennes with the same

Candellara

Original Poster:

1,310 posts

138 months

Saturday 19th January 2013
quotequote all
Have any 996 (non metzger) engines been affected?

premio

1,020 posts

120 months

Saturday 19th January 2013
quotequote all
Candellara said:
Have any 996 (non metzger) engines been affected?
I think the 2004 onwards ones have had problems yes, there are some references online

s_mcneil

896 posts

151 months

Saturday 19th January 2013
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Read the Hartech guides, affects all 996 carrera engines.

paulmoonraker

2,843 posts

119 months

Saturday 19th January 2013
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s_mcneil said:
Read the Hartech guides, affects all 996 carrera engines.
And Cayman S engines, and Boxster S engines, and 997 engines... It seems to be more common with larger cylinder sizes.

itsybitsy

3,964 posts

141 months

Saturday 19th January 2013
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[quote=premio]My 2.9 gen 2 cayman on an 09 plate had it, all 6 cylinders after 6k miles. No idea what the actual engine is called though. The engine was written off and replaced.

this is more to do with the piston rings which porsche upgraded 3 times on the 2.9
most pre 9A1 engines ie m96/m97 engines pre 2009 suffer at some time although the 2.7 does not seem to suffer and the 3.8 most affected but can happen to 3.6 and 3.4 boxster/cayman.the new gen2 cars ie 2009 onwards have different liners and are not known to suffer although i think i read according to hartech the rings coating may wear and cause scoring at high mileages 100k+


Edited by itsybitsy on Saturday 19th January 19:26

Pope

2,359 posts

203 months

Saturday 19th January 2013
quotequote all
itsybitsy]remio said:
My 2.9 gen 2 cayman on an 09 plate had it, all 6 cylinders after 6k miles. No idea what the actual engine is called though. The engine was written off and replaced.

this is more to do with the piston rings which porsche upgraded 3 times on the 2.9
most pre MA1 engines ie m96/m97 engines pre 2009 suffer at some time although the 2.7 does not seem to suffer and the 3.8 most affected but can happen to 3.6 and 3.4 boxster/cayman.the new gen2 cars ie 2009 onwards have different liners and are not known to suffer although i think i read according to hartech the rings coating may wear and cause scoring at high mileages 100k+
EFA

Helicopter123

8,122 posts

112 months

Saturday 19th January 2013
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Just to clarify, the 997.1 can be affected but NOT the the 997.2 that has the newer Dfi lump.

metsta

425 posts

179 months

Saturday 19th January 2013
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997.2 is from 06 on? carrera and s?

Helicopter123

8,122 posts

112 months

Saturday 19th January 2013
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2010 I think

hartech

1,873 posts

173 months

Saturday 19th January 2013
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Lokasil was a new silicon based bore material that is similar but not the same as Alusil, but which was regarded and reported as the same by the manufacturers and sub contracting suppliers - which we now question.

As a result - the piston coatings that were found to be reliable in Alusil were assumed to work as well in Lokasil and did so initially until the piston loads were increased with the introduction of larger capacity examples and variable valve lift increasing low speed loads and torque. Unfortunately at the same time as the piston loads were increased - so the coolant jacket capacity was reduced and the piston coating was changed to a more environmentally acceptable and less expensive alternative. This moved three factors on a converging path - higher piston wall temperatures and loads together with a less durable piston coating. During this period the specification of the Lokasil silicon distribution in the cast in preform was altered resulting in there being a Lokasil 1 and Lokasil 2 specification. It was found to be a very expensive process and so I can only guess that this change was to reduce costs (but cannot verify that and may be wrong).

With the original piston coating the Lokasil material proved very long lasting. For some reason the cylinder tubes were made thinner and weaker on engines larger than 3.2 capacity gradually distorting into an oval shape that increased bore clearances on the thrust side and increased blow by and piston temperatures with time while reducing oil film strength. Eventually this lead to cylinder cracks when the elastic limit of the bore tube was exceeded or the fatigue effect of multiple cylinder pressure changes started crack initiation.

Despite all this it was almost OK (most examples statistically lasting a reasonable length of time) but as the safety factor had been reduced - so the limits were reached and exceeded in some circumstances of driving, quality control and perhaps even general bad luck. Furthermore as mileages increased - so did bore clearances (not through wear but through distortion) - while radiators and coolant pumps became less efficient - all contributing to moving that narrow new reduced safety margin in the design closer to a failure limit.

With two banks of three cylinders on opposite sides of the engine the coolant path is different between one side and the other. On bank 2 (cylinders 4, 5 and 6) the coolant is hotter on the thrust side of the bore (where almost all of the scoring occurs). Furthermore the oil spray jets on that side of the engine distribute cylinder lubrication more to the non thrust side - so there are two differences here that make bank 2 more vulnerable by reducing the effectiveness of the oil film on the piston thrust side by different distribution and a higher temperature lowering the viscosity.

Bore scoring is usually caused by either too much load at low revs for the piston coating and the oil film strength between it and the cylinder wall material - to resist the abrasive nature of the Lokasil bore material or by wearing through over a longer period - exposing the piston base alloy to that bore material insufficiently lubricated to survive - although it does seem generally OK on bank 1 (apart from eventual cracking - which we can prevent by fitting restraining rings).

Early engines like Boxster 2.5, 2.7, 3.2 and the early 3.4 996 were OK. Later 996 3.4 engines could fail but didn't create enough torque to do so in numbers. (and had KS or Mahle pistons with a different piston coating).

Later engines (3.6, 3.8 Boxster 3.4 and Cayman engines all had the newer piston coating from Mahle.

To trace through the gradual increase in failures through piston scoring you can see that the following changes also occured.

Some of the issues that make bank two more vulnerable worked OK with the original piston coating. The hotter thrust face coolant temperatures, coolant jacket capacity, unbalanced oil spray jet distribution, reduced coolant flow etc - seem to become a lot of contributory changes all conspiring to work together to make the later and larger engines less likely to work reliably anyway - whatever piston coating was employed. It was as if as loads were increased other changes made them more likely to fail rather than increase reliability to cope with those extra loads applied. Different piston coating, different Lokasil specification, higher coolant temperatures, unbalanced coolant flow etc.

In addition any engine with a worn (but useable) coolant pump will flow less at low revs (when scoring through low speed piston loads are highest) or a leaking radiator or pressure cap (that lowers coolant bubble formation temperatures as the boiling point lowers with pressure reduction) will move an already vulnerable engine closer to a failure point or increase wear rates and reduce serviceable life.

All air cooled engines (apart from very early examples) had Nikasil bore coatings on alloy (as did the later turbos and GT versions) and these work well with almost any piston coating. When we repair engines with damaged or cracked bores we use a manufactured alloy cylinder with Nikasil bore finish in exactly the same way as all those engines that did not have bore scoring problems.

The 924S, 944 and 968 used Alusil bores with the same piston coating as did the early successful Boxster 2.5, 2.7 and 3.2 S and early 996's. It is difficult co compare how those engines now last with newer coatings that some piston specialists provide as they were well cooled and most with replaced pistons have not yet covered enough miles to judge the outcome.

Alusil (as used in Gen 2 engines) does seem to work Ok with the newer type of piston coating (also in use) although some Cayennes with this combination have scored despite the lower loads evident. I think the new piston coatings will not last as long even in Alusil as the older coating did - but it is proving difficult to find anyone to apply that original coating these days. I guess if the coolant paths and flows are corrected so that they do not result in one bank running basically hotter than the other - plus using a closed deck design (as used now in Gen 2 engines) they probably will last OK.

Despite all the changes - bank 1 seems to survive well (apart from the gradual ovality that will increase piston temperatures and blow by) so during a repair - we offer to change all cylinders to a closed deck design and re-round those that have become oval - and increase the coolant flow to the cylinders and provide a lower temperature thermostat. All these changes are designed to move the original design parameters of the whole engine to be more like the engines that worked OK and away from the changes that made them less reliable - and the results have been extremely successful.

Many customers now prefer all 6 cylinders to be replaced with the bullet proof closed deck Nikasil alloy cylinders and this is still possible well inside the cost of a replacement new engine that still incorporates those weaknesses in design.

It would of course be less expensive to close the deck and re-bore the original Lokasil cylinders and we have been working on this solution for several years with different piston suppliers and coatings.

It takes a long time and high mileages to be sure this alternative is good enough to offer the general public and to help we have track tested our ideas (to speed up wear rates and analysis and conclusions (as a race engine will show up weaknesses quicker than anything else) and road tested these options for around 2 years or 24K. This has been on-going for several years and still we are trying newer solutions are under test.

Results have been too mixed to lead to a permanent conclusion yet (with 3.6's seeming OK but still a few problems with some 3.8 versions and their extra torque at low revs).

We will not offer the general public solutions that we are not yet 100% confident in and have managed to test for long enough and high enough mileages to be sure of the outcome - so all our present rebuilds have individual or 6 cylinder replacements with closed deck alloy Nikasil plated bores - or a mixture of Nikasil and Lokasil (with closed deck options) etc but the full 6 cylinder Nikasil solution is best long term - but more expensive.

Despite claims to the contrary by our competitors and the inclusion of different cylinder materials in the Porsche Championship regulations - all our 100% reliable Boxster S racing engines in 2012 had original Lokasil bores - proving that the other changes can make these engines survive extreme conditions. This will not necessarily be the case if we race 996's one day - that jury is still out and time and testing in the future will tell.

To reduce costs some competitors are fitting steel liners claiming that this is OK because many racing engines use them - but those racing engines do not incorporate the in-balance in coolant flow and thrust face temperatures - and have cylinder coolant capacities and head sealing systems to allow for the differential expansion that occurs. Furthermore steel does not transfer heat as well (one of the problem issues especially with bank 2) so it is no surprise to us that we are increasingly re-manufacturing engines recently fitted with steel liners elsewhere - so this alternative seems to us to be a short term false economy.

At some time during the next 12 months we will have completed high mileage tests on two cars and three different new piston coatings and a mixture of Nikasil and Lokasil bores - to add to the results we already have for two other piston coatings in several different cars and engines and can then decide which is best and what the technical issues are with each alternative.

We wish we could do all this sooner but if the original manufacturers did not provide a solution in over 10 years - with all their resources - we hope the public will be patient and understand that if the original problems took several years to materialise and then only in a relatively small number of engines - it takes a long time and a huge effort to cover enough tests and be sure of the results.

I think it is fair to say - we have done far more than any other business Worldwide to help owners with good service and reliable affordable solutions with cost options and a trustworthy guarantee and back-up.

This examination of the problem and testing of alternatives has been hugely expensive and frustratingly time consuming for us (buying cars to test, stripping and rebuilding engines, having special small quantity parts manufactured, road testing etc) but we anticipate numbers rising as more cars reach higher mileages and age and during this expected increase in rebuild numbers we have installed an engine dyno (to enable high running test hours efficiently under IN HOUSE test conditions) and CNC machining capacity to our existing internal precision machine shop) to maintain the highest quality, quickest turn round times and keep costs reasonable - expecting to recover those development costs eventually through increased numbers.

For those interested in finding out more - there is additional information and photos on our web site www.hartech.org under engine rebuild sections and buyers guide sections 4 and 5.

Baz












Edited by hartech on Saturday 19th January 14:19


Edited by hartech on Saturday 19th January 15:06

Trev450

5,932 posts

128 months

Saturday 19th January 2013
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As always, Baz, a very enlightning post.

It's comforting to know that there are skilled people such as yourself out there should the worst happen.

jvr

746 posts

203 months

Saturday 19th January 2013
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But what would you term as low revs and how much throttle?
P

paulmoonraker

2,843 posts

119 months

Saturday 19th January 2013
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Temendous knowledge and insight Baz...

Candellara

Original Poster:

1,310 posts

138 months

Saturday 19th January 2013
quotequote all
Baz

Insightful post.

Does this mean that Turbo and GT3 cars are unaffected?

Porkupine

1,642 posts

121 months

Saturday 19th January 2013
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Candellara said:
Baz

Insightful post.

Does this mean that Turbo and GT3 cars are unaffected?
Yes.

blade7

9,528 posts

172 months

Saturday 19th January 2013
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hartech said:
The 924S, 944 and 968 used Alusil bores with the same piston coating as did the early successful Boxster 2.5, 2.7 and 3.2 S and early 996's. It is difficult co compare how those engines now last with newer coatings that some piston specialists provide as they were well cooled and most with replaced pistons have not yet covered enough miles to judge the outcome.


Baz
Would it be a more durable option to Nikasil plate the 944 bores and more cost effective than fitting plated liners ?.

Waitforme

592 posts

120 months

Sunday 20th January 2013
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Baz, potential numpty question..... Why does it matter what coating the pistons have ? I thought the only part of the piston that touched the bores were the rings ?

itsybitsy

3,964 posts

141 months

Sunday 20th January 2013
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i think i can answer that.the coating comes off under heat/stress hence causing bore scoring.over to hartech