Porsche 996 3.4 bore to cylinder clearence

Hi Everyone!! Im looking for the min and max clearance cylinder to bore , I've had my piston skirts Teflon coated and want to check that there is still enough clearance, they basically apply the coating and then it's down to me to ensure they are down to correct size buffing down the coating. I'm sure someone knows, I idd get a general sizing from someone of 3 thou per inch diameter of cylinder bore but I've got a feeling it will be tighter on the Porsche. Any help much appreciated!! Thanks !!!!!

Quotes source post: 11980440

Not too sure I'm ok with this. If the pistons are stock the teflon coating will have to come off to get the clearance you need. Assuming the pistons are not stock, but undersize to allow for the growth from the teflon coating then you should size the coated pistons to the same diameter as the stock pistons. The above assumes the bores are stock and have the correct diameter. If you remove to much teflon what's left can flake or peel. Oh, I don't think you are going to have much luck buffing the teflon down as this method of material removal has too much variability. The way to remove the material is to machine it away or better yet to get the proper surface finish and better control the amount of material removed precision grinding is called for. BTW, the stock pistons are coated with some kind of iron. Not sure what teflon buys you. As for the clearances Porsche doesn't publish the piston/cylinder clearances. You might send an email to Jake Raby and he might share with you what he knows. IIRC he believes this info to be a business confidential -- I can understand his thinking on this -- so he might not be willing to share, but ask. All he can say is no.

Noz, Teflon? Are you sure? I have just had my skirts(actually the pistons skirts !) coated and the routine is that YOU tell the 'coater' what finished size you want. That is where the fun(and skill) starts .You need to measure to less than 1 thou accuracy the ovality and taper of each cylinder. If they are within spec (anther problem -what are the specs?), decide where in the size range you want the o.d. of the coating to be. If you follow all that is implied with what I just said ,you will have many,many questions and doubts. Those answers are way beyond the skill level of someone like me who is competent to rebuild 'normal' engines -like Cummins,Jaguar,Miata etc. The difficulty is that you get so deep into the rebuild that you just have to make an (uninformed) choice. My kingdom for - a book about rebuilding the M96.............

Since Macster has come to your rescue, please ignore all I said and follow his time-proven advice. I'll leave you with one hint: search for "abradable piston coating" BTW the stock coating IS iron -yes, really!

The Pistons are stock and the coat is between 1 and 2 thou thick , the coater has said to me that as long as I buff down by hand evenly and identically across both sides of the piston it should be ok, any slight irregularities will be addressed as the engine beds in, just leaving behind a sort of filling for any previous wear but I'm sort of wishing I'd left it alone. Macster, think I will buff most of it off and leave a slight coating or try to get it machined of , other option is a good set of used Pistons from my local supplier and stick these in the bin???

Never adjust clearances for coatings. This is Hi Performance coating 101. If the coating is consistent at .0005" you'll have no issues.

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Assuming the cylinder bores have the stock diameter if the pistons are also stock with that much teflon to get the proper clearance you will have to remove most if not all of the teflon. In a previous career (machinist) where removing material was my primary job function. I can assure you you will *not* remove the material evenly by buffing. Even if you manage to get the teflon reduced in thickness by some other method of material removal (buffing doesn't really remove material, it repositions what is there to smooth the surface) the remaining teflon material will be prone to flaking once the pistons are in use. If you insist on using the pistons with their coating of teflon then you need to 1) ensure the piston diameter is precise and consistent and that the pistons have the same geometry as the stock pistons with the same (if any) ovality or taper; 2) the cylinder diameter is increased to provide for the proper piston/cylinder wall clearance, with the proper clearance possibly affected by the presence of teflon. That is the presence of teflon may require more (or less) clearance than the stock piston/cylinders have. The cylinder wall finish may need to be customized, too. The coating company should be able to advise you regarding any special piston/cylinder wall clearance or cylinder wall finish requirements. Really though, for the best chance of a successful outcome, my advice is you bin these coated pistons and fall back to the stock pistons provided the cylinder bores have their stock diameter and stock pistons in these stock bores will result in at least what the factory believes to be the best piston/cylinder fit.

Hi Macster/jake Think I'm going to play it safe and pick up some good used stock Pistons and put it down to experience, anyone interested in a Teflon coated ashtray!! Just for curiosity, does anyone know the stock measurement of a new 3.4 piston, I want to measure the used set I get and asses wear against new , new size out the box and smallest useable size would be a great help. Thanks guys!!

I always thought you were supposed to measure new pistons and then size bores accordingly.

Ok so I've buffed the coating as advised by the coater and I'm getting 95.92mm on the skirt with the calipers, does anyone know how this compares to standard?? Thanks Lee

According to what size grouping your pistons were to begin with, that could be very close to stock..... BUT calipers are not precision measuring devices and can't be trusted for this measurement. You need a real micrometer and a bore gauge. Measure the piston and then use that measurement to set up the micrometer to be the standard for the bore gauge. Then measure the bore in 3 axes and determine your true running clearance. It can't be accurately done in any other way .

Ok I've done all bore to piston measurements No1 piston 3.778" No1 Cyl 3.781 gap .003" No2 piston 3.777" No2 Cyl 3.782 gap .005" No3 piston 3.778" No3 Cyl 3.782 gap .004" No4 piston 3.777" No4 Cyl 3.781 gap .004 No5 piston 3.777" No5 Cyl 3.780 gap .003" No6 piston 3.777" No6 Cyl 3.781 gap .004" Jake , do you think these are ok to build back up?? Thanks Lee

Jake Measurements above still with calipers, only just seen your reply, I'll have to also check with mic/ bore gauge If measurement is same do the measurement look ok??

Code on piston is +96P21

piston to cylinder clearance is usually finished by a final cylinder hone.i have never buffed clearance on a piston.imo your "piston coater" should have given you a spec. wiseco [iirc] offered coated pistons years ago for harleys........they sucked. old school piston/cylinder clearance is .001 for every 1 inch bore. .005 imho is too much for a rebuilt motor. do you plan on keeping this car long ?

Quotes source post: 11984304

Those clearances look a might large to me. For all but one engine rebuild I had the block bored oversize and fitted with oversized pistons. The clearances were obtained with the automotive machine shop having the new pistons to use to size the cylinders. Also, the shop and I had access to official piston/cylinder wall clearances published by the factory, which offered the oversized pistons. For these rebuilds I do not recall the allowable clearances, but seem to recall the clearances were on the order of 0.001" to 0.002". My auto mechanic buddies advised me a street engine wanted to be built to the lower clearances as this made for a quieter engine. The only engine rebuild from which I can recall any clearance info was a 318ci V8 I rebuilt and was under severe budget constraints so I had to reuse the stock pistons. I honed the cylinder bores just enough to give them the right finish so the new rings would seat. I recall the cylinder/piston clearances were up around 0.005" which was right at the high end of what was acceptable. But I had not choice and figured I'd risk it. The engine ran just fine afterwards, so fine, so quiet that more than once I tried to start the engine when it was already running. However, I have to note this was a low revving V8 coupled to a big old Torqueflite automatic and with 4.56:1 rear gears. The engine didn't need to run much above idle to accelerate briskly and I used it for around town driving. (The rear diff ratio made the few times I took the car out on the highway -- I drove it once to LA and back on business -- a real fun experience with my having to limit the truck's speed to keep engine RPMs reasonable. 50mph "top" speed to LA and back made the trip seem like it took forever.) Also, I have to note I do not know how the engine held up over time with these piston/cylinder wall clearances for a few years or so after rebuilding the engine the pick up truck it was in was no longer necessary for my vehicle needs and I bought a new car and stopped using the pick up truck. I eventually sold it.

Hi Extanker Yes I want to keep the car, I can't hone the bore however as it is Lokasil which is an aluminium/silicon compound and needs special machine, according to your 001 per inch , would give me almost 0004 so I got 1 piston 0001 out?? Might live with that!! Thanks for your help!!

There are some very odd comments here if they are for an M96. 1.You need a dial bore gauge and a micrometer. 2.You need multiple measurements,replicated to way less than 1 thou up & down & around the bore. 3.The issue is taper and ovality not just 'clearance'. Making .replicating and checking these measurements is beyond the skill level of many of us(including me). This is precision machinist work. Until you have all these measurements ,i don't understand how valid recommendations can be made for an M96. Most M96 engines with over 100K would fail this pre-qualifier and need ........? Forgive me if I am missing something.

Here is what you need. http://www.amazon.com/Fowler-74-646-401-Cylinder-Bore-Gage/dp/B000GTIL9C http://www.amazon.com/Fowler-74-646-401-Cylinder-Bore-Gage/dp/B000GTIL9C

And the substantial skill to use it !

Schnell Gelb - Noz1974 started this thread trying to find a specification .Your comment that all engines over 100,000 miles will probably fail leads me to believe you have a good idea of what it should be .Although Macster has a Boxster running strong with over 250,0000 miles so there may be room for debate or interpretations .

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Might mention in the case of that V8 rebuild I did have precision dial bore gages (which measured to 0.0001" resolution) and an accurate and high resolution micrometer (it would measure to 0.0001") since at the time I was a machinist. I also had a suitable surface plate and gage blocks with which to check the accuracy of the micrometer and dial bore gages. Thus I was able to determine the geometry of each piston and its cylinder. I also had access to a very nice adjustable in size Sunnen Hone stone set with the right grain size of alum. oxide stones and a very powerful 1/2" drill motor with which to drive it so I could put the proper hone finish on the cylinder bores. In the shop we never used dial calipers. We used some vernier calipers some quite large (6 feet in size) and I still have a 2 foot caliper. Later I bought one or two sets of digital calipers but didn't use them for anything that close.

No, I Ihave no data. Jake has and mentioned some recently in a thread here on ovality and taper. You are confusing gap( OP) and cylinder bore tolerances. They are related but not the same.With ovality and taper ,ring gap is a moot data point.Perhaps I am missing something else? This subject has been covered several times with no conclusive answers . I am rebuilding an M96 this afternoon.Just about too do the ultimate test of blind co-ordination skills #6 circlip.

Thats nice Macster but as you know the M96 is not as easy as an old American V8 . The instruments and skills you mention are the minimum required for the M96. Buying the tools is easy. Acquiring the skills you have(& most of us don't have) is another story.Then interpreting it all and relating it to published(?) specs ,yet another.

I was not confused on gap clearance

Gap and clearance are two different engineering terms. Piston ring end gap / cylinder to bore clearance.

This discussion has always been about piston clearances .You introduced ring gap .I believe the other point being missed here is that his "gap" is half of what several have said is too large . Your points are multiple measurements etc are valid .

So, are we talking ring gap or cylinder clearance? If someone only has .004" of ring gap, they'll have some bigger problems... By the same token, if you have .004" of cylinder to piston clearance, the engine will sound like a diesel.

Exactly !

I am not sure why I am bothering with this but .............It's about piston to cylinder clearance as the poster documented in post 16 .Ring gap is a Gelb inclusion because the poster used the word gap instead of clearance .Sort of like sorting fly**** from pepper .Chart 16 by the poster doesn't tell the whole story as Gelb pointed out .The gap he shows (actually clearance ) does not take into consideration centering the piston in the cylinder so he is overstating the clearance or his words gap .This is my last post so no rebuttal needed or requested

"centering the piston in the cylinder so he is overstating the clearance " How,where and why would you try to center a piston in a tapered,oval bore? What tool would measure this ? And even if you did, how would the 'measurement' be useful? IMHO this centering/single measurement per bore advice could result in an expensive misunderstanding for future M96 rebuilders. For clarity's sake and to avoid misleading others in future ,I'll repeat the wisdom I have gleaned by reading and studying this issue - and actually using all the correct tools and equipment many times on the M96. And if this is wrong ,please correct it based on knowledge and experience because this is too expensive to screw up with. One fundamental issue with determining if an M96 can be rebuilt to a particular standard is measuring all cylinder bores in multiple locations for ovality and taper,particularly 2 & 5. Without those measurements ,discussion of any piston/wall clearance or ring gap is meaningless. First - we need to use the correct engineering terms. Gap/clearance of what/where? This isn't pedantry ,it is for accurate communication and fundamental to the task. Second- there is no meaningful single gap nor any single clearance measurement because of the ovality/taper of the cylinder bores. This is a well reported issue here on Rennlist . Multiple measurements are required for each piston/bore relationship. And clearance is not measured, it is calculated by multiple measurements of both the piston and the bore. And measurement to merely 1 thou is inadequate. Rebuilding an M96 demands large amounts of time,money on parts ,special tools,equipment. Therefore the risk of unintentionally giving inaccurate/misleading information is a serious issue. For my part, I want to be clear and share my experience of an M96 rebuild currently underway to help others do better work in future. Isn't that the Rennlist at it's best ? So please, if the info I have offered above is incorrect,please correct it and cite your source if you can.It becomes more important as the number of higher mileage rebuild candidates arrive because ovality and taper increase with heat cycling and mileage. I hope this helps those who follow this path in future.

Now let's advance the discussion for the OP to get accurate,meaningful measurements & help him interpret them. Macster is your clue. Find an (ex)Machinist who understands the M96. Get him to provide the ovality/taper measurements with particular attention to 2 & 5. The average machinist /engine rebuilder may not be appropriate. The cost of this measurement work is way less than the cost of the necessary instruments and the education/skill/experience required to use them.Post the results here, we'll help you interpret. I have the equipment but was not confident of my rusty skills for this task. The consequence of an error was far too expensive to just hope. So I asked my M96 machinist to make the measurements. He is brilliant but a bit of an enigma. He tersely assured me that the measurements he took were good enough for my build. I made my own measurements out of curiosity to see what 'good enough' was. Based on my measurements, he seems to imply I will have bigger issues than ovality and taper :-). I'll review the subject on the Forum when I have run the engine - for a few thousand miles. I hope the Machinist suggestion above is helpful to those who follow and will avoid wasting time/money. My self depreciating anecdote may amuse you all at my expense.

Ok guys a quick update Firstly my cylinders have had strengthening rings fitted to stop d chunking and been re rounded as much as possible by Hartech , so the ovality and taper will have been addressed as much as possible by them I've had my piston skirts Teflon coated and was looking for the correct clearence to make sure they were the right fit in the bores I only measured with calipers which I now see is not accurate enough so I have a micrometer and dial bore gauge on order to carry out proper measurements which I'm hoping will be a bit tighter. Hartech have advised an ideal tolerance of 2 thou So I will measure the relevant piston, zero out the dial bore gauge from the micrometer after locking it for the piston measurement, then place in the bore and and readings will show me the Tolerance hopefully Sorry for using the word gap , the shame of it lol!!

Poor choice of illustration on my part -center the piston in the bore .The point is that in chart 16 you need to divide by 2 to get the clearance on each side of the piston . I am not sure why you wouldn't just verify the Hartech advice .By the way that's the first time I have ever seen any tolerance recommendation in print .Based on your crude measurements you might be off to the races . However, there are specific points on the piston and the cylinders to be measured to obtain accurate results .Perhaps Hsrtech could share those with you .

Ok dcdrechsel So what your saying is that if I zero my bore gauge on the piston when I put it in the bore I will see .004 reading to get the .002 clearance I'm looking for I.e .002 on left skirt + .002 on right skirt = total .004 Is that correct??

Google-Goodson.com -they have an article on how to use the bore gauge .Again the starting point is the right place on the piston and I don't know on this engine where that is

Ok thanks, I'll update you with progress next week!!

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Using the micrometer to zero the dial bore gage is a bit fiddly and one has to be very careful he doesn't make a mistake. You can use snap bore gage to get the bore/cylinder diameter and then use your outside micrometer to measure the bore gage and from this know the cylinder diameter. 'course, manipulating a snap bore gage and getting a good reading from it is not fall off the fence easy either. The piston/cylinder wall clearance will be one half the difference in diameter of the piston diameter subtracted from the cylinder wall diameter. Say the piston measures 3.1566" (just a number). The cylinder wall measures 3.1604" (again just a number). 3.1604" minus 3.1566" is 0.0038". Divide this by 2 gives a clearance of 0.0019". You have to be sure the piston or cylinder has no taper or ovality. To this end you want to take sufficient measurements around the piston from the crown to the skirt to develop a real feel for the piston's shape. You want to do the same for each cylinder. (Make a diagram and a chart for each piston and cylinder.) Also, you want to be sure both the block with the cylinders and the pistons are at the same temperature. When you are measuring to ten thousandths of an inch temperature differences play a role. Do not hold the micrometer any more than you have to. I used to place it on a surface plate which acted to remove any heat my handling of the micrometer might have transferred to the micrometer. After you have nailed down the piston to cylinder wall clearances you want to check ring gap of each ring in its cylinder to ensure the ring gap is correct. Carefully use the cylinder's piston to push the ring into the cylinder and make it square in the bore. Use a feeler gage to measure the ring gap to make sure the ring gap is to specification, whatever that is. The gap can be increased by filing or sanding. Remember. You can always take more metal off. You can't put it back. Check and re-check your measurements. You must keep each piston with its cylinder and the piston's population of rings with their piston in case there are any variations in cylinder diameters and piston diameters.

so far i see the advice going in 2 directions......one side will make you use straight STP and the other side will make you use a 24 volt starter to turn over the motor. time to embrace the horror......box it up and send it to raby while you can.

If its all apart, its way too late for that.

extanker what is the point of your posts ?First your post on the clearances are too big when they may not be was just snide . .Second the only real advice the poster got is from Hartech on the piston to cylinder clearance .I think Macster's post was quite informative on how to measure .

Let's all remember that the factory has provided zero in regard to internal clearances for these engines. Anyone who has values has had to derive them from imperial data, like Hartech and I have. Its confusing when clearances are not stated to be measured "on the radius" or "overall". Too tight and it fails quickly. Too loose and it sounds like a diesel and takes longer to fail. Just right is a real b_i_t_c_h to find. Most factory cylinders don't measure up well enough with ovality and taper to even worry about it.

I think if the OP needs this much instructions on how to make these measurements then he is better off to just take the block and pistons to a shop/person that knows how to do this and ask them to measure it while he is observing the procedure. Maybe after a couple cylinders they will let him try to take the measurements himself. He needs hands on instructions as opposed to a write up.

Imo You are a wise man. My posts above said much the same but perhaps I was too polite/subtle? Even IF I made perfectly accurate measurements (& I have been doing this since I was an engineering apprentice), it is meaningless because I have no idea(and only a few within the "Inner Circle" do) what the specs are. This is partially why unmolested , very low mileage M96 engines with good crankcase halves and crankshafts are valuable .Since the ovality/taper is miles/heat cycle related, the chances of finding a high mileage rebuildable(to a particular standard) core is slim. I was unaware of the precise detail of this when I started my rebuild.The information to make this dismal assessment is available from many sources but is scattered and buried in threads like this. Jaguar had a similar problem in the 1970's ,so Ford/Chevy V8's were substituted. Obstructive 'smog' laws in California block this logical and expedient alternative(LS1) for the M96. The Raby-standard reconstruction(or similar) is the only logical alternative I am aware of for the higher mileage M96 engines. My diy build may run well(or not) but whichever it is, it is a fools errand imho.It is certainly an ill-informed financial/time gamble.

I agree with Imo... I'd also recommend that all measurements be done in a room thats 72* and allow the parts to normalize in the room for 24 hours before being measured. Everything matters, especially in the winter, and a few degrees can really impact things. Never hold a piston in your hand while measuring it, that little bit of heat is all thats required to throw off your numbers a bit.

Hi everyone Sorry it's been a while coming back but I ordered a digital dial bore gauge that took ages to come but finally it arrived and I have been able to do some measurements to work out my clearances. It looks as though I'm getting .002" to .0025" clearance in total (ie .0001 on each side)so is coming up a lot more like it now I'm doing more accurate measuring. I've done the process a few times firstly locking the piston diameter with the micrometer, then zeroing out the dial bore gauge on the mic and inserting it into the relevant bores. I know I originally mentioned gap in error but now I am mentioning it in the correct format, piston ring end gap. I have new factory rings to install, will they be good to go or must I still check the gap?? If so what sort of gap should I be looking for Also do I Need to check both of the compression rings on each piston?? Thanks all

I admire your perseverance Noz. The bore ovality and taper issue is depressing. What are you going to do if it is excessive? And most engines over 100kmiles are reputed to have excessive ovality/taper. The cynics will say that the ovality/taper 'scare' is just a ploy to sell Nickies. I would prefer to ignore that & try to understand the consequences of specific degrees of ovality/taper. So let's try to develop a knowledge base of that issue? Why - because most M96/97 engines are going to reach/exceed the 100k limit soon & case halves are scarce and expensive. And so are Nickies. We need a data base of ovality/taper measurements by mileage. We also need to know the measured ring gap in standardized locations in the bore. Then we need compression test results /other performance measures after rebuild in engines with bore issues. What I am trying to figure out is -o.k. ovality and taper are bad, but how bad and at what limits ? And what does 'bad' mean ? I can live with slightly below spec compression IF it is even, a bit of blow by o.k. - but what else significant happens and how fast does it deteriorate ? This may help you- sadly I have no other answers for you: http://www.ringspacers.com/tips.htm http://forums.pelicanparts.com/911-e...g-end-gap.html

Hi Schnell I understand your concern and I have done my best to address this situation with the standard bores, they have been re rounded and strengthened at the top by Hartech who is maybe the best place in the UK at working with standard bores and also re sleeving! I just wanted to know what the standard manufacturer ring gap is so I can check before I install the Pistons. Maybe in future I may have the engine re sleeved and go bigger with capacity but at the moment I have already spent quite a lot going through the engine and can't justify it, especially with 996 prices not being fantastic at the moment . Noz

Noz, That is very interesting to hear that Hartech are somehow reducing the ovality & taper .Thank you for mentioning it.I would love to know exactly what they do. So far everyone has dismissed the idea of honeing the M96 because of the characteristics of the alloy. I have always admired the style and skills at Hartech. Just a pity they didn't apply them to a British brand. No whining about 996 prices - be glad you don't have a Boxster .In future ,I think Boxsters may be simply sources of used parts for the more valuable 996 -unless it is a Track-boxster.

A relatively minor issue, but I think for "0.0001" you meant to type "0.001". You must check the ring gap for every ring in the cylinder in which it will be used. However, I do not know the ring gaps for the compression and oil control rings. I might add that if the cylinder is tapered you must make sure the ring gap is gapped right for where the gap will be the smallest. If the ring ends come together this can fracture the ring and that ain't good. Also maybe it is your writing, or more likely my reading comprehension but I'm not sure you are measuring the cylinder diameters accurately enough. You must have accuracy/repeatability down to a very small amount, ideally down to 0.0001 of an inch. IOWs, when you measure a bore diameter of say 3.1565" (just a number) you should be very confident the actual diameter is within 3.1564" to 3.1566". If you think it would be off by say 0.0005" that's not knowing the actual/true diameter well enough. As for taper and ovality this could arise from the uncertainty of your measuring. If the taper and ovality is real I do not know how much is tolerable. While in some cases auto makers provide this info for their engines, as you have learned Porsche does not.

Schell As far as I know the strengthening rings are fitted first, I think these are 'sweated on' ie heated up to fit then as they cool they go tight. This extra force around the top of the cylinder reduces clearances slightly so the cylinder is honed using a special honing tool suitable for lockasil , also the rest of the cylinder is ' re rounded as far as possible with careful measurement and honing where necessary. The honing tool re dresses the bore with a finish similar to the factory finish. Obviously the liners would be better but they are expensive !! I believe Hartech say the standard bores hold their shape better after the above treatment has been carried out, I hope so!! Macster with regard to measurement , yes your right I did type wrong it was .001, I have done numerous measurements down each bore in comparison with the relevant piston , each cylinder seems in tolerance with what Hartech has told me and the measurements seem consistent down the bore so fingers crossed all should be good, basically I just wanted to check I still had enough clearence after having my piston skirts Teflon coated, it's more too little clearence than too much which worried me. Just a mad off the wall thought, we are all sh****ng our selves about ovality and taper, I recently read an article somewhere where a guy had an engine which scored its bores , obviously it was knocking like hell and using oil but strangely he said he hadn't noticed any loss in power it was just the sound of the engine which alerted him to the problem, imagine the taper on those bores!!

Noz, Thank you for your information. Very interesting. I was unaware of what you described:" strengthening rings are fitted first, I think these are 'sweated on' ie heated up to fit then as they cool they go tight". This does seem different to what LN Engineering call "Nickies". The only related(?) product I could find on the Hartech website is this: http://www.hartech.org/porsche_996_9...er_liners.html But this is a liner and is different from your description ? Do you have any tech info on the process you describe? What do Hartech call it?

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In one of the UK Porsche mags I read I came across a guy with a Cayman which was diagnosed with bore scoring. Had the symptoms and a check with a borescope found "scoring" sign. The engine was running fine otherwise. I do not recall if oil consumption was high or not, though. The Cayman owner ecided to just drive the car and when the engine got bad enough to do something then. So he continued to drive the car. He even tracked the car. The bore scoring symptoms went away after a while. I do not know what Porsche allows regarding bore ovality and taper. I recall with other engines there was some allowed. Not much but some. While one would like to have the bores round and straight the engine can be ok with some ovality and taper. 'course, the question becomes how much and I don't know the answer to that. One might find a similar sized engine, or least one with the similar bore diameter and stroke and if he can find what the factory allows that could be something to go on. Not much, admittedly, but something. Of course, if one isn't or can't do anything about this what difference does it make? Either the engine goes together with whatever ovality and taper it has or the bores get machined round and straight before it goes together or the engine builder doesn't put the engine back together and the parts spend the rest of eternity in a box.

Quotes source post: 12048133

Schnell... There is more info re strengthening rings for the M96 engine in the Hartech webpage ( www.hartech.org ) Buyers Guide section, Part 4. See page #136 onwards regarding bore ovality.

not po specs just motor 101 [well harley davidson] ring gap .015/.019 all rings. i like to fit each ring set to a cylinder and mark it down. i forget ring/piston groove clearance.i you use a ring gap grinding tool be VERY care full they really remove metal fast. i used to use a small hone. your piston maker may have the ring specs. more 101...... imho use a good assembly lube on just about everything. old days we would dip the ring area of the piston in oil prior to going into the cylinder .DO NOT USE SYNTHETIC OIL FOR BREAK IN. snap on tools make an easy to use ring compressor

Thank you Stephen ! It is page 137 . I had printed out Barry Hart's magnum opus a long time ago. He seems to be offering the rings as a stand-alone repair? The rings make a 'closed deck ' for the crankcase halves.Then there is the secret squeeze locations to re-form the bore. Impressive. Interesting that the 'closed-deck ring fitment' job is not offered by LN Engineering?I guess they think the Nickies solution is the ultimate answer in view of the r&r costs? Thanks for the reference http://www.hartech.org/docs/buyers%2...20part%204.pdf see page 137

Hi guys Just asked Hartech about my new factory rings, the recommend that they are fine to go in as they come. Just thought I'd let you all know in case you are due to fit your own Noz

put a ring in the cylinder and give us the ..."fine to go"...specs. please

The Hartech advice makes sense to me. Because the rings supplied are intended for a specific bore-variant of the M96.They would be made to the new assembly spec. In the U.S.A. we are accustomed to buying generic aftermarket rings and grinding to fit.Two utterly different markets. WAG The other issue is that typically the ring gap on engines like the M96 are much wider than more conventional engines .I wonder if the thinking was - too small a gap =bang! Too wide (but wide enough to tolerate the closure that may occur in extreme situations = a little blow-by/compression loss. That would be no problem - so they err on the safe side ? If so, the 'correct'(?)ring gap could grow to over 0.006 per inch ? So 15 thou+ for a 93mm bore?