996 Piston Skirt Recoating

Hi, I recently disassembled my engine. The cylinder bores appear to be in good condition, but I noticed that the coating on the piston skirts is beginning to wear. Rather than replacing the pistons, I�m interested in having the skirts recoated to help prevent future cylinder scoring. I have a few questions: What type of piston-skirt coating would you recommend for this application? Can the existing pistons be stripped and recoated? Are there any shops you recommend for this work? I�m in the Phoenix area, so a local shop where I can drop off the pistons would be ideal, though I�m also open to shipping them to a reputable shop. Thanks, Taylor

There are good JE and Mahle Motorsport new piston options that may not be that much more than reliably re-coating the OE pistons. On that note, have you measured the bores to confirm they are round and cylindrical, as well as smooth? Not knowing what prompted the rebuild, or the current scope, I�m not sure where the line will be between reinstalling the pistons with no coating, new pistons in old cylinders, having the cylinders re-lined (Millennium?), and all out sleeves/bore replacement.

The engine looks like it had an IMS failure, which caused one timing chain guide to break, but I'm not experienced with these engines at all, so that's a guess. The cylinders have some minor wear; I would say blown-up pictures of them make them look much worse than they are. The goal is a reasonable rebuild that I would feel comfortable driving cross-country and reselling down the line without feeling like I'm selling a ticking time bomb. I'll be providing all the pictures / some video to whoever buys it from the rebuild. As for checking out-of-round, which accuracy dial bore gauge do I need? I'm heading out of town for a vacation, but I can order a measuring tool and have it waiting for when I get home. I'm assuming it's just the outer race of the IMS bearing in there, but the outside of the IMS was pulled when I got the motor with the car. This piston had two small horizontal scratches that I can't catch my finger on but can feel lightly under the pad. I'd seen someone else post something similar, saying it hadn't been an issue for them for years. I just noticed this odd, discolored line; I can't feel it at all. This is probably the most "worn" cylinder. The worst offending piston. The rest either seem fine or are just starting to have the coating come off.

Has anyone used Line2line? It seems more reasonable than buying new pistons. I would need to ensure the cylinders are round and be able to determine the required clearance.

I was waiting for more seasoned and experienced folks to chime in on measurements for you, I hope they do. Line2Line sounds like they could be ok, maybe confirm with them their coatings won�t exacerbate issues in Lokasil bores during the coating run-in process. There is a Porsche engine gurus group on Facebook with some smart fellas that might help with measurements if no one pipes up here. I can see from your other threads you are way down the rabbit hole here (good for you) - it may be worth getting Jake Raby�s/LN�s comprehensive video and book series on rebuilding these. It�s a few hundred bucks but you�ll be able to avoid a few simple gotchas and review what everything should look like and what may be missing in the mean time. I would just as likely take it off your hands when you are done with it too.

Those original pistons tend to hold very well and typically do not have any dimensional issues. Line2Line, Polydyn, etc. can provide appropriate skirt coating if required. However, the real issue is the cylinders. Even if the surfaces can be properly prepared for new rings, you will find they are all out of round by upwards of .005" to .008". The cylinders should really be addressed and there are plenty of options. Feel free to reach out for more information.

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m96 engine is not like a normal Chevy/Toyota/Ford engine as far as rebuilding it is concerned. And can confirm that as I've built hundreds of all types engines in my many decades of work. Last engine I built with a defective block was a Chevy Vega, and what do ya know,,, it also was a lokasil type block... Its really to bad and a major factor that affects the resale of all m96 cars.... Ya, sorry to say the m96 cylinder block is a 1 time use piece. As its Lokasil cylinders are not able to be bored or honed to fit new or old pistons. And to top that off, the pistons have to be coated with a ferrous type coating that has been regulated out of production for epa reasons. Porsche tried a plastic type coating afterwards that was not working well at all. The direct contact of the aluminum pistons on the Lokasil cylinder walls (without the correct not available coating) caused a rapid abrasive wear in the cylinders/pistons. Then Porsche went away from Lokasil cylinders. Also to top it off, and it gives me no pleasure saying this, ALL m96 blocks will be way way out of round specs by 100,000 miles. This has been stated by the biggest m96 engine builders and techs, right here on our forum and other published papers. I've measured m96 blocks with 60,000 miles that looked perfect, not a scratch on them. They measured out at .00265" out of round/taper. Charles from LN says the max out of round specs for a m96 is .0035". So at 60K miles cyl were less than .001" from being out of spec. This was on a meticulously well maintained m96. I know from many other engines I built that have .0015" max OOR specs. Most engine max specs do not exceed .002" OOR Taper. So what I'm saying is the OOR/Taper specs for the m96 (wherever they originate from..?) are way high comparatively speaking. Unfortunately Porsche does not publish these specs. This is a block structure problem Porsche knew/knows about. Hence the reason for sleeved cylinders in all proper m96 builds that last without blowby/oil usage/smoke/piston slap... Some have had limited success with not disturbing the original ring position on the pistons and keeping the original piston hole location the same. Not removing the pistons is the best way to keep ring seal the same as it was, but seems that ship left port. So to help answer your questions, no known piston coating currently available works for original lokasil m96 cylinders/pistons. New pistons in oor cyl with or without any type of available coatings never seem to seal up. So, for a cross country car, not sure of a solution for you other than a nice set of aluminum sleeved nikisil cylinders from LN or Slakker/HarTech. Steel sleeves done correctly will work with a substantial HP decrease. Make sure to have crank magnafluxed carefully for cracks as thats a common problem on m96 engines that have failed. Suppose if you're doing this yourself for a resale and are a gambling man, could just clean it up best as possible making sure all bearings are good/in spec, and hope for the best. R&R all other parts as needed. Worst case would be rings don't seal up causing loss in HP, blowby, oil usage, smoke, piston slap, or D-chunk/cracked cyl wall from the worn oor cyl structure, I'd be very surprised if this fix lasted for any extended period, but maybe..? Then again,,, the time and effort to build the engine and R&R into car, makes more sense to me to spend the $6000-7500 for nik'd alum cyl with new performance pistons and now have a big bore 4.1L, stout, great sealing,. full HP, piston/cylinder assembly. jmho

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This is one of my less expensive bore gauges that goes to .0000, which is plenty good enough to show the m96 block structure problems.

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There is no reusing those cylinders as is. If you do, you'll be pulling it back apart in to time.

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I have spoken to Line2Line and their process puts on a heavy coating, much thicker than what most aftermarket pistons come with, usually .0002-.0004" thick per side. Line2Line would be several thousandths (.002-.004"). Problem is the coating is way thicker than the total piston to wall clearance. There's a good chance it will seize before it self clearances. They recommended against it for our application. Back when we did the RND Engine program 10+ years ago for SSF Auto Parts, we reused original pistons, but had the factory coating removed and then sent the pistons to Calico Coatings to have their skirt coatings put on, which they can do much thinner than Line2Line.

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I'm pretty sure I know which one that is. Although it goes down to four decimal places, it's resolution is 0.0005 in. If it's the one I think it is, it's not good enough to inspect cylinder bores. I'd say it's adequate for general bore size checks and for looking at ovality/taper. We have something similar for quick and dirty measurements on bores when honing when we have a ton of material to remove from the bores. When the measurement matters, a Sunnen or Mitutoyo with accuracy down to .0001" is needed. We set them with either our Sunnen pre-setter or precision ring gauges.

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Thanks for sharing. I am having trouble identifying what it is from the picture, though. Does it go to .0001 or .0005, there seems to be some back and forth online about what is needed for checking work vs actual machinining.

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That's fantastic information, thank you! Any chance you remember which "coating" would be appropriate if I were to use the original cylinders and not move to Nikasil?

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One of the things I'm trying to wrap my head around is that all these cars running around 60-150k miles are slightly out of round, and they still run okay slightly out of round. If I were to go buy a 2001 996 manual for 30k that has anywhere between 60-100k miles on it, it would have out-of-round cylinders, as you said; that's a known fact. If I'm putting together an engine that didn't fail because my "cylinders" were out of round, and instead it looks like an IMS failure, what reason do I have to drop an extra 6-7k unless the engine won't run? I'm still going to clean everything out, replace all the bearings and gaskets, and upgrade the IMS, do a valve job, etc., but what you're talking about more than doubles that cost. With the failure scenario resulting in my replacing the at-risk part for the same 6-7k, it doesn't appear to be risking the rest of the engine rebuild if I have blowby and need to take it apart, and at that time sleeve the thing. The biggest issue is likely me just messing up assembling the block and grenading the whole thing anyways, I might as well get some practice in. I should probably buy that book... I know there are extremely good builders {on this forum in fact}, and they need to put out extremely good engines with absolutely perfect tolerance, because that's what their reputation is built on. That's what you're paying for from them, a bulletproof kickass motor.

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Not that I have any experience with them, but Swain Tech claims to have a skirt coating that is compatible with Alusil and Lokasil. I'd give them a call.

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What we have told people who are rebuilding the engine without reworking the cylinders is to just reuse the old rings in their original positions. There is no way a new ring will conform to the bore being out of round. Although in all these years I've never been able to find a wear limit for the M96, I did find it for the Cayenne V8 years ago. That same value should be your limit for ovality and taper on an M96 or M97 engine.

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This is great and exactly the information I was looking for. Based on other conversations I'd read, my plan has been to use the old pistons and old rings that are formed to the cylinders. I'll order something to measure and use this as a guide to make sure I'm not too out of round. It'll be a couple weeks as I'm on vacation. Thanks Charles, you've been super helpful.

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Agree on this... Whats your guesstimate on the miles on that engine and the amount of OOR/Taper, just from looking at the pic...? What do you think of direct plating those cylinders using existing pistons new rings, and coated skirts, for a semi-temporary build that will at least seal up for a while...? How fast you think it will go back out of round..?

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There is no way from looking at the picture, but I would bet it's at least .005" OOR/tapered. No clue on expected lifespan with just direct plating it. Not something we have done or have offered.

Not an experienced engine building myself I'm wondering the following, if you decide to reuse the existing pistons/rings with the untreated cylinders as some have suggested, albeit less than ideal, it appears it can be done. How do the rings 'realign' with the out of round cylinders? Is the assumption that they will work themselves around the piston during the first few (100?) strokes to eventually find their 'spot' and seal? This makes no sense to me but again, I'm no engine builder...

Aichele, that Cayenne spec has cylinder dims on it as well, you could use that to inform what coating thickness you want to match your bore. You�ll need a micrometer for the pistons as well.

Well, an inexperienced home DIYer used a Shars .0001 dial bore gauge and an .0001 Accusize set of outside Micrometers to measure. I think my biggest issue is that the Accusize doesn't "feel" completely flat, so I'm hesitant to call my numbers below "accurate" with how precise things need to be. On top of whatever deviation there might be in the micrometer. I was intending to use this in a YouTube video, but now I'm a little perplexed about the best approach.

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I have next to zero knowledge on how to read/interpret some number-chart-results I received from having my 3.6 -03 engine rebuilt. I just know that at 110k miles, the cylinders were junk, not to mention starting to bore-score also. Only 1 of 6 cylinders was "deemed OK'. If someone can interpret these numbers on the chart, go for it. I am replacing all with job being done by a top-tier shop. These are OOR cylinders, but so what? All are being replaced with new Ps & Cs. In addition, what are measurement/specs for crankshaft, rods, oil pump, valves, and so on? It is a big list of what has been wearing out (yes/no) for a long time. My view putting everything back together in a "used parts" condition is not wise. It is a lot of labor, DIY or paid, to simply take the engine out and replace it in the car. After all of the ter-down and "fix" work is added, it does not make sense to put engine back together with more than likely failure-prone-parts. Having to do the whole procedure all over again at possibly a short time is not something that would inspire confidence. Any Porsche engine is not a cheap assembly of inferior parts. Sure, P made stupid cost-effective choices with IMSB, AOS, and RMS (for examples) but the resulting engine was/is a high performance engine. The known "stupid cost-effective" parts can be replaced with proven known far superior parts - again, not cheap. Whatever, your engine, $$$, and decision. FWIW: YMMV

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Well, with Charles max spec of .0032" OOR or taper, you seem to be considerably over that on 4 out of 6 cylinders and at or close to max spec on the best 1. OOR or taper in engine cylinders doesn't have to be/go from top to bottom to be out of spec. Just 1 small area out of spec is all it takes to be 'out of spec'... So when dragging the bore gauge up and down cylinder after gauge is zeroed out, if gauge goes out from zero to .0045" even for only a small area, thats all you need to know to condemn block. I check at 4 positions around bore, 12 o'clock/3/6/9, depending on what I find might then go to more... Its really not needed to use inside mic with the bore gauge. Its really not critical to be so accurate for this measurement when checking used blocks for out of spec bores. Now when honing block to size, thats a much more critical measurement. We got a very liberal, huge, .0032 max spec. Anywhere close to that max spec would be absolutely terrible for almost any other type/brand engine that many use .0015-.002" as absolute max out of round/taper. OOR taper on new cylinders should be under .001" if the machine shop is decent. A great machine shop, with Sunnen equipment, will get it close to .000"... That makes for great cylinder sealing, easier ring break in, a engine that can deliver full HP/torque, and a quite engine that uses/burns no oil. jmo