Piston to cylinder clearance for Mahle

2001 996 non tubro I am looking for the piston to cylinder clearance for a 2001 996 3.4l . This is the bore size - piston size. I have seen some referenced as new to be 0.0008" to 0.0012" and even 0.002" . I have Mahle pistons and will be re-coating the skirts. Case bores are being refinishes. I find a lot of un-certain quotes but no data? Anyone have any experience or data to share?

I don’t have the specs you are looking for but I am curious about the piston skirt recasting. Can you explain what this involves and where you are having it done?

The cylinder refinishing will make a huge impact on the optimum cylinder to piston clearance. What is specifically being done to the cylinders? What coating is being applied to the piston skirts?

Line2line is doing the piston skirts and US Chrome in WI is doing the cylinders with Nikasil . Had very slight scoring and 0.003" out of round . I am considering going with new Mahle stock pistons for $800 more. ​​​​​​ https://www.line2linecoatings.com/

This is Line2Line claim, several friends have used it with good results. My pistons are in good shape with no scoring and only the skirt film slightly wore through on one side. How long will the skirt and crown coatings last? Many dry film lubricants are designed to be "sacrificial" and eventually wear out. APC is designed to clearance itself until the optimal fit is achieved. The porosity of the coating helps maintain the oil film that the piston is riding on. Theoretically, the coating could last indefinitely under these conditions. Barring any catastrophic failures, ceramic thermal barrier coatings are virtually permanent.

Just found this link to Mahle technical site gives 0.055mm for cylinder to piston clearance https://catalog.mahle-aftermarket.co...63339657592904

I had my original pistons coated by Camcoat in the Uk , you literally only need like one or two thou thickness on each side of the skirt , the clearance between bore and piston is normally about two thou so I'm told ! I struggled to find this info, I measured my piston skirts with a micrometer , then zeroed a dial bore gauge on the mic and dropped it into the bore to give a clearance reading, took some practice to get consistent readings I can tel you!

Quotes source post: 17113866

is that on the radius or diameter?

I assume diameter? Doesn't say it just says piston skirt clearance. I have alway understood that to get the clearance it is the diameter of the bore minus the diameter of the piston.

Quotes source post: 17114519

well that would mean .001 inch actual clearance between the skirt and the cylinder wall. That's just small.....that's why I asked

Quotes source post: 17114716

maybe that's not how to look at it

I have used line2line with DFI/ 9a1 pistons in the past. The issue with just plating a stock cylinder is the strength of the parent material, coupled to adhesion. Every attempt I made at doing that in the early days was met with great disappointment. Surface finish, and hone angle are everything when it comes to Nikisil.

Your looking at two tho each side so 4 thou in total for the diameter , you can go six thou total !

My understanding is the dimension specification is total gap so 0.001" each side? Jake when you give a dimension (0.055mm or 0.002") for skirt clearance is it actually 0.001" for each side or 0.002" on each side???? Total of 0.004" seems loose to me for a low expansion piston? Happy holiday everyone

Quotes source post: 17117919

That's according to who is doing the measurement, and how they measure/ set clearance. This is why it's critical that the person running the hone has the actual piston that will be used, and determines where said piston should be measured from. If this is done, they can then use the same micrometer that is used to measure the piston, and set up their bore gauge to measure the clearance/ adjust it. If this isn't done, you instantly experience tolerance stack up between measuring devices, and everything is out the window. If they don't want to do this, or say that it isn't necessary- find a new machine shop.

Differently agree Jake, The shop has requested the pistons so they have them in the shop at the same temperature as the case . The Question I have is (as a general rule) how do you read a the specification as given by Mahle. Is it 0.055mm for each side of the piston making the number of total clearance doubled, or is the 0.055mm the total of clearance in the cylinder making each side of the piston clearance of 0.0275mm. The pistons are being measured before the skirt coating is removed so it will shed light on this. But in general, how should you read the specification as written for the Piston Skirt clearance ??? Thanks and Happy Holiday

Quotes source post: 17119468

What's more important is the changes that you may see due to a non- lokasil cylinder being used. You can only use the Mahle spec if you are keeping the cylinders the way they were from the factory. The other variable is the piston skirt coating that is used from the factory, Vs what you plan to apply. These things are where the person doing the boring/ honing work must use their own experience to come up with a specification to shoot for. Given that you are keeping the Mahle piston, be heavily concerned with cylinder surface finish after the mining is complete. This is a critical, but often overlooked (and sorely misunderstood) factor in cylinder finishing. At the end of the day, the person operating the hone needs to know, understand, and apply all of these things, and do what their experience tells them is best. They shouldn't even allow you to have any input on the subject. Manufacturers typically only publish total clearances, when it comes to any running clearance. I'm doing SUMEBore surface finishing today as part of the developmental process. We have air-cooled, M9X, and 9a1 engines using SUMEBore for developmental purposes, and it has been controlling my life for the last few months.

Thanks for everyones feedback. The final response was from the rep from Mahle was:Total clearance between piston and bore. .0275mm on each side. As Jake mentioned it is always more complicated that that but this gives me a base line. Unless you have unlimited funds the building process is to try and not do any harm. I was trained as a 911 transmission and engine unit shop mechanic back in the 70's by Porsche of America and worked for 15 years rebuilding engines. Left the field to do research for the department of defense. I turned 70 and finally purchased my first Porsche, of all things a 996. I have been using it as a daily driver in the summers. But a rebuild was necessary due to low oil pressure creeping in at 3/4 bar at idle ( main bearing by flywheel). Thanks Everyone Have a Happy Holiday

[QUOTE=Lyso;17126032]Thanks for everyones feedback. The final response was from the rep from Mahle was:Total clearance between piston and bore. .0275mm on each side. that surprises me. I don't know anything about this subject but that aint much for sure

Quotes source post: 17126032

[QUOTE=dougn;17126347] I'll bet 50 bucks that guy ate Mahle has never built an engine on his own. My 9 year old daughter likely has more engine building experience than he does. This is pretty much the norm with "tech reps" at parts suppliers/ manufacturers. They are generally guys that are hired based on their degree, and then they sit around and read SAE papers describing things. Sooner or later they catch on, but in my experience, they are totally lost and have little to no direct experience with what they are describing. Listening to them usually sends me down the wrong path.