Gneurling piston skirts..

[NV Stig]

Have you guys heard about this gneurling procedure? You make cross cuts on the piston skirts to help enhance the piston oil scavenging capability, and significantly enhances the life of the engine. Apparantly it became a Porsche SOP in the late 70's at some point.
That is so cool. I ask a lot of stupid questions, and was amazed such a little thing could do so much. I apologize if I'm the last one in this group to know about this, but thought it was cool and wanted to pass it along if you are working on your air cooled engines.

[jcge]

I cannot speak as to Porsche practice or purpose in this area

However, I have seen piston knurling done to reduce piston to bore clearance - the knurl upsets the surface slightly - it would not be my first option to achieve this end.

Having said that, Porsche did knurl the bores of its early Chromal racing cylinders (dimples, rather than hatches) to retain oil.

What is the origin and history of the parts you have shown ?

John

[NV Stig]

I'm getting my engine rebuilt at this time. I have another thread on here with a question on cylinder honing, pistons, and heads. I was showed this procedure on my "Judgement Day" with the car. It was all apart, and this is where we need to go and what to do. there were several other 911's in the shop, and I had my camera handy. I learned about knurling, and liked the principle behind it. The guy working on my engine does it for you when you get your engine rebuilt. Sorry about the spelling on knurling.
It's just cool to me to see all this stuff from the inside: all the how, why, and upgrades made over the years. It's becoming more than a cool car, old man stories, and a bunch of stats in Road and Track or Excellence article to me. I'm just getting immersed in all this stuff now, and having fun all the way to the poor house, ha ha.

[Flieger]

Chromal honing

[jcge]

NV - you also need to know that every one of those knurl markings is a stress raiser....and on a cast piston...not good from a fatigue/structural integrity perspective.

"Enhances the life of the engine" - yes.....by avoiding replacement of parts that probably should be replaced.

I have never seen new pistons, of any make, with knurling.....I'm sure the OEM's would do it if there were some advantage.

Suggest you look at many of the pics of other professionally rebuilt engines/pistons on Early S and Pelican, and note the absence of knurled pistons.

[911pcars]

Knurling is an oft-used machine process to restore the running clearance between two objects by raising or displacing the metal adjacent to the "knurl" in order to increase its diameter. It's been known as a repair measure that may or may not have the service life of a proper-fitting part - usually not. The pro engine builders here can chime in with their experience.

Sherwood

[pu911rsr]

I had a 914 race engine once with knurled pistons in it, lasted about 10 minutes.
Phil

[tom1394racing]

Here's a set of Arias 95mm RSR style pistons with something similar on the skirts.

I have never seen anything like this on OEM Porsche pistons.

[jcge]

Interesting Arias pistons Tom - thanks for posting a pic - I'm guessing those skirts are lathe cut, rather than knurled (deformed under pressure) and oil retention would appear to be their primary purpose.

NV
As to the picture in the first post in this thread - I was trying to move you away from installing, what appear to me, to be 6 hand grenades in your motor.... I'm imagining a thin wall of (cast ?)aluminium piston skirt, with XXX thousand miles of running, being manually knurled in a lathe, with little or no process control, and micro fractures extending in every direction.

Regards,
John

[NV Stig]

Hello, John,
No worries about losing my grenades It was determined that they were too worn to be used again. I do now have 6 ash trays for anyone interested.
We are going with honing cylinders, and new JE pistons, unless I get that 2.2 MFI pump. then it's going to have 2.2's installed.
I was going to knurl the new pistons prior to the install. I was under the impression that it helped with more efficient oil scavenging, increasing the life of the engine. Looks like there may be some differing insights here I would love to hear more about.
Phil, was it determined that the 914 died due to knurling the pistons? What happened? Sounds like a lousy experience. Inquiring minds want to know.