Piston/cylinder clearance question

Mysterytrain · 03-09-2007, 05:22 AM

Ok..I have a question for you all.
Lets say you have a new set of custom made Billet/nikasil 98mm cylinder that has a bore of 98.039 mm and is dead nuts round. Now, lets say you want to match thess cylinders up with a Mahle Max Moritz piston. The Mahle vendor is willing to cull the herd for the largest 6 pistons he has in stock..which of course would not be 98mm actual measurement.
And lets say your goal was a 100k mile street engine...would you proceed with this marriage?
Also..the cylinders are not ' nikkies' so no slamming good people/vendors.

I'm just trying to base the 'go/or no go' on the the numbers presented.

patkeefe · 03-09-2007, 08:46 AM

Ron:
Far from an expert, but I would want to look at the expansion rates of the cylinders vs the pistons, so as to be able to compare for the purpose of establishing a clearance value. From what I can tell, the Mahle piston alloys are pretty well matched up expansion wise with the Mahle cylinders. What would the piston diameters be?
Pat

911pcars · 03-09-2007, 10:03 AM

If the cylinders are 98.039mm and the desired piston-to-wall clearance is .0381mm (0.0015"), then the ideal piston OD should be 98.009mm.

Sherwood

Mysterytrain · 03-09-2007, 10:35 AM

Pat...I was told [ and I trust this person ] that when the they embarked on this project [10 years ago or so]the goal was to make cylinders that would match the mahle expansion rates. They had the resources for testing, did so and matched the rates. I wondered about that too.
But as Sherwood mention..If I'm shooting for a .0015" clearance then I would need a Mahle piston that is 98.009mm ...and they don't make them. I'm also under the impression that I wouldn't want to shoot for a .003" clearance.

patkeefe · 03-09-2007, 12:45 PM

Ron:
What were the design criteria for the cylinders? Evidently, they were developed for a certain size piston. Perhaps someone had a JE in mind? Or, is this a case of the available (first choice) Mahles being of the wrong diameter for the application?
Pat

Mysterytrain · 03-09-2007, 04:41 PM

Pat..I think that they had racer's in mind at least for this bore. It is my understanding that it's common to run larger clearances on engines that spend their time near the redline.

Eagledriver · 03-09-2007, 05:28 PM

The stock sets that I have tried to measure (just sticking a feeler guage between the pistons and cylinders) seem to be about .004 inches. That's on used sets with minimal wear showing. I think .003 would be a good number. Tighter than that would worry me. I haven't measured any new sets however so maybe they are tighter than that.

-Andy

Mysterytrain · 03-09-2007, 05:36 PM

Correct me if I'm wrong but I've always thought that with new Mahle P & C's you shot for .001 or.0015 side clearance and that if you were pulling apart a used set for a re-ring then .004 or .005 was at the wear limit.

patkeefe · 03-09-2007, 05:38 PM

Yes, that's my understanding. Hotter running in a race engine. What are the actual bores and piston diameters? I have to go read my sheets in the basement, but I think I'm still less than 0.002" on the SC engine currently on the stand, which has the original 160,000 mile pistons and cylinders.

This evidently is why Porsche chose the Nikasil and piston alloys; they could run the tighter clearances in the production engines. Somewhere in this forum is a very interesting discussion of the clearance topic, but I do not have it bookmarked.

911pcars · 03-09-2007, 07:34 PM

I think you always want to get away with as little clearance as possible yet not so tight that normal hi temp. heat expansion won't scuff the piston/cylinder wall. Small clearances results in less piston rocking and thus better ring sealing and longer ring life. In a street engine, the recommended clearances reduce piston slap when the engine is cold.

In a track-specific engine, I think you'll want to use a premium (i.e. synthetic oil) for the high temp protection it provides regardless of the clearance you eventually decide to have.

Sherwood