Are my pistons worn out or barely worn? Wayne?

[Scott Clarke]

Way back in the dark ages (before Wayne's book was released) I tore down my 2.2E motor and made decisons concerning a course of action for a rebuild. I decided that the pistons were shot based on measuring the top ring side clearance with a feeler gauge. The results were all at .005". I was using the old rings, as I had no information that they could wear significantly (I learned this in Wayne's book). The information I had was from Anderson's book. On page 114 of "Performance Handbook" Bruce shows a piston being measured. The caption reads that if the resulting clearance is greater than .004" then the pistons should be junked. I saw a thread talking about the difference between clearances and wear limits, and out of curiosity looked at the appendix of Wayne's book, page 210. The top ring clearance is listed at 1.478mm -1.490mm. 1.490mm=.0042", so, according to Bruce, some pistons were worn out before they left the factory! Am I right, or am I misunderstanding what clearances are? Wayne lists the wear limit for top ring clearance at .2mm. This is nearly .008" inch! Are my pistons gold or future soda cans? Input would be greatly appreciated.

-Scott

[Tim Walsh]

most interstings.. I looked at wayne's engine book but I can't figure out exactly what he has indicated. Is the wear limit 0.20mm between the ring and the land? I really don't get how to measure that at all and how to tell if it is in spec or not. I know that the gap between the top of the land and the top of the ring is critical but is this the 0.20 that is indicated in waynes book? If so what are the other measurements?

[Scott Clarke]

Tim-
See the thread titled "Parts tolerances and wear limits" for a discusion of the dimensional issue. It would be great if someone could verify my interpretaion of that infomation as it applies to my situation.
-Scott

[Wayne 962]

The info found in the Appendices is compiled almost verbatim from the Porsche Technical Spec books for all the 911s from 1965 through the 964. That said, and this is off the top of my head as I don't have the info in front of me, the wear limit would be the gap between the ring and the groove, as measured with the feeler gauge. From the factory, this should be 0.075 - 0.107mm when measured. The pistons and/or rings are worn when the groove measures 0.20mm.

This is off the top of my head, and I'm also a bit confused about l33t9eek's question - the ring gap information is different, and is shown in the table below the ring land info?

-Wayne

[Scott Clarke]

Wayne-
Thanks for your response. You seem to confirm my observation that the caption I mentioned above allows for the possiblity of pistons that are "worn out" before they have even been used. This, of course, doesn't make sense. Ultimately, my own ignorance has got me to this point, but I can't help feeling a little put out that Anderson's book steered me wrong. I made a host of decisions based on the assumption that my pistons were worn out. I purchased a 7R case with spigots for 90mm cylinders and had quite a bit of work done to that case. I also had my heads machined to accept the larger cylinders. I should end up with a nice short-stroke, E-spec, high compression 2.5. Trouble is, I can't afford it! At least I didn't throw out the old Ps and Cs. End of Vent.
-Scott

[Wayne 962]

Okay, there is some confusion here - and I have looked at the books in question (including my own), and I have these following observations:

- The Porsche Tech Spec books list the clearances for the space between the ring and the groove to be 0.107mm MAX when new. This converts into 0.0042", as the maximum allowable clearances when new.

- Bruce is obviously reocommending that you do not use pistons with grooves that are larger than this allowable clearance when new. Some rings/piston combos will come in smaller when new, and some will approach this limit. In general, following Bruce's recommendations is a good idea.

- As for wear limit, this is a term used by the factory (and duplicated in my book). It's not explained in detail in any of the books, but I have always understood it to be the maximum clerances allowed or tolerated prior to replacement. For instance, brake discs come new with tolerances, and have wear limits that would equal the minimum thickness of the disc. In this case, the factory spec books are stating that a maximum of 0.20mm can be tolerated before replacement.

In actuality, this number is a bit confusing. So, what if you tear down your engine, and the clearance is 0.19mm? Do you replace them, or not? If you put them back into your engine and run them, then in a very short while, they will exceed this clearance, and be out-of-spec. I suppose if you were running a race motor that would run for a short number of hours and then come apart, it would be fine. But for doing a street rebuild that you want to last for 200,000 miles, then this would not be a good decision.

What Bruce is recommending is conservative, and technically correct (well, sort of). The measurements of cleances specify a range - any good engineering manufacturing class will teach you that clearances will be all over the map due to variances in machining (tools, temperature, operators, etc.). The .004" spec is on the outside of this clearance range. This is where engineering gets fuzzy - where manufacturing tolerances and random noise get injected into the science.

Bruce's statement of "don't use the pistons if the groove is 0.004" or greater" while conservative, is also misleading too. By that statement, Bruce would say basically to send back to Mahle the pistons and rings that don't meet this spec. Following the specs in the tech spec books, a small number of pistons and cylinders would not meet Bruce's requirement (Bruce = 0.004", Tech Spec = 0.0042"). I would guess that Bruce simply rounded down the number, since the .0002" is almost imeasurable anyways.

So what is the clear answer? There really isn't one. Bruce's recommendation is probably based upon what Jerry Wood's recommends, which is don't use any pistons in an engine that don't meet the original Porsche OEM specs when they were new. I know from talking to many other people that you can indeed use them up to the wear limit. But how much wear? Depends upon your application, and what you are trying to achieve.

To answer your original question, I would probably feel comfortable reusing your pistons, especially considering that I don't think that the 2.2Es are available right now (I could be mistaken).

-Wayne

P.S. This is a good question - this is exactly what this board has been created for. Stuff like this that's not 100% clear in any book...