specific piston measurements (bear with me people ;))

Ok here we go...tonights topic is piston measurements... most of the time I post questions without first checking my tech. data...then Warren quickly tells me to go read everything first before wasteing his time... heh heh heh.., so tonight I have my little spec book right here and I want ot get a few things straight.

Ok, I'm checking my pistons to see if they are still reuseable.

On page 41 of my SC spec book it gives some information on how to measure the piston ring gap and ring height clearance.

First off am I to understand that the ring gap is the measurement taken with a new ring slid into a cylinder and then use a feel er blade to measure the gap (at the open end)?
Then the piston ring "height clearance" is actually the one that relates to installing a new ring into a piston groove and measuring the gap between the ring and piston groove with a feeler blade?
Am I right so far?

[This message has been edited by Leland Pate (edited 05-07-2001).]

Ok part two...

I bought a feeler blade set from a local parts store but it is an SAE set with Metric conversions stamped on it.

From what I can understand the wear limit for the compression rings (from my spec book) says 0,2 mm. (note the "," comma) instead of the normal "." ...just a german thing?)

I have on my feeler blade set a .178 mm and a .203mm blade. Neither of them will fit in my piston ring gap. Does this mean that they are in spec???!!!
I think so and this might seem elementary but I JUST WANT TO MAKE SURE!

Ok, why does my Factory spec book say the height limit fo the compression ring is 0,2 mm and Bruce Andersons book says the limit is 0.115 mm?
This is really starting to give me some heart ache. That is a big difference!

Ok, I feel like i'm talking to myself but I want to add that in my Haynes manual (supplementary chapter) and Bentley 3.2 manual says the side clearance for the two compression rings wear limit is 0.2mm as well. So why does Bruce Anderson say the wear limit is 0.115mm?
Well I've go three out of four rooting for me but I am still concerned.

[This message has been edited by Leland Pate (edited 05-07-2001).]

Leland, I'm not going to ignore you, even if the others do. I can't account for BS's differing specs, but I'd agree that your measuring methods are correct. Sounds like your piston grooves are 'tight.'

And yes, the ring gap is just as you described. Put a ring, by itself, into the bore and measure the gap. Please note that the cylinder will have a tapered bore. Just under the top of the cylinder you'll find a lip. You should be able to feel it with your fingernail. This is a highest point taht the upper compression ring could get to. The diameter of the bore above this ridge is the original bore. If the bore is wider below this ridge, it is cause by wear. I believe the greatest wear will be near the top, just below the ridge.

If you have a tool that actually measures bore, I'd use it to measure all cylinders everywhere that the rings were touching, particularly the cylinder whose stud was broke, to see if the wear pattern is uniform, or "goofy" (a professional, technical term). Finally, there's piston measurements.

I am very interested in your measurements, Leland. I mean, your engine parts measurements. Whew!

I wish I could hold my parts in my hand like you can. Again, engine parts is what I mean.

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'83 SC

Jim, you kill me!

Well I was starting to think that the last "dead rabbit" joke really did me in...
All I wanted was a second opinion about my technique, and thank you for that Jim.

The Haynes, Bentley and Spec book all say 0.2mm so that is what I am going to go with.

I also did something very exciting today!!!!!!!!!!!!!!!!!!!!!

I put the layshaft and crank back in the case to measure the layshaft gear clearance with my new spiffy dial indicator.
With the aid of my supper spiffy computer conversion program I was able to figure that my layshaft has 0.0254mm of travel...
BA says no more than 0.049mm is allowable so I'm in the nook!
I'll post pics later tonight!
I love it!

Lee,

Your piston and ring measurements sound great, so far! So, try a 0.002" or 0.003" feeler gauge for the top height gap! BA probably recommends not reusing pistons with more than 0.004" height clearance because that would mean the pistons are more than 50% 'towards' the wear limit, and since new rings tend to 'bite into' the old pistons and wear more ... tending to break, it just is risking a problem needing another teardown sooner rather than later!

What marking does your intermediate shaft gear and crankcase have, 0 or 1? You will find the end play specs for different cases and gears on p. 33 of your spec book! Your measurements are fine, I just wanted you to be aware that there is not just one spec!

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Warren Hall
1973 911S Targa

Leland,
Although I think you will get excellent recommendations and suggestions from the many experienced people on the forum, there are many factors to consider when rebuilding an engine; a lot having to do with experience and that which must be visually inspected for signs of normal as well as abnormal wear.

I'm not sure how much you'll have invested in your rebuild, but I'd hate to think of the consequences if you overlooked something; for you and/or the next owner. Would it be possible for you to pay an experienced wrench to peek in once in awhile to see the progress and for you to benefit from his experience? Or as an alternative, rebuild something similar, like an air-cooled Vdub and see what happens. I hate to discourage you, in fact I'm impressed by your willingness to dive in and learn. I just don't want dire consequences due to your lack of experience. If you proceed, read all you can on the subject, take it slow and easy and don't be afraid to ask the "stupid" questions (hint: there's no such thing).

Best wishes,
Sherwood Lee
http://members.rennlist.org/911pcars

Sherwood, ...
I live in Idaho....

There is one (1), one Porsche mechanic in this entire state.
And he really gave me the attitude of "just replace everything" so I have not seeked his guidance since. I know all too well that I am inexperienced. In fact it scares the hell out of me. But this is the best way to learn. I can do it, I am confidant of that, I just need alot of reassurance that whatever I am doing is correct because i have never done this before.
IT'S TOO LATE TO TURN BACK NOW BABY!!!!

And yes Warren, I noticed the different specs for the different gear combos.
Thanks for your concern Sherwood...I only wish I had that Luxury

Leland:I recently bought a rebuilt 2.7 engine that on the rebuild reciept it said that the cylinders had been "acid etched prior to new rings being installed" I asked the shop that I bought the engine from and they had told me that it was a process that his german engine guy had always used when rebuilding porche engines as it makes sure the rings have a good surface to seal against and they seat alot better..Maybe Warren or somebody else could shed some light on this process and whether or not its worthwile doing.

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Grant 75 911s targa

Well I'm skeptical, but let's hear some more about this acid etching thing. And yes, Leland is building one of the more exacting and unforgiving engines ever designed for a productin car but....do I look worried. Well okay, you can't see me, but I don't look worried, trust me.

Besides, at the rate Leland is going gasoline will be a controlled substances when he's done. And Leland will be the Controller, probably.

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'83 SC

Acid etching is an inherent part of manufacturing or prepping reconditioned Alusil cylinders prior to installing new pistons or rings. It leaves the base aluminum a few thousand microns below the level of the chunks of silicon that actually are the wearing surface!

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Warren Hall
1973 911S Targa