Ring Gap
Copyright 1997, by Harry Pellow, All Rights Reserved.

ON THE HOLE, IT SMOKES

.....Maybe you saw the Middle-Column Story on June 13, 1994, wherein the "Wall Street Journal" interviewed a College Professor who had "made up" the official "Klingon" language. That's Klingon- you know- as in "Star Trek"- THAT kind of Klingon.

.....Anyhow, in the article, when asked ifin making up a language for an entirely fictional race living hundreds of years in the Future seemed bizarre, the Professor of Klingon said it's:

....."No more bizarre than sports trivia or knowing the details of engines of cars that haven't been manufactured in 20 years."

.....WHAT???!!! Knowing the Details of 20-year-old car engines is Bizarre???

.....Well, then the Maestro MUST be Bizarre, 'cause his whole LIVELIHOOD depends upon knowing Intimate Details not just of Twenty-year-old cars, but of Twenty-FIVE to FORTY-FIVE year-old cars! (The First 356 was made circa 1950- that's 45 years. Almost. The last 912 was made in 1969. That's 25 years- at least.) And they most definitely aren't in production today. And that's a cryin' shame! Not Bizarre.

.....Speaking of Bizarre, the Maestro's been asked many times ifin he hasn't seen It All. Meaning that, in his Collection of Broken Porsche Engine Parts, he must have every possible Bizarre failure. There must not be much new under the Sun?

.....Don't you Believe it!

.....Because just when you're arrogant enough to think you've seen it all, what walks through the door but some Unique Failure you've never seen before. And because things always Come In Pairs, you'll see an IDENTICAL failure within two weeks. Maybe this Unique Failure will even happen to YOU after you read this Story. Affect you personally. Especially if you have a '68 912.

.....Because, yes, this Bizarre problem happened on a '68 912, of course. What else. Those poor '68 912's, with the awful Smog Pump and cold air injection into a hot head that quickly causes cracking. Though they only made '68 912's for one year, it now seems like 2/3 of all the 912's On The Road are '68's. As was this one.

.....This '68 912 was so Original that it still had a WORKING smog pump- with all the plumbing intact! That's only the third one the Maestro's seen.

.....Actually, the 912 wasn't quite Original, having had an overhaul at 120,000 miles and now with 160,000 miles, needing another.

.....The Owner was a man formerly employed by a famous Computer Company named for a Fruit, and was the Reason that a MaestroMassaged, Gold-Plated, Super-90 Engine appears on Video as background enhancement in the "garage" where the Guru's of the Macintosh dispense advice through UGTV in '93. After all, either of the Two Steves can afford anything he wants for HIS garage!

.....Anyhow, the Director of the Video had a '68 912 that had a Unique Problem (he also had a nice '63 356B Coupe, so he knew Real Quality. The 912 was for his wife.)

.....One Typically Beautiful California June Day, in walks the Director with his '68 912 engine in a bunch of paper bags. And yes, it had the usual cracked '68 912 Cylinder Head- but only 1 crack in only one head- a New Low for '68 912 heads whose crack expectation is now approaching 1.5 cracks per head at the 90% confidence level. That's 3 cracks out of a possible 4.

.....It also had a Repro camshaft- of C1 casting infamy- whose lobes had been reduced to Molehills from the aftermarket Mountains.

.....The Original Ate Cam followers however, having wore the lobes of the aftermarket cam down to mere Twiggy-like nubs, (remember Twiggy?) suffered little for the ordeal. See, character (and Quality) counts.

.....But the Real Reason for the teardown, lay with the Original, Expensive SC/912 Mahle Pistons and Cylinders!

.....Ironically, the Maestro randomly grabbed the one afflicted piston from the Customer's shopping bag of 4. And noticed a very un-nice hole in the side of it- with melted piston funneling through the V-shape hole to the Lower end, to be distributed throughout the engine by the oil pump, bypassing the bypass oil filter, to contaminate the rod & main bearings.

.....But the hole in THIS piston was not caused by the usual broken ring/vibrating ring pieces/ring-eating monster/chunked top of piston/moon-like cylinder head sequence. No, not this one! This one was different.

.....Both of the two Top Compression Rings were intact- NOT broken! There was NO "ring eating monster" effect! The top of the piston HADN'T chunked.

.....There was merely a fairly large hole blow-torched through the side of the piston!

.....The Owner asked, "So, what do you think happened Maestro?"

.....Funny you should ask, thought the Maestro, as he eyeballed the piston. "This piston failed in a most unusual manner," noting first that which is painfully obvious. "How the hell do you blowtorch a hole through #4 piston (NOT #3), withOUT having Broken Rings or broken ring lands or some such?" he asked rhetorically.

.....Yes, that is the Question.

.....The Maestro turned the piston over and eyeballed the inside of the piston, in the hope this might give him a clue. He could see the four pairs of little oil return holes drilled through the 912 piston under the second (and last) Top Compression Ring.

.....Near where the burned hole was, one little oil return hole, still little, was intact. But the other oil return hole was now a BIG hole, enlarged to over a quarter-inch!

.....Enlarged by a verrrryyy Interesting Blowtorch/Saturn-V rocket exhaust effect. But HOW did it happen???

.....At first the Maestro thought- how the hell do you get the exhaust gas to bypass behind BOTH Compression rings to get to the oil return hole?

.....The Maestro though about that. And thought about that, and thought about that. The Customer tapped his foot, waiting impatiently for an answer.

.....The Maestro, clearly under pressure, tried to rotate the two intact top rings. They wouldn't rotate. They were "stuck" in one position, ironically, with their ring gaps lined up over the Oil hole,... With The Ring Gaps Lined Up!

.....WITH THE RING GAPS LINED UP!!!

....."THAT'S what happened!" shouted the Maestro, almost knocking the Customer off his feet.

....."The RING GAPS got lined up- AND, more importantly, the ring gaps got lined up right ON TOP of THE OIL RETURN HOLE!!"

.....The Maestro's somewhat twisted brain flashed back to a time Heap Many Moons ago when he was reading an ATE Instruction Sheet for installing replacement rings on a Piston. In the text there was a little note. "Note: do NOT align the ring gap of the lower Compression ring directly over an oil return hole," it said.

....."Now, why would they be worried about that?" thought the then proto-Maestro. There are usually THREE top compression rings. So what if the gap of the lowest ring is aligned over the oil return hole? The other two ring's gap would have to be aligned too- and that's a very Unlikely Event, as we used to say in the Nuclear Industry."

.....The proto-Maestro's conclusions are probably correct- for a three-top-compression ring piston. But ifin the piston only had TWO top Compression Rings, like a 912 does, things might be a little different.

.....With only TWO compression rings, a 912 piston is much more likely to have both ring gaps aligned. (It's part of the price you pay for lower friction & higher RPM. Speed & RPM cost money. How fast do you want to go?)

.....And if both ring gaps happen to line up above an oil return hole, a continuous tunnel or "hole" is momentarily formed. Hot exhaust gases channel through the hole, and there you are.

.....Now, getting both ring gaps to line up atop an oil hole sounds like a pretty damned unlikely event- unless one or both of the rings were INSTALLED with the gaps aligned or, by Murphy the rings rotated randomly and came together at Random times on top of the random oil hole.

.....And if the rings do move around then why doesn't the ring KEEP rotating and close off the "hole"? Could it be that maybe the hot exhaust gas flowing through the channel deposits a bit of debris that keeps the rings from rotating any more?

.....In any event, the Maestro holds in his hand proof that this ring-gap/oil-hole lineup DOES form a mechanical worm-hole that's permanently a part of your piston! Locked in place so that the escaping hot exhaust gas can blow-torch a BIGGER hole through the piston. Which allows more hot gas to pass. Which enlarges the hole, which causes more hot gas to pass. You can see where this is going.

.....Passing gas is not good. Passing hot exhaust gas is even worse.

.....And, boy, you have to be a real unlucky Porsche owner (or a '68 912 owner) to have that kind of bad luck!

.....So, let this be a lesson for you- when the Instructions say you should NEVER ALIGN THE RINGS GAPS when you're putting a piston into a cylinder (or in the case of a 356/912, when you're putting a Cylinder OVER a piston), there IS a Good Reason!

.....You've always known that aligned ring gaps can lead to excessive blow-by, but in this case, aligned ring gaps led to pieces of piston blowing by!

.....Yet another Lesson in Porsche Pathology.

.....And the proof of the old adage: when the Old Sage tells you to do something in a Strange or slightly Bizarre way, like DON'T ALIGN THE RING GAPS, it would be best ifin you'd BELIEVE him!!!

.....Actually, just do it, you don't have to believe it. Belief may take 30 years. Or the next Millennium. Whichever comes first.

.....But believe you me, you'd better:

..... KEEP THE 356/912 FAITH!!!

..... P.S.:

.....The Maestro started to calculate the Probability of the End Gap of both 912 compression rings aligning on top of an OIl Hole, and quickly realized that his 386 wasn't up to the task. He needs a Cray. Or a Pentium.

.....Easy problem, you say, eh? OK, then YOU solve it! The Failure looked like this:

.....TOP RING GAP==> ___________________ _____________________

..... LOWER RING GAP ==> ------------------- -------------------

..... O <======== OIL RETURN HOLE ------------------ --------------------

.....

.....So, here is the Challenge:

.....A 912 piston has 2 top compression rings. Each ring has an "end gap" that can vary from .006" to .016" when new to maybe a millimeter when old.

.....Below the second Ring are 8 little oil return holes, sorta evenly spaced around the piston.

.....The Stock 912 Piston is 82.5mm Diameter and the 8 little oil return holes are 2.5mm (0.100") in Diameter, and arranged around the circumference of the Piston, below the lower ring.

.....Part 1. What is the probability that the Ring Gaps of BOTH Compression Rings will Align on top of an Oil Return Hole if:

.....A. Both rings move at the same angular velocity?

.....B. Both rings move at different velocities? (Show velocity spectrum.)

.....C. One ring is stuck

.....1. over an oil hole

.....2. not over an oil hole

.....Part 2. With a ring gap of Oh, .010" to .040 (1mm)(worn rings), and both rings aligned over the Oil Return Hole, estimate the Exhaust Gas Flow Rate through the opening, calculate the Heat Transfer Coefficients, Piston Temperature profile, and enlargement rate of hole.

.....Part 3. Predict time and mileage to Failure.

.....Extra credit given for model which shows actual funnel-shaped erosion.

.....Use whatever computer facilities your employer may have available.

.....And:

.....KEEP THE 356/912 FAITH!!!
 

 

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