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THE MAESTRO'S PERPLEXING PISTON PROBLEM QUIZ
The Maestro's told you about Piston Problems before- notably the three sets of Brand NEW 911 2-liter Mahle Pistons at the Ural-Asia Importers that were stuffed inside 912 Cylinders.
Now you should know that a 911 Piston is 80mm diameter, but a 912 Cylinder is 82.5mm.
That's a silly 2.5 millimeters Running Clearance! Which is about 0.100"- almost an EIGHT OF AN INCH!
Which means that when you tipped the cylinder up, the piston fell out! Very embarrassing for Inspector 69 to miss that. And no need for a ring compressor here!
That was one heck of a Factory-Installed Error! Where's CU and the FTC when you really need 'em?
Or take the L&R Oil Cooler, please, that was COMPLETELY CLOGGED with 1.78 POUND Of SOLDER, rendering it incapable of passing (or cooling) any oil. (And blocking off an oil passageway to boot.)
Today, another Piston problem was pointed out by Chuck Stoddard (Yes, THE Chuck Stoddard- the Founder & former owner of Stoddard Imported Cars, and a fellow graduate of the Maestro's Alma Mater.)
Seems as he was going through his collection of NOS Pistons and noticed something Interesting.
On a Stock Mahle or Kolbenschmidt Piston there is a "Pictograph" stamped into the top of the Piston. It's a "stick-figure" squiggly Crankshaft with a "Flywheel" attached, near an exhaust valve cut-out.
The Flywheel end of the Pictograph is how you're supposed to arrange the Piston on the Rod, with that part of the Piston facing towards the Flywheel.
Later on, the Factory realized that the average Intelligence was declining worldwide, and decided to put an ARROW on the piston too- just in case the Pictograph is too difficult for VW mechanics to interpret.
(And the Maestro DID have one wise-ass Customer who thought assembling Porsche Engines was a piece of cake. Until the Maestro asked ifin he noticed the Pictograph on top of the SC pistons.
"Sure he said- it's a crooked arrow."
"No" said the Maestro. "It's a Pictograph of a Crank with a Flywheel," showing him an SC piston with exactly those markings on it.
The guy ran home and disassembled his very recently assembled engine to find that, yes he HAD put EVERY piston on BACKWARDS! (Kind of like the Mercedes Mechanic who Pictured in his mind putting a Porsche 356 engine into the front of a Mercedes. And thus, put every piston on the Porsche bass-ackwards.)
But the Piston Problem Mr. Stoddard called the Maestro about could affect you personally!
For, on top of all four Pistons in the set was the usual Pictograph of the Crank & Flywheel.
And there was the "Arrow" too.
But both the Pictograph and the Arrow were pointing in OPPOSITE Directions!
So, either the Arrow was pointing in the "wrong" direction.
Or the Crankshaft Pictograph was pointing in the wrong direction.
One thing for certain- ONE of the two was wrong!
The Question from Mr. Stoddard, and for YOU- is How do you tell Which one is right? The Arrow or the Pictograph???
Will the Right Direction please stand up?
Better guess right. Your engine's life may depend on it! Keep the 356 Faith Maestro
Well, you guys are pretty smart. Two of you got that there's a "bump" or "protuberance" on the inside of most (but not all) Pistons- on one half of the Wrist Pin Boss- that's also on the same side as the "Arrow" or Flywheel in the Pictograph.
And that's the side that should point towards the Flywheel! But that's not what the Maestro was really after- after all, not all Pistons have this "bump" on the inside of the wrist pin! Yeah, one guy claimed you could actually see the "offset" ifin you look hard enough. Right. And NO Distributor Drives are ever put in Bass-Ackwards. And it don't rain in Indianapolis in the Summertime. What the Maestro was after was a simple, but effective technique to measure the wrist pin "offset" at home. And Mr. Stoddard, being a fellow Alumnus of a rather good East Coast Engineering School, made a fixture to do just that.
Take a Vice. Most any vice. Install a protective covering over the teeth of the Vice. Push wrist pin out of Piston until 2/3 of the wrist pin is sticking out. Stick the section that's sticking out into the covered jaws of the vice. Snug jaws of Vice around Wrist pin. Place one "leg" of a Carpenter's Square or equivalent on the table where the Vice is. Place the other "leg" of the Square up against the side of the Piston. (Adjust the Piston if necessary). Without moving the Square, Rotate the piston until the OTHER side of the Piston is facing the Square. Note that there is now a noticeable "gap" between the leg of the Square and the Piston. That "gap" is a measure of the "offset". (Actually, the gap is TWICE the wrist pin "offset", and it is left to the Student to prove why this is so.) Oh, so when you rotated the Piston, the Piston moved the Square back a little? No problem. Line the Square up again against the side of the Piston. Then Rotate the piston until the other side of the Piston is facing the Square. NOW, note the "gap". When you see the "gap", that side of the Piston is the one where the Wrist Pin is closest and that side should be "down", when installed on the #1/#2 side of the Engine. And "up" on the #3/#4 side. (Think of the "offset" as increasing or decreasing the angle of the "lever" arm of the Connecting Rod on the Crankshaft. If, on the #1/#2 (passenger's) side, you were to put the offset "up" then the Lever arm and Torque would be increased, to the detriment of piston "slap". If the "offset" were put "down", then the Lever Arm and Torque is DEcreased, but the Piston "slap" is reduced, as intended. (Some Formula V racers purposely put their VW pistons in backwards- to INCREASE the Lever arm and Torque- piston slap be damned- especially if you win the race.) A Tip o' the Maestro's Hat to Mr. Stoddard for this simple but effective, handy-dandy little homemade tool to measure piston "offset". And: Keep the 356 Faith Maestro Here's an attempt to explain this in PseudoGraphics, although it'll probably come out awful in Word or AOL.
_____________ / Piston \ / \ _____ | | | | | offset wrist | (note: wrist pin | | | pin | held in soft-jawed | | | O | Vice. Piston then | | | -------- | rotated around | | | / \ | wrist pin to find & | | |/ \| measure offset) __________| | "T" Square | ______________| =====gap==== gap= twice offset
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