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356/912 Quiz of the Week: The Small Problem
Copyright 1997, by Harry Pellow, All Rights Reserved.

OK, guys. You think you're so smart. Then try diagnosing the following itty bitty problem that cropped up today on the Maestro's Telephone. ..... A Customer calls (408) 727-1864. The Maestro answers. The Customer has a 1960 356B Super Engine that came, already rebuilt, in the car he bought. ..... The Customer had removed the Flywheel to surface and balance it as part of doing a clutch job The Right Way, the First Time. ..... The Customer, before sticking the flywheel back on, checked the End Play like a good mechanic should do. ..... Lo, a good thing too, for the end play is Not Right. With a .035" shim in place the End Play is over 12 thou (0.012"). Since the End Play spec is .004-.007", (and the Maestro likes .0055"), the Customer figures he needs about a 0.042" shim which he ordered from Stoddard. ..... The .042" shim arrived. The Customer installed it and torqued the flywheel to measure the End Play. As the flywheel nut got tight, so did the end play! As the flywheel nut got really tight, the engine SEIZED! ..... How can this be, asks the Customer. A .035" shim has .012" End Play, so a .042" shim should provide .012-.007= .005" End Play. But the engine seizes instead! ..... The Customer wracked his brain trying to figure out this puzzle, inside a Conundrum, wrapped around an enigma, but to Know A Vail. So he did the only thing he could Under the Circumstances- and called the Maestro at (408) 727-1864 and asked: How come, Maestro? ..... The Maestro at first, couldn't believe how 12 thou of end play could disappear by adding 7 thou of shim thickness. But, Curious that he is about weird Porsche Problems, he devised an Experimental Approach on the fly, over the phone. ..... Given that the Customer had 2 End Play Shims, one .035" the other .042", and TWO Soft Iron Gaskets. Soooooooo. ..... The Maestro's Experimental Procedure: Part 1. OK, so you got Twelve Thou End Pay with the .035" shim- with the Soft Iron Gasket in place. Now, try that SAME .035" shim withOUT the Soft Iron Gasket in place! (No, we won't put it together this way- we're doing an Experiment here!) ..... Since the Soft Iron Gasket is .004-.005" thick, leaving it OUT will REDUCE the end play from .012" to maybe .007-.008. It's the equivalent of making the .035" shim "thicker" by .004", about equal to a .039" shim. Part 2. Since the .042" shim SEIZES the engine, try putting an EXTRA Soft Iron Gasket in with the .042" shim. With TWO soft Iron Gaskets, it's like making the .042" shim into a .038" shim. ..... The Maestro figured the results of t His two Experiments oughta be Interesting. ..... More than that, they were Fascinating! ..... 1. With the .035" shim an NO Soft Iron Gasket, the End Play was STILL .012". ..... 2. And with the .042" shim and TWO Soft Iron Gaskets, the engine still Seized! ..... So, what was wrong? And Why? ..... Just then the Light Bulb clicked on in the Maestro's somewhat twisted brain. ..... "OK," said the Maestro. "So now you have a SEIZED engine with the .042" shim and TWO Soft Iron Gaskets. This is gonna sound Crazy- but go to the Engine and see ifin there's any End Play NOW! (Aside: obviously, Under Normal Circumstances, an engine seized from too thick an End Play shim will have, obviously, Zero end play! But this is not a Normal Circumstance.) ..... The Maestro asked the Customer to do something Weird. (No, not THAT weird!), and go to the seized engine and measure the End Play. ..... The Customer did as he was told, albeit a bit reluctantly, and went back to check the End Play on the Seized engine. On the phone, the Maestro could hear the pitter-patter of work boots walking out to the engine. Followed by a pregnant pause. Followed by the RAPID pitter-patter of work boots RUNNING back to the phone! ..... Excitedly, the Customer told the Maestro: ..... "Maestro, Maestro, said the Customer. "You won't believe what I found!" ..... "The Seized engine STILL has "end play", huh?" said the Maestro. ..... "Why that's really AMAZING, Maestro. Yes! The "seized" engine still has Twelve Thou End Play! ..... "That's too bad," said the Maestro. "I was hoping it would be something else." ..... "Why?, said the Customer, now apprehensive. "Is having 12 thou end play on a seized engine bad?" ..... "Yes," said the Maestro. "What your measurements show is that the Flywheel Main Bearing is "walking"- moving back and forth axially in the case. A "regular" engine seizes from an end play shim that's larger than the the gap between the Flywheel and the #1 Main Bearing. ..... But your problem is that the Flywheel Main Bearing has a worn Thrust Surface and the whole locked assembly of Flywheel, Crank and Bearing are clunking back and forth as a unit. ..... The "End Play" you measured on your "seized" engine is how much the Thrust Surface of the Flywheel Main Bearing is worn! About twelve thou." ..... "Is that bad?", asked the Customer. ..... "Well, it's not good. The fact that the FWM Bearing's Thrust surface is worn usually means that the bearing is loose in the case- and that the case therefore needs an Align Bore. And if the FWM Bearing gets REALLY loose, it'll pound the side of the CASE badly enough that a new Align Bore Bearings won't be tight in the case and the problem will repeat itself again and again. Ad infinitum. ..... If the Case does have a worn Thrust Surface, it can be welded up and the Thrust recut (messy and stress-inducing). Or, if one can obtain a FWM bearing with an "Oversize Thrust" (A thicker thrust flange on the FWM), then one can trim the case and cut the Bearing's thrust to fit the trimmed case. And Voila! There you are. Until the next overhaul. ..... Oh, but there's only one minor problem with obtaining an Oversize Thrust Flywheel Main Bearing- Porsche never made a FWM bearing with an Oversize Thrust! ..... "So how can I get one if the Factory never made one???" ..... "The Good News is that Volkswagen DID make a FWM with the oversize thrust you need. The Bad News is that these were only available in Align Bore sizes! ..... The Good, Bad news is that with a "walking" bearing, you're probably gonna need an align bore anyway, so you'll need Align Bore bearings too. ..... But weren't VW and Porsche Case sizes DIFFERENT? ..... Yes. The VW had a Case Bore of 60.00mm. Porsches have a Case Bore of 60.24mm, a silly quarter millimeter larger. (The Maestro figures that, In The Beginning, when Porsche used VW 36 HP Cases that Porsche either (A.) didn't really trust VW's Align Boring, or (B.) (more likely), took VW's rejected Cases and rebored them THEIR way! Hence, the silly little quarter millimeter difference- and the reason why 2-Piece Porsche Cases can NOT use Standard VW 36HP Bearings, despite the assertions of many a VW mechanic.) ..... Anyhow, VW Align Bore Bearings go in steps of 0.5mm thanks to their more easily deformed Magnesium Cases that require half a millimeter to clean up. ..... Porsche Align Bore sizes go in steps of 0.25mm, thanks to the much stronger Luftwaffe Aluminum used in all 3-Piece Porsche Cases, and usually clean up nicely at .25mm over. ..... The Good News is that, by a Fortuitous Coincidence, First Over VW (60.00mm+0.50mm= 60.5mm) is the same as 1st Over Porsche (60.24mm+0.25mm=60.49mm). ..... (Note, however that this is NOT true for Second Oversize! Second Over Porsche (60.24mm+0.50mm=60.74mm) does not equal Second Over VW (60.00mm+1.00mm=61mm). Second Over Porsche is a Nowhere Land Oversize. ..... However, THIRD over Porsche (60.24+0.75=60.99mm) is the same as 2nd over VW (60.0mm+1.0mm=61.00mm).) But Engines built with Third Over Cases never seem to run well for long. ..... Anyhow, should YOU be faced with an End Play problem like the above, you now know what the Solution is. ..... An Engine Overhaul!




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