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79 911SC Euro Piston Failure

After loud bang coming from engine compartment after attempting to start, engine ceased turning (aka full load starter current with full battery charge). Partial drop on engine and inspected timing chain/tensioners which were pristine.
Dropped engine and tore down and found that #2 piston destroyed itself. the piston pin failed on one side of the rod bearing, creating a torque at the pin to piston connection and snapped the entire bottom half of the piston out. this happened while trying to start the engine! Note attached pics.
Any thoughts on the cause of this failure would be greatly appreciated.
I had about 150 k on the motor and did not see any evidence of lack of lubrication. There was no evidence of scoring or other damage to the cylinder wall.
I am baffled by this one. I have a used set of 9.3 pistons and cylinders on the way and am reluctant to put the beast back together until I know what caused this mess.
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Old 02-13-2006, 06:40 PM
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Pure speculation but....backfire on starting attempt with already cracked piston? Raw gas in cylinder causing hydraulic lock? Terrorist retaliation for anti-Muslim cartoons in German newspaper?
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Old 02-13-2006, 09:56 PM
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the hydraulic lock option sounds promising.....tell me more. Not sure what this is and what the mechanism is.
thanx
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Old 02-14-2006, 04:59 PM
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A hydraulic lock is when enough liquid gets into a cylinder to cause the piston to run into a solid wall of water on it's way to TDC. It only takes 50cc's or so. Most likely would be gas from a runaway injector. (liquids are usually not compressible).

-Andy
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Old 02-14-2006, 05:34 PM
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Thanx, Andy
For this to occur, the piston would have to fail under compression. When you look closely at the piston, it failed under tension; i.e. the rod had to be pullng away from the piston somehow...note attached pic of the piston pin which appeared to initiate the disaster
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Old 02-14-2006, 09:03 PM
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Totally ametuer speculation but:

-couldn't the pin have broken in the same manner with enough force pushing up?

-did you manually crank the engine after dropping it to see whether the piston in question was going up or down?

-did you fit everything back together to see if you have any extra pieces that may have come from elsewhere?

-are the retainer clips still in the piston? could one have come out and wedged against the cylinder wall? (oops, just looked again and saw they are awol; still, might one have popped out somehow?)

-did any rings break and if so could a piece of the rings have lodged against the cylinder wall?

Erik

Last edited by berettafan; 02-15-2006 at 07:36 AM..
Old 02-15-2006, 07:33 AM
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The pin could have broken on the compression stroke. That would cause it to "bend" up against the dome and break off the parts of the piston that the pin goes through. After that the parts could go anywhere. Still could be hydraulic lock IMO.

-Andy
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Old 02-15-2006, 10:14 AM
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Looking at your first picture makes it look even more like the failure was during compression. There is a crack in the piston part where the pin was that looks like it was pushed toward the dome. On the "skinny" side there is a piece missing in the same location.
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Old 02-15-2006, 10:17 AM
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We had a similar failure a few years ago. In our case is appeared the piston siezed in the bore, and the rod ripped out the bottom of the piston.

How easily did that piston remnant come out of the jug?

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Old 02-15-2006, 04:00 PM
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Great feedback guys, thankyou
I did not see any evidence that the piston seized along the cylinder wall. there was only one tiny spot that had some metal gouged out. I guess it could have been hydraulic lock now that I look more closely at the pictures. the fuel injection was running like crap at the time and I had had numerous backfires or detonations in the airbox. I remember that I tried for a long time to get it to start before I heard the bang, so it is feasible. And you are correct, it could have been while the piston was going up.
thanx again. BTW, found a great set of 9.3 CR pistons and cylinders with pretty low mileage on them. I probably should replace the one rod that was connected to the piston. Are these rods the same for all 911SCs from 78 to 83? Does anyone know where I can get one in good shape?
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Old 02-15-2006, 05:37 PM
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Could the crank have been stressed in the process as well?
Old 02-16-2006, 05:47 AM
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Yes, I will have the crank checked out at my machine shop. I did notice that one ournal had minor scoring. Is it permissible to machine these things and put different bearings inserts in or do you screw up the factory hardening by doing this??
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Old 02-16-2006, 07:05 PM
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what? me worry?
 
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You may read in some of the 911 engine rebuilding guides that recommend against doing suchmachining on 911 cranks..

But there are more than a few places here in the states that can grind, polish, offset grind, resize, and the properly harden the crank.

We have a number of engines out there that we have cross drilled the center main, turned the rod throws so we can use Clevite NASCAR bearings so the engine can live at 9000rpm, had the rod throw radii modified to suit, etc.

Had cranks coated with DFL and bearing coated also.. that is beyond the scope of this thread though...
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who are those guys? <<< ( Butch Cassidy and the Sundance Kid)
Rent a GT3RS from us!! Call or e-mail.
Transportation and Track support
Rudtners Racing

'69 911 GT-5
'75 914 GT-3
Authorised Haltech seller and installer
Authorised Unichip installer
BBS wheels
Fikse wheels
Redline motors oils
Swepco
1500 hp chassis dyno
Old 02-16-2006, 07:15 PM
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