Failure of incorrectly bushed connecting rod.

Builders,

Here is a photo report on the Corvair Engine which mechanically failed after departing Sun n Fun. 

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EVENT:  Zenith 601XL-B with a 2,850 Corvair departed SNF and climbed to 5,200′, The engine experienced a sudden stoppage after running very rough for 2-3 seconds. Pilot declared and emergency and glided back to the airport to an uneventful landing with no damage to the airframe. 

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QUICK SUMMARY: Tearing the engine down showed the aftermarket billet connecting rods, set up for floating wrist pins had the bushings in the small ends set up incorrectly. The engine builder sourced them from a non-aviation supplier.  The bushings seized on the pins, rotated in the rods, and blocked the oil delivery holes. The #6 rod broke from this at the wrist pin centerline. The broken rod  destroyed the case, dislodged the starter, and stopped the engine from rotating by pushing the starter into the ring gear. Estimated cost of the replacement parts to fix the engine is approximately $4,000. 

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BACKGROUND: The engine was built in California in 2007. It had approximately 565 hours on it, and had been flown coast to coast several times. The builder was a highly experienced engine person, and had extensive contacts in the automotive racing industry. He elected to use new billet rods in place of the original Corvair forged rods. The Billet rods were a relatively new option for Corvair car owners. They were imported in s semi-finished stated and needed to have the bushings installed in the US. Because no aviation companies were yet doing this, the builder opted to have an automotive machine shop in his home state of California perform the operation. The pictures will show how the bushings became dislodged. with the #4 rod from the same engine as the photo sample.  At the time of the event, the new owner of the aircraft was flying it,  He has 275hrs TT, and 65hrs in the plane, including flying it from California to Florida 40 hours ago. 

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INFORMATION ON CORVAIR RODS:  GM made 12 million Corvair rods, The are a 4/5ths scale model of a Small Block Chevrolet rod.  They are forged, and just like the 400 million rods original small blocks, they use fixed, non-floating wrist pins. (Only very rare small blocks like 1967-69 DZ code 302cid Z-28’s had floating pins, no factory Corvairs ever had floating pins.)  Corvair rods have an excellent reputation, with only one known failure in flight, a 1999 event traced to a machining error when the rods were being rebuilt and modified for ARP bolts.   That aircraft was landed without damage.  Billet Rods became available through aviation suppliers approximately 2009.  There are hundreds of sets in aircraft with displacements ranging from 2,775cc to 3,300 cc, there are no documented failures.  Both the rebuilt original forged rods and the aftermarket new billet rods use new ARP fatteners. The GM rods have a bolt and nut set, the billet rods use a 12 point cap screw threaded into the body of the rod.  All billet rods are set up for floating pins. Virtually all GM rods are set up for fixed pins. ( with the exception of ‘dune buggy’ intended Corvairs with VW pistons, where GM rods were modified to accept the 22mm VW pins, run without bushings. It was a questionable system, and has not been promoted in nearly 20 years., very few are in aircraft. )

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ENGINE MAINTENCE BEFORE EVENT: The engine was given a condition inspection in California by IA with Corvair specific experience 60 Hours previous. to the event. The compressions were in the high 70’s on all cylinders. The engine was in a continuous oil analysis program by Lab One, who maintains the largest Corvair comparative database in the world for oil samples.  The samples actually showed a slight reduction in wear particles before the event. The failure in the event was not detectable in advance by normal methods typically used in the Corvair fleet. 

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OBSERVATION:  Although the top of #6 rod was missing, The other rods in the engine were examined, and it was determined the original bushings were installed incorrectly. The common pin to bushing clearance is .0008″. The other examined rods showed no clearance between the bushings and the pins.  With spiral locks for the floating pins centering the pins, and the tight fit keeping the piston centered, extreme stress was being placed on the spinal locks on the other cylinders.  However, all spiral locks were still in place, even the ones in the #6 piston, which had its pin still in place. 

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CONCLUSION: Because of the original  source of the aftermarket rods, was an automotive shop, and comparing the perfect track record of aviation sourced aftermarket rods with correctly installed bushings, this event is considered to be a one time, non-reoccurring failure. Perhaps the only operators who should be cautious are second owners working with older engine build with aftermarket rods from an unknown source.  Such engines can not count on early warning from standard testing, and should strongly consider a top end tear down to inspect for proper rod to wrist pin clearance and fit. 

 

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Head on look at #6. Notice the top of the case, the small end of the rod, and the lifter bores are completely missing.  This damage was done by the small end of the rod moving free of the pin.

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Three Corvair rods, Left, Stock forged GM rod, right a billet rod. The center rod looks like a billet rod, but is actually a GM rod with polished beams. It can be identified as a GM rod by looking for the ‘bolt and nut’ fasteners. Notice the billet rod does not use nuts. 

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Number 4 rod from event engine.  notice position of oil hole.

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Same rod, slightly different angle. Notice the bushing oil hole does not line up with the oil hole in the rod, thus the pin was starving for oil. The tight fit on the bushing to pin is the cause of the bushing becoming dislodged and slightly rotated. 

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Same rod flipped 180 degrees. Note how the bushing is migrating out of the rod. In a correct installation the bushing is exactly centered in the small end. 

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Remains of #6 rod on crank, showing smooth motion of rod bearing on crank. Crank appeared undamaged and rotated freely after starter was removed.  Crank will be retired and scrapped on principle. It could be magnafluxed, but that process is best for discovering stress cracks developed over time. This engine made perhaps 150 revolutions after the rod broke. It is enough to have the crank stressed, but not long enough for cracks from that stress to develop and propagate enough to be reliably detected by magnaflux.  The $1,000 cost to replace the crank is inexpensive in comparison to other engines.. 

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Airframe with engine removed. I did the removal to inspect for any available clues to a possible contributing factor. There were none. 

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Engine transported to my hangar, in the process of being torn down. 

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Remains of #6 cylinder. This was a Clarks’s aftermarket cylinder. In spite of the damage, the top of the cylinder was still round, fit the head gasket area, and the piston smoothly moved in the bore. 

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Slot was cut in top cover by the motion of the big end of the rod, unrestrained by the pin. 

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Underside of the starter showing how hard the big end of the rod struck the starter. This dislodged the starter up and forward into the ring gear, stopping the rotation of the engine.  The top cover with the rectangular slot was between the rod and the starter. 

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This shows the piston still moved freely in the bore, and the rings were undamaged. If this failure and been detonation related, the piston would not be in this condition. Over the years, several  engines have been detonated to death, but no matter how much pistons damage they suffered, the rods never broke.  This was not a detonation related failure.

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View of #4 piston, where side loads on the stuck piston pin produced very high loads on the spiral lock rings. notice they are being extruded here. 

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Number 4 pin and rod. It should push out with your finger tips, but this was driven out with a brass drift. 

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A look at the condition of the #6  piston, note the pin still in place, both spiral locks in place. 

 

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Thank you,

William Wynne. 

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