Broken cylinder base studs IO-470

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dmount76

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Got a puzzeling one I'd appreciate any input on. 1962 Baron A 55. IO 470-L with 1,600 hours. Had a miss and diagnosed it to #3 on the left side. Plugs showed dark sooty appearance suggesting rich mixture. No CHT, but have EGT and EGT was running about 200-300 degrees back of other cylinders especially at lower power settings. Went to get a better look suspecting ignition or fuel problem. Pulled cowl to discover broken off cylinder base nuts on the #3. Pulled lower cowl. All 6 cylinder studs broken off. Only through bolts holding on cylinder! Cylinder has 1,600 hours on it with no other known problems. Pulled cylinder off today and extracted studs. No sign of detonation that we can see. Just signs of rich running condition. Will inspect cylinder base flange and case mounting service more closely but nothing obvious at the moment now. Wondering what to do? Replace cylinder? What could cause this? Thanks in advance.
 
dmount76 said:
What could cause this?
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Probably the through bolts lost their torque.
How many of the broken stud ends were shiny vs darkened?
May want to measure out the entire cylinder and especially the flange and check for cracks, or send it for an IRAN.
Might also check the opposite cylinder as well.
Definitely check the torque on all cylinders.
 
Removing the nuts on the remaining through bolts, they were extremely tight. All of the broken studs looked shiny and relatively fresh. I'll check again to confirm next trip.
 
misread torque spec, used (higher) value for different engine?
(gotta wonder about the other cylinders, now - regardless of what theories you get here)
 
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Bell206 said:
Probably the through bolts lost their torque.
How many of the broken stud ends were shiny vs darkened?
May want to measure out the entire cylinder and especially the flange and check for cracks, or send it for an IRAN.
Might also check the opposite cylinder as well.
Definitely check the torque on all cylinders.
Click to expand...
If that cylinder was moving, it might have been cocking the piston on the con rod and cracking both the rod and piston.

Continental has a Service Bulletin on it: https://fadodge.com/wp-content/uplo...aft_engine_service_bulletin_torque_limits.pdf

The SB has been superseded by SB96-7D, which leaves out the salient details and refers you to the latest engine overhaul manual. AN excerpt from the above SB:

1789952170944.png
 
I don't think there's even a question of replacing with that same cylinder given its flange participated in the failure of all cylinder hold down studs. This sounds like a rapidly cascading failure where one stud failure led to the next and on to the next if all had shiny un-abraded failure surfaces.

Without knowing the history of the engine and/or cylinder other than TT, it's hard to really know why this happen. I'd consult with an engine builder, as while having an occasional broken stud happens, having all go nearly at once after a long time in service (unknown here is how long since it was on the engine regardless of TIS - possible never since rebuild) suggests a fundamental change in operation.

Since for a (short?) time the engine ran with just the through bolt(s) taking up the loads, I'd say the whole situation unfortunately warrants more expert and likely more involved tear down/measurement/replacement/overhaul.
 
dmount76 said:
All of the broken studs looked shiny and relatively fresh. I'll check again to confirm next trip
Click to expand...
Interesting. If all the broken stud faces were shiny and possibly broken at the same time points to a potential larger underlying issue, than just a loss of torque and stud failure over a period. As mentioned above need to expand your checks and pull the opposite cylinder, then have your mechanic do a detailed check of both cylinder assys, etc. Would also check those through bolts for stretch and ensure the crankcase around the cylinder mount areas for any issues. And just to add, if I were looking into this I'd get a call to the OEM tech support for some input.
 
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Impressive that the engine kept running and didn't self destruct. And good for you for catching it, saving the engine and maybe yourself.

What is the history of that cylinder? Were all cylinders put on at same time, or was that one installed separately for some reason? What is different about those studs? If bad torques or defective bolts were used, you'd think it would affect the other cylinders as well. Was the cylinder mating surface clean, or was there any contamination, "accidental" sealant, etc.?
 
bonefishpete said:
I would think there is some fretting on the case?
Click to expand...
Should be some signs of it. That cylinder, held by two bolts on one side, had to be rocking sideways a bit, at least. Likely the only other thing holding it would be the exhaust system, and that might be distorted or cracked, too.
 
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