Sheared cylinder studs

Witmo

Witmo

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Tim
Pilot heard a noise sounding like a backfire about 3K agl after takeoff. Landed and parked it for me to look at. Didn't sound like any afterfire I'd ever heard so I grounded airplane pending investigation. Found four studs of the eight holding the #1 cylinder on the Lycoming O-360 had sheared off. The engine had less than 50 hours SMOH.

Other than mis-torqueing the nuts, does anything else come to mind that would cause the studs to break? Engine was running fine up til then. Airplane is used to tow gliders and this happened on the eighth tow of the day.
 
Over-torquing the four that broke, or under-torquing the four that didn't, comes to mind, unless you find a piston welded to the cylinder!
 
Can you tell if there is paint between the cylinder flange and case half?
 
I want to know if they're through bolts as well. But for what it's worth, not all the large studs are through bolts so a closer examination of what fasteners are broken will be necessary than just "big" or "little".

Depending on the history of the case and the thoroughness of the overhaul, there could be a number of problems leading up to broken studs.
 
We had two studs sheer on one cylinder on our IO-360 many many years ago, I'd have to look at the logs. I can't find the posts from then in the search function, so they must be gone with the wind. Anyway, we had the A&P replace all of the thru studs on that cylinder as well as all the short ones for that cylinder, and button it back up. That engine ran happily for at least 6 years after that. Sadly I cannot remember if it was the shorts or thrus that broke or if it was a mixture.
 
All new cylinder attaching hardware, studs, nuts and through studs run about $1k for an O320.
 
All hardware Thru-studs should be changed at overhaul.
every time they are torqued they stretch, eventually they will crack.

Who knows when they will fail?
 
Cylinder studs are supposed to be measured for spec dimensions for maximum stretch. A lot of shops don't. Few rarely use recently calibrated torque wrenches and some apply the torque incorrectly.
 
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NordicDave said:
Cylinder stubs are supposed to be measured for spec dimensions for maximum stretch. A lot of shops don't. Few rarely use recently calibrated torque wrenches and some apply the torque incorrectly.
Click to expand...
Define a lot of shops.

FYI: If it's a Part 145 CRS shop then they're busting the regs if their torque wrenches are not properly calibrated or they don't follow the OEM procedures. Don't know about non-CRS shops as I've only dealt with 145 shops for engine work.
 
Tom-D said:
All hardware Thru-studs should be changed at overhaul. every time they are torqued they stretch, eventually they will crack.
Who knows when they will fail?
Click to expand...

How about even before then. The Mike Buschs among us (me included to some extent) may now be running their engines to over 3000 hrs between oh's. Maybe the ones involved in every cylinder change; or at least all of them, every top?

NordicDave said:
Cylinder stubs are supposed to be measured for spec dimensions for maximum stretch. A lot of shops don't. Few rarely use recently calibrated torque wrenches and some apply the torque incorrectly.
Click to expand...

I did not know that, I think I read Lycoming conn rods are torqued by measuring stretch but through-bolts too?
Or maybe you mean they should be measured for length before reusing them?
 
Let'sgoflying! said:
...you mean they should be measured for length before reusing them?
Click to expand...

Yes, this one. The studs are supposed to be measured for max stretch and diameter before reuse. The Continental manual says the studs must have zinc plating present on the threads and lubed with specific instructions for oil type. Nuts supposed to lubed as well. Using a Calibrated torque wrench (not Harbor Freight), final tightening must be one sweeping movement and not taking bites out of the tightening until the click is heard.

Surprising how many A&P's don't follow the service manual specific to an engine model; when replacing cylinders.
 
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NordicDave said:
Yes, this one. The studs are supposed to be measured for max stretch and diameter before reuse.
Click to expand...
When you use the proper repair station that is all supposed to happen.
How many times has this cylinder been changed?
 
Tom-D said:
When you use the proper repair station that is all supposed to happen.
How many times has this cylinder been changed?
Click to expand...
This is an exchanged engine so I don't know the history in much detail.
 
NordicDave said:
Cylinder studs are supposed to be measured for spec dimensions for maximum stretch. A lot of shops don't. Few rarely use recently calibrated torque wrenches and some apply the torque incorrectly.
Click to expand...
CRS are supposed to be in a cal program.
This is Why I do not trust field overhauled engine.
 
Witmo said:
This is an exchanged engine so I don't know the history in much detail.
Click to expand...

We exchanged our O360 which was nearing TBO and had had a sudden stoppage, for a newly overhauled O360.
 
Just so you know, "shear" is the wrong term for that stud failure. Those studs are not loaded in shear; they're loaded in tension, and the failure is a tensile failure.
serveimage
 
The shop that installed the engine says they will contact the overhauler, until we hear from them...
 
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