scsmith
Well Known Member
I just read Mike Busch's write-up about the factors that can impede achieving proper pre-load in the cylinder hold-down stud and through bolts when installing a cylinder.
He makes it sound like every engine that has ever had a cylinder swap with the engine mounted on the airplane is at high risk for failure and death! How many cylinders are changed on the airplane? How many have led to failures?
A million to one might be an exaggeration, but it is **** rare, for sure. I generally like Mike's discussions and learn from them, but I think this one is a bit over-dramatic.
Reading through his arguments, the real issue, and the real take-away from the story, is that the threads need to be very clean and very well lubricated. If the original assembly was done with generous lubrication, then his other big concern, thread wear from repeated assembly, is minimized. A second assembly, again with generous lubrication, can fairly be expected to produce proper preload. Lycoming's recommendation to mix 10% STP in with the lubricating oil is a very good protection against thread wear, as would other high-pressure lubes. In the car world, I always used an assembly lube that had some moly-sulfide in it.
The other take-away that is useful in his arguments is to be sure to have enough clear swing of the torque wrench to approach the right torque while the nut is turning. It would seem necessary to remove most of the accessories and other things that get in the way to do a cylinder job anyway, so in principle, the access shouldn't be much different between having the engine on an engine stand and having it on the airplane. Not to say that access isn't complicated in any case - there is a whole family of odd-shaped box-end wrenches that aren't called "Lycoming wrenches" for nothing! But fundamentally, it can be done on the airplane to the same degree of care as on an engine stand.
I do agree with Mike that many shops are too eager to pull a cylinder when other options should be explored first. No doubt his overly dramatic approach to a discussion about achieving proper pre-load is motivated by hoping to add some deterrent threat to those that would unreasonably want to pull cylinders off.
He makes it sound like every engine that has ever had a cylinder swap with the engine mounted on the airplane is at high risk for failure and death! How many cylinders are changed on the airplane? How many have led to failures?
A million to one might be an exaggeration, but it is **** rare, for sure. I generally like Mike's discussions and learn from them, but I think this one is a bit over-dramatic.
Reading through his arguments, the real issue, and the real take-away from the story, is that the threads need to be very clean and very well lubricated. If the original assembly was done with generous lubrication, then his other big concern, thread wear from repeated assembly, is minimized. A second assembly, again with generous lubrication, can fairly be expected to produce proper preload. Lycoming's recommendation to mix 10% STP in with the lubricating oil is a very good protection against thread wear, as would other high-pressure lubes. In the car world, I always used an assembly lube that had some moly-sulfide in it.
The other take-away that is useful in his arguments is to be sure to have enough clear swing of the torque wrench to approach the right torque while the nut is turning. It would seem necessary to remove most of the accessories and other things that get in the way to do a cylinder job anyway, so in principle, the access shouldn't be much different between having the engine on an engine stand and having it on the airplane. Not to say that access isn't complicated in any case - there is a whole family of odd-shaped box-end wrenches that aren't called "Lycoming wrenches" for nothing! But fundamentally, it can be done on the airplane to the same degree of care as on an engine stand.
I do agree with Mike that many shops are too eager to pull a cylinder when other options should be explored first. No doubt his overly dramatic approach to a discussion about achieving proper pre-load is motivated by hoping to add some deterrent threat to those that would unreasonably want to pull cylinders off.