Incorrect use can lead to disaster
By Joe Escobar
In the assembly procedure of a piston engine, it is standard practice to use silk thread and a thin film of sealant to seal the crankcase parting surfaces. Engine manufacturers outline this procedure in their overhaul manuals, and it is an acceptable method of sealing these surfaces. But if silk thread is used in other areas of the engine during assembly, namely around the bearing saddles at the thru-studs, it can lead to disaster. This article will shed a little light on why this is not a good practice.
More on sealants
The latest revision of Textron Lycoming Service Instruction No. 1125 specifies POB No. 4 Perfect Seal and silk thread as the generally used items for sealing crankcase finished parting surfaces that do not employ gaskets. Two other products, RTV-102 or LOCTITE-515 applied as a very thin film without silk thread, may be used as alternate materials for sealing crankcase parting surfaces. Other sealants have not been tested and approved for this purpose, and NONE are approved for other uses in the assembly of Lycoming engines. Improper use of these and other sealing compounds can create serious problems. As an example, a red colored sealant was used by a mechanic to hold the pressure screen gasket in place. As the material solidified, pieces broke loose and eventually blocked the engine's small oil passages causing oil starvation and engine failure. The bottom line: only use those sealant materials approved by the Lycoming Overhaul manual and other service publications, and only use them for approved purposes.
Lycoming Flyer Key Reprints, 19
With the good intention of preventing possible oil leaks, some mechanics have gotten in the habit of applying sealer and a ring of silk thread around the thru-studs. While this may seem like a harmless practice to try and reduce oil leaks, sealer and thread applied at this area can cause a catastrophic engine failure.
James L. Tubbs, vice president of engineering at Engine Components Inc., has been involved in numerous accident and incident investigations where the failures stem from the addition of materials between the bearing saddles. He points out, "We have conducted comprehensive engineering analysis of these phenomena, and the results are clear!"
It can be hard for a mechanic who is not an engineer to understand why such a harmless seeming procedure can be so detrimental to the engine. A little background on preload and torque can come in handy in understanding this problem.
Torque and preload
When engineers design an engine, they perform analysis on all of the stresses
involved with each part. These stresses as well as temperature changes and fatigue are taken into consideration when figuring how much preload must be applied to the part. This preload is the clamping force that the bolt will be applying to the part.
In order to achieve the desired preload, the engineers take the preload that is required and convert that into a torque value to use when torquing the bolt. When figuring out this torque, they take all of the factors like the bolt size, material, and friction into account. Varying any of these factors will affect the effective preload that is being applied. For example, if the procedure calls for lubricating a bolt before torquing and it is not, although the torque wrench may indicate that the desired torque was achieved, the actual preload will be lower because of the added friction when torquing. In addition, damaged or dirty threads can reduce the amount of preload applied.
In the same manner, the bolted joint preload for the thru-studs is a critical factor. When torquing these thru-studs, if gasket material and silk thread are added to the thru-studs, although the indicated torque may be reached on the torque wrench, the effective preload will be lower. Instead of a firm preload being applied to the part, the gasket material acts like microscopic springs. The result is an effective loss of preload on the assembly.
What happens next?
So what is the result of inadequate preload? Basically, failure can occur
because either the undertorqued bolt or nut backs out or because of a stress fracture. A stress fracture can propagate on the bolt because it is subjected to stresses that it otherwise wouldn't if the proper preload was applied. It starts off as a small crack at a localized area of the bolt, usually around the collar. With each successive operation, the crack propagates slightly due to the cyclic stress it is subjected to. Over time, the crack will be large enough that the bolt completely fractures.
Tubbs offers the following engine failure reports where the failures are all similar:
- The through-studs failed and exhibit fatigue fractures just under a cylinder hold down nut.
- The cylinder hold down short studs either fail in fatigue, or the nuts back off.
- The engine quits when the cylinders move far enough out from the crankcase to sever the intake system or from piston structural failure.
- Upon teardown, the crankcase halves show considerable fretting and evidence of sealer and/or thread or other material between the bearing bosses.
Tubbs says that many of these failures are attributed to improper torque when the engine is assembled. Obviously, torque variations during assembly can cause these problems. However, accusations of improper torquing are erroneous more often than not.
As shown, even if proper torquing procedures are followed, the gasket changes the dynamics of the bolted joint and leads to an inadequate preload situation. Failure is almost inevitable. This practice, no matter how harmless it may seem, can be detrimental to the life of the engine.
