Thursday, June 18, 2009

Sealing Aircraft Engine Cases

The sealing of engine cases seems to come up on aircraft groups often. Lycoming says to use POB #4 and some other materials. I have tried to ask question about where to purchase these materials and cannot get an answer. Some people say it is made by Perfect seal, and some say Permatex. Could you clearify this and show where and how to use these materials in a section? You could have pictures showing the thread and showing how much of the sealant is put on these areas.


I am reluctant to discuss crankcase sealing as my method (the traditional method) conflicts with both Lycoming and Continental. The way almost all overhaul shops did it 50 years ago is time proven - Titeseal and silk thread.

The Titeseal (usually medium weight) is used only to provide a tack surface for the thread; it does no sealing in itself. Thus the line of Titeseal need not extend completely across the surface, just wide enough to lay your thread. It should be absolutely thin.

The silk thread that Continental sells is the correct diameter (gage?) as too large a diameter will make a small divot into the crankcase parting surface.

The one problem with this method is that it's slow and large shops or factories think it takes too much time. They would rather slap some goop on the surfaces and torque it up.

Why Titeseal? It remains tacky so you have unlimited work time to lay down the thread. It doesn't cure into small balls or particles that can plug an oil passage (usually the oil passage through the rod bearing - only a few thousandths clearance). Also, Titeseal is a great NPT thread sealing compound so it has another purpose in the shop.

A possible shortcoming of silk thread is that silk thread is not tolerant to damaged or scored parting surfaces. I might consider a more elastic product when working with damaged faying surfaces.

Whatever method is used you have several objectives:
  • Thin is better as it retains the clamping force produced by torque. Thick gaskets or fluids that compress will loosen the joint and cause all kinds of problems.
  • Anti-creep. Product should not "run away from stress" i.e. it should be anti-extrusion and anti-oozing for the same reason as above. This eliminates many of the RTV style sealants unless applied extremely thin.
  • Product should not be capable of contaminating system. This eliminates RTV style sealants.
  • Product should be easy to remove at next repair.
  • Product should not create corrosion.

"The mark of an expert is decidedly not a big wad of hardened silicone out of every joint, but proper preparation of sealing surfaces." Greg McConiga, Motor Service, Feb. 2002.

Improper and dangerous application of RTV.  See Burdett v. Teledyne Continental Motors Plane Crash and $14,967,413 jury verdict.

Silk Thread part number from Continental: 641543
Titeseal is available from most aircraft parts houses. Lightweight titeseal works well on gaskets to keep them from leaking. Doesn't harden so the gasket removes easily during later repairs.

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