Sealing Aircraft Engine Cases

John:
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.

Don,

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.

Inspecting the magneto coil

I received this email inquiry:

John, Recently we have found, during a 500hr Slick Mag Inspections, defective coils. The engines in both of these cases were running OK when brought in for the inspection. When coils were tested found primary coil to be within tolerances but the secondary coil showed open resistance. In both cases we replaced coils. We just want to understand why the mags were still operating so we can give an "intelligent" answer to the customer as to why we had to replace coils when all seemed to be operating fine.

Cross-section of Slick coil. Large primary windings next to core. Smaller secondary windings on outside.

Interesting question as sometimes inspection standards seem arbitrary without an explanation of what their intended purpose. by "open resistance' I assume that you had infinite resistance or a open secondary circuit.

Your ohm meter does its check by flowing a tiny bit of current through the secondary wire at a fraction of a volt. Any broken wire will stop the current flow and indicate an open circuit. However, when the engine is operating the voltage in the secondary wires is high enough to jump a spark plug gap so it will easily jump across a small break in the secondary wire inside the coil and the magneto operates fine - for awhile.

The arching inside the coil causes the coil to get hot. On a high-voltage coil tester when you pick the coil up it's like a hot potato! The burning inside the coil starts to melt insulation and burn the secondary wires. Gradually the number of secondary windings reduces as the current shorts across adjacent wires. Now the ratio of primary windings to secondary windings is reduced so the transformer effect of stepping-up of voltage is reduced.

The first indication for the pilot is hard starting; at the slow rotation speed during start there is not enough voltage to spark the plug. Get it started and it runs fine for awhile as the high rpm creates enough current in the primary to off-set the degradation in the secondary. Eventually, the burning inside the coil reaches the outside by burning a hole out through one end of the coil.

It is amazing when you see this because you know that the aircraft was operated with all that internal coil damage. Eventually, enough secondary wires are damaged that the magneto won't produce enough voltage to fire the plugs. At that point the aircraft is grounded in some hole-in-the-wall airport with an angry wife and screaming kids.

Checking secondary resistance on a Bendix S-1200 series magneto

Editorial on inspections: The I-35W bridge that collapsed into the Mississippi river and killed 13 people was operating fine the moment before it collapsed. It had failed previous inspections and was rated as "structurally deficient". These inspections were ignored because it was operating fine. Operating fine - don't fix it or "if its not broke don't fix it - kills innocents