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Cooling box fin breakage

After 100 hours or so, pilots may notice that one or both of these fins (located at the bottom of the cylinder on each side) that help direct cooling air through the engine may be separated from the rest of the cooling box.  The intense vibration of the Top 80 is what causes this metal fatigue and breakage.  The consequence is less air circulating past the cylinder and head of the engine.  This results in higher engine running temperatures.  It is not too important that the fin is rattling around but it could come completely loose and go into the propeller.  In addition, big pilots that are operating near sea level and at full throttle could exceed maximum operating temperatures for the engine.

The way to prevent this problem is to prevent the cooling box fins from becoming, essentially, free-moving tuning forks that will eventually break loose.

If the fins are already loose, you will have to reattach them to the cooling box with a new strip of aluminum and some rivets.  The cylinder head will have to be removed to do this.  The head "O" ring will have to be replaced but, if you are very careful, you can reuse the cylinder gasket at its base.  If you tear the base gasket, you are out of luck.  When reassembling things, apply a very thin coat of RTV to the exposed portion of the cylinder head base gasket.  If you do not loosen the cylinder head from the cylinder when undoing everything, you may get away with not replacing any of the gaskets.  If you go this route, be certain to check the cylinder top and bottom for leaks after you have run the engine a bit.

YOU DO NOT WANT LEAKS BECAUSE IT AFFECTS THE AIR/FUEL MIXTURE!

To prevent the cooling box fins from breakage, apply automotive RTV Blue sealant cement between the cooling box fin and the engine cooling fin that is right above it.  The closer the cement bead is to the tip of the cooling box fin, the less the fin will vibrate.  It is easier to do this is with the cylinder removed from the engine.  Otherwise, you will have to glob the RTV on the fins to ensure good contact with each surface.  All surfaces must be 100% free of any oil or grease to ensure that the RTV does not loosen.  Cleaning is easily done with brake cleaner.  A 1 1/2" long bead of RTV between the fins is sufficient.  Be certain that the bead of cement is in full contact with the cooling box and engine fins.  Let the engine sit overnight after the cement is applied.

The downside of this preventative measure is that every time the cylinder is removed, you will have to reapply the RTV to the fins.

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