Exhaust port gasket, nuts, springs, and studs

by Had Robinson
updated January 2, 2020

A. How to tighten the Top 80 exhaust flange nuts against the springs

Quick ref

Spring length, new: 12mm
Spring length, minimum: 9.5mm

Adjustment, ideal: 1.7mm (0.066") between widest coil
Adjustment, minimum: 0.5mm (0.020") between widest coil.

Note: If the gasket leaks when set to minimum spring adjustment or less, springs should be replaced.  This assumes that the gasket is in good condition and has been installed (seated) correctly.  See below....

Spring adjustment

Tools needed: (1) metric/U.S. feeler gauge.

Ordinary feeler gauges (such as the above) do not have a blade that is 1.7mm (0.066") thick.  To get around this, double-up adjacent blades so that you can get within a few thousandths of the needed value.

New springs that have not been previously over-tightened should be tightened so that there is a space of 1.7mm (0.066") between the widest coil.  This value is usually sufficient to stop oil leaks with new springs.

Top 80 exhaust port spring installation
photo courtesy of Miniplane

WARNING: The tighter the exhaust springs, the shorter the life of the gasket and the more strain that is put on the exhaust system from engine vibration.  Pilots routinely over-tighten these springs and ruin them thinking that this will fix leaking at the exhaust port.  Only tighten them as needed.  If the springs are compressed too tightly, the steel exhaust pipe will crack the soft copper exhaust flange gasket.  A cracked/bent gasket will leak no matter how tight the springs are compressed.

If the inside surface of the cylinder exhaust port is damaged (has ridges or grooves), it can be repaired with the exhaust port repair tool that is available from Miniplane-USA.

If case nuts were provided that are not the locking type (or are worn out), put a drop of blue threadlock on the studs about 14mm out from the cylinder head before installing the nuts.

As the springs age or have been over-tightened, this space needs to be decreased in order to keep the correct amount of tension so that the exhaust port does not leak.  Tighten the nuts ONLY enough to stop leaks.  If there are leaks, tighten the nuts a 1/4 turn at a time but do not allow the space to be less than the 0.5mm minimum (Top 80).  However, the copper exhaust flange gasket (on the exhaust pipe side) will always leak just a little but it should not drip copiously.  If it does, the gasket or exhaust port surface is damaged or the gasket is improperly installed.  Further tightening of the nuts will not help.  Replace the springs and the gasket if leaks still occur after the nuts are correctly tightened.

B. Installing the exhaust port gasket on the Top 80

The copper exhaust gasket on the Top 80 will eventually split/crack after many hours (100+) and should be checked periodically.  For minimal leaking around the exhaust port here is the best way to do it.  If these steps are not followed exactly, there will be leaks!  Remember that serious leaks in the exhaust system will lean out the fuel mixture.

Special materials needed: Permatex Ultra Grey RTV sealant (or equivalent)

Steps

  1. Remove the (2) button-head screws that hold the exhaust system to the engine.
  2. Remove the (2) nuts and springs that hold the exhaust pipe to the exhaust port on the engine.
  3. Remove the complete exhaust system and lay it aside.
  4. Remove the copper gasket and clean it thoroughly with brake cleaner.  Dry with compressed air.  Examine the cleaned gasket carefully.  If it has any cracks or pieces missing, discard and replace with a new one.
  5. Rotate the flywheel so that the piston covers the inner exhaust port (you do not want debris getting into the cylinder).
  6. Tightly stuff some tissue into the exhaust port only enough to cover the exposed piston.
  7. If necessary, clean the exhaust port.
  8. Use brake cleaner to remove all traces of oil from the outer part of the exhaust port (the area contacted by the copper gasket).  Dry with compressed air.  If the exhaust port is uneven or badly worn from the exhaust pipe chewing up things, the port should be reconditioned with the special tool.
  9. After everything is clean, seat the gasket.  Install the exhaust system without sealant and all parts EXCEPT THE SPRINGS that go underneath the nuts on both sides.  Torque the exhaust port nuts to 5 Nm.  This will force the copper gasket to take the shape of the exhaust pipe and the aluminum head.  BE CERTAIN TO PUT A MARK ON THE TOP SIDE OF THE GASKET SO IT CAN BE INSTALLED THE SAME WAY AFTER YOU APPLY THE SEALANT.  The top and bottom of the gasket will be different now.  Note the missing springs in the photo below.  This is the only way to seat the gasket so things do not leak!  Remove the exhaust system and the now properly-shaped copper gasket.

    seating the Top 80 exhaust port gasket
  10. Apply copious amounts of sealant ONLY to the side of the copper gasket that is against the cylinder.  DO NOT PUT CEMENT ON THE EXHAUST PIPE SIDE OF THE GASKET.  This is because the exhaust pipe and gasket form a flexible joint and the parts must be able to move with respect to each other.
  11. Install the copper gasket.
  12. Install the (2) nuts directly onto the studs without the springs or exhaust pipe.  Tighten them evenly to 1-2 Nm or just enough to force the uncured sealant to ooze out from around the inside and outside of the gasket.  The gap between the gasket and the cylinder should be about 0.25mm (0.010").
  13. Clean the excess sealant from around the inside of the gasket and the cylinder.  We do not need anything obstructing the exhaust port.
  14. WAIT ONE HOUR!
  15. Remove the nuts and remount the entire exhaust system.  Start with the (2) button-head screws that hold the muffler to the engine.  Use BLUE threadlock on these screws.  Tighten just snug.  Button-head screws cannot be tightened like ordinary bolts!  The threadlock holds them in place.
  16. Install the (2) exhaust nuts and springs according to the steps in "A" above.  If this is done correctly, exhaust system leaks will be almost nil.  That is, there will always be traces of black gunk around the port but it should not be enough to flow around the engine.

C. Exhaust port studs

All studs in the cylinder must be installed with RED (high strength) threadlock and then tightened to the correct torque.  The engine can be assembled completely and run immediately.  That is, there is no "cure" time for threadlock.  It does its job within minutes.

I have noticed that Miniplane is now using tempered steel (black) studs which are installed with threadlock.  These will last much longer and will not break as easily as the commonly used stainless steel studs.  Always check to be sure these studs are installed properly, regardless.  NOTE:  LOOSE STUDS WILL RUIN WHATEVER THEY ARE SCREWED IN TO!

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