Exhaust port gasket, nuts, springs, and studs – Top 80
by Had Robinson
A. How to tighten the exhaust flange nuts against the springs
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.
Spring adjustment – detail
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.
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. This is why it is best to 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.
Pilots routinely over-tighten these springs and ruin them. 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. Exhaust port gasket on the Top 80
The copper exhaust flange gasket on the Top 80 will eventually split/crack after many hours (100+) and should be checked periodically.
When changing out the gasket or re-installing the old one, install it with Permatex Ultra Grey RTV sealant (or equivalent) on the side that is towards the cylinder. DO NOT PUT CEMENT BETWEEN THE COPPER GASKET AND THE EXHAUST HEADER PIPE. These two parts must be able to move with respect to each other. Clean both surfaces thoroughly with brake cleaner and dry with compressed air. Stuff the exhaust port with tissue in order to keep the brake cleaner out of the cylinder.
After everything is clean, seat the gasket. Install the exhaust system without sealant and using all parts EXCEPT THE SPRINGS that go underneath the nuts on both sides. Torque the exhaust port nuts to about 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 safe way to seat the gasket.
Remove everything completely. No apply apply sealant with small stick or screwdriver to the side of the gasket that contacts the cylinder. Use plenty of sealant (the only time this is a good idea). Install the gasket pushing it hard into the cylinder opening. Remove the excess sealant, if any, that is INSIDE the exhaust port with your finger. Excess sealant that oozed to the outside does no harm and helps seal the joint. Let dry overnight before operating the engine.
C. Exhaust port studs
All studs in the cylinder must be installed with RED (high strength)
threadlock to the proper torque (see the SPEC page). 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!