Exhaust port gasket, springs, and studs
by Had Robinson
updated July 26, 2021
A. How to tighten the Top 80 exhaust flange nuts against the springs
Different springs supplied by Miniplane
Miniplane's technique for adjusting the spring tension with a feeler gauge should not be used because they have supplied exhaust springs that are not alike. Here is a sample of (2) different styles of springs supplied by Miniplane Italy.
The gap between the widest two coils on the left set is 4mm ± and the gap on the right set is 2mm ±. There may be other styles that may vary from these dimensions. Therefore, using a feeler gauge to measure the gap will not work.
The important measurement is NOT the gap but the amount of spring tension necessary to seat the exhaust pipe in order to prevent a significant leak.
Note: the copper exhaust port gasket must be installed correctly or the join will leak. Cylinders that do not have the copper gasket installed will also leak and eventually be ruined from the vibration and rubbing between the steel exhaust pipe and the soft aluminum cylinder.
Spring length (un-mounted) new: 12mm
Spring length (un-mounted) minimum: 9.5mm
- Install copper gasket correctly, if not already installed. The gasket must also be seated correctly or it will leak.
- Measure spring length, replace if shorter than the minimum
- Ensure that the exhaust port studs are tight in the cylinder. They should be installed with red threadlock so that they cannot loosen! Double up the exhaust nuts on each of the studs and see if they can be loosened easily (do NOT attempt to use a hex bit to loosen the studs). If they are secure, continue. If not, remove the studs and reinstall them with red threadlock.
- If the surface of the aluminum 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.
- Mount the muffler system. Tighten the two buttonhead screws "hand-tighten" before tightening the nuts for the springs.
- Push the exhaust pipe flange with pipe firmly against the copper gasket. The flange should be equidistant from the cylinder on both sides. You may need to adjust the exhaust system joints to do this.
- Install both springs over studs
- Hand-tighten both nuts so that they touch the springs.
- In case the exhaust flange nuts are not of the locking type (or are worn out), put a drop of blue threadlock on the studs about 14mm out from the cylinder head before re-installing the nuts. Remember that threadlock will not work if there is any grease or oil on the parts being treated.
- Using a 10mm socket, tighten each nut 1.0-1.5 turns. 1.0 turns is usually sufficient. For the curious: the thread pitch on the studs is 1.25 mm. One turn on the nut compresses the spring 1.25mm.
- Reinstall muffler buttonhead screws with blue threadlock. Remember that it is the threadlock that holds the screws, NOT the application of torque.
Note: the copper gasket (on the exhaust pipe side) will always leak just a little but it should not drip.
The more the exhaust springs are compressed, the shorter the life of the gasket and the more strain is put on the exhaust system from engine vibration. Pilots routinely over-tighten these springs and ruin them (including other parts of the engine) thinking that this will fix leaking at the exhaust port joint. If the springs are compressed too tightly, the steel exhaust pipe can crack the soft copper gasket. A cracked/bent gasket will leak no matter how tightly the springs are compressed.
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 may lean out the fuel mixture and this can burn up the engine.
Special materials needed: Permatex Ultra Grey RTV sealant (or equivalent)
- Remove the (2) button-head screws that hold the exhaust system to the engine.
- Remove the (2) nuts and springs that hold the exhaust pipe to the exhaust port on the engine.
- Remove the complete exhaust system and lay it aside.
- Remove the copper gasket (or a new 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.
- Rotate the flywheel so that the piston covers the inner exhaust port (you do not want debris getting into the cylinder).
- Tightly stuff some tissue into the exhaust port only enough to cover the exposed piston.
- If necessary, clean the exhaust port.
- 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.
- After everything is clean, the gasket must be properly seated. 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 IS INSTALLED THE SAME WAY AFTER YOU APPLY THE SEALANT. The top and bottom of the gasket are different! This is the only way to seat the gasket so things do not leak. Remove the exhaust system and the properly seated copper gasket.
- 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.
- Install the copper gasket making sure it oriented correctly.
- Install the exhaust system. Hand-tighten the (2) buttonhead screws. Install the (2) nuts directly onto the studs without the springs,
as in the photo above. 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").
- Clean the excess sealant from around the inside of the gasket and the cylinder. We do not need anything obstructing the exhaust port.
- WAIT ONE HOUR and the remove the exhaust nets.
- Re-install 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.
- 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 engine should 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 INTO.