Engine, exhaust, and redrive leaks

by Had Robinson
updated February 8, 2021

Note: The Top 80 redrive will leak if the fill plug relief valve is clogged.  See paragraph "C" below.


It is the nature of (2) stroke engines to “spit” fuel/oil from the throttle and choke shafts on the carburetor. Drops of oil form where these shafts exit the carburetor body because the gasoline evaporates, leaving only oil. When the drops get big enough, they go somewhere – and it can be anywhere. Often they fall on the muffler and stain it. They can also splash on parts of the engine which I think has happened here in your photo. The air is moving everywhere around the engine and drops of oil can land anywhere.

In a word, (2) stroke engines are messy. Pilots can always apply a degreaser and then wash them down to clean the mess off that accumulates but I do not recommend it often because the water gets inside the propeller hub and, when necessary, makes removing the hub difficult, even with heat, because of the formation of rust. Where I live it is extremely dry so it’s not a problem but everywhere else, it can be.  If you must remove the oil, use a rag soaked with mineral spirits.

Here are two photographs of oil leaks.  The first photo of a leak is of no consequence and is not likely to be a leak from the crankshaft seal.  The second photo is of a serious leak of the crankshaft seal on the clutch side of the engine which should be fixed.

Top 80 minor oil leak

Top 80 failed crankshaft seal on the clutch side

A. How to detect where the leaks are

It is much easier to detect the source of the leaks if the engine has been cleaned thoroughly.  Oil tends to move downwards and back on the engine surfaces.  This is due to gravity and the air moving past the engine.

Engine & exhaust port oil leaks

Oil leaking from the engine crankcase or the redrive (if there is one) will be the same color as the oil used in the fuel mix, not black.  It could be a leak in a crankshaft seal but this is not as common as a leak in a redrive seal or from a fill plug.  Engine oil is thinner than the heavier oil used in the redrive.  As the oil from both the redrive and the engine has not been burned (run through the engine), it is hard to tell them apart.  The easiest way to tell the difference is to remove the redrive and pressure test it (about 5 psi) or to heat it with a heat gun (see test with pressure on this page).  If there are no leaks in the redrive, the source of the leak is likely a bad crankshaft seal.  Leaks in the redrive must be repaired promptly.

Sometimes, the factory or a pilot does not assemble the crankcase properly, leaving out or misplacing the RTV sealant.  The photo below is of a Miniplane Top 80 that has a serious leak.  Soap solution was used to illustrate how bad the leak is.  The engine will have to be completely disassembled in order to fix this, likely caused by the poor QC at most Italian manufacturers.  A leak like this will affect the fuel/air mixture.  Photo courtesy of Jay Wildman

Top 80 leaking crankcase

A leaking crankshaft seal is not worth repairing unless it is more than a few drops every time you fly.  The oil will typically appear at the lower joint of the redrive (Top 80 – others similar) and the engine crankcase.  However, more than a trace of leakage can wind up in the clutch and cause it to slip.  Clutch slippage is more an annoyance than a problem.  If the oil from the leak is black and icky, it is a leak from the exhaust port (most of the time) or from the cylinder/head joint (more rare).  The oil gets black from having been burned (run through the engine).  Leaks from the redrive, the carburetor, the base of the cylinder, and the crankshaft seals will always be clear.

The most common area of black oil leaks is just below the exhaust outlet on the cylinder and/or at the top of the muffler.  Below is a photo of the exhaust port on a Top 80.  NOTE: the exhaust springs are missing in order to correctly seat the gasket.  On any engine, a correctly installed exhaust gasket will not leak any oil.

Top 80 exhaust port

The exhaust port gasket and springs are critical to sealing the exhaust system in the Top 80.  If the springs are not tightened properly or the copper exhaust flange gasket is damaged/missing/deformed, there will be black oil in the area.  It can also get on the propeller and/or into the redrive.  Go to this page for detailed instructions on how to correctly tighten the exhaust port nuts/springs or install the exhaust port gasket on the Top 80.  If the installation is not done correctly, the exhaust port is guaranteed to leak oil.  It is NOT as simple as it looks.

Black oil leaks in any engine also indicate an air leak.  That is, there is both PRESSURE and SUCTION in the exhaust stream up to the muffler.  If oil is coming OUT then air is also coming IN.  An air leak will cause the engine to run leaner and, in certain situations, can cause it to overheat and seize.  It is another reason why pilots should never attach smoke devices anywhere on a 2-stroke engine except at the muffler.  The smoke fluid (unlike in a 4-stroke engine) will get into the cylinder to be burned and can overheat the engine.

With other engines, like the Thor or Minari, be sure to use red threadlock on the exhaust flange studs and nuts.  It is an area of the engine under radical stress from the variation in temperatures which can go from ambient temperature to over 1,200 degrees F quickly.

Even with care, the exhaust flange gasket may disintegrate over time from the INTENSE heat at the exhaust port.  The heat can eat away at the copper materials in the gasket (Polini Thor).  The gasket will leak no matter how carefully it is initially installed.  We just have to replace the gasket and write it off to normal wear and tear.  A good time to replace the gasket is when the top end of the engine is repaired/replaced.  Remember to thoroughly clean and degrease the area.  Use gray RTV to seal the gasket (a 1/4" (6mm) bead or more should be applied), red threadlock on the studs and nuts, be certain to tighten the nuts to the specified torque.  The side of the gasket with the greater metal-faced area goes away from the cylinder head.

Note: if the exhaust port leaks but a few drops now and then, this is not important.

On the Top 80, over-tightening the exhaust springs to "stop a leak" will ruin them and cause premature wear of the copper exhaust flange gasket.  On other engines, it is likely that the nuts which tighten down the exhaust flange to the cylinder head did not have threadlock applied nor were they tightened to the proper torque.  Once the gasket starts to leak, it must be replaced as no amount of torque or sealant will fix it.

Exhaust system joint leaks

If you see any signs of oil around the exhaust system joints (NOT at the exhaust port), the O-rings are bad.  Most Top 80 exhaust systems have (2) joints sealed with O-rings.  Some newer models, however, do not have an O-ring in the smaller top joint which, generally, will never leak unless the springs are defective/weak.    Leakage of these joints allows air into the engine fuel stream which may lean out the air/fuel mixture and potentially burn up the engine.  The small upper O-ring disintegrates after about 100 hours, depending on the type of fuel used.  Note: some Top 80 mufflers have (2) upper O-rings.  Replacement O-ring sets (upper and lower) are available from Miniplane-USA.

This Top 80 model does not have an O-ring in the top joint yet should not leak.

Top 80 exhaust joints

This exhaust system has O-rings in both joints.  On some models the upper joint can have (1) or (2) O-rings.

Top 80 exhaust joints

The larger muffler O-ring rarely, if ever, goes bad.  However, it should be replaced every 150 hours.

Remove the exhaust springs with a line.  If tools are used, the springs will be ruined.  Here is a video on how to properly do it.  Note how Dyneema line was used to contain the springs in case they break.  You do NOT want spring pieces to get into the propeller (the place where EVERYTHING goes that gets loose on a paramotor).

When replacing the O-rings, remove all traces of gunk and the old O ring from the female part of the pipe before installing the new one.  Thoroughly clean the grooves that hold the O-ring with brake cleaner and compressed air.  The O-rings are  best installed with silicone grease (ordinary grease cakes up when it gets hot).  If silicone grease is not available, use 2-stroke oil.  Smear the O-ring with the grease and work it into the groove in the pipe.  Rub grease around the tip of the male section of the pipe.  The sections must be brought FULLY together.  Gentle twisting helps.  Reinstall the springs with a new Dyneema line and fix the knot with Super-Glue.

Finding hard to detect oil leaks (not the redrive)

If it is not obvious where the leak is coming from, pressurizing the crankcase with no more than 3-5 psi of air can help.  This is a last resort method because the flywheel must be removed and the engine re-timed, no small task.  Here is the basic way to do this, minus some details.

Replacing the engine seals is not that difficult.  However, if one is failing it may be due to excessive wear of a main bearing.  Engines with more than 200 hours should be overhauled if one or both of the main seals are leaking.

Leaks of the reed valve body must be repaired ASAP.

Redrive oil leaks

Some redrives operate in an oil bath like the Top 80 or the Polini Thor models.  If a Top 80 has light-colored (not black) oil around the base of the engine, on the redrive or muffler, it is a leak in the redrive seal or, much less often, a leak in the redrive housing itself.  The Thor models have a wet clutch so the redrive oil quickly becomes black.  If there is black oil around the propeller shaft, there is a leak in the propeller shaft seal.  Redrive oil of a Top 80 falling on the muffler, however, may become black in color from being burned.

An easy way to check for leaks in the redrive is to take a heat gun and heat up the redrive quickly. (If pressurizing a Top 80 redrive, see the Top 80 note below.)  This will pressurize the air inside it safely and heat up the oil.  Alternatively, use regulated air (no more than 3-5 psi) to pressurize the redrive through the fill plug.  Submerse the entire redrive in a bucket of water except, of course, the area around the fill plug.  Water will not hurt the redrive and you can detect very small leaks e.g. around the clutch bell shaft without having to guess the source of the leak.  After you do a water test, it is a good idea to change the oil in the redrive if you have any doubts that you failed to maintain pressure inside the redrive at any time while it was submerged in water. 

Top 80: The yellow seal plug must be installed for this to work.  The standard fill plug is used, it will automatically relieve the built-up pressure inside the redrive.  The oil will leak out of the smallest crack.  If the seal behind the clutch bell is leaking, it will take a few minutes for the oil to run down inside to a place where you can see it.

Below is an example of a new redrive (Top 80) that was assembled without sealant applied between the case halves.  Whoever assembled the redrive at the Miniplane factory missed the mating surfaces of the cases halves, applying the sealant where it was useless. Did the tech have too much to drink that night before?  Is there any sort of quality control at Miniplane?  Who knows....  Unfortunately, the poor pilot who purchased the engine thought either his fill or drain plug was leaking and tightened them more which, of course, had no effect.  He sent me the redrive and  I pressure tested it.  The oil came pouring out between the case halves (photo below).  It was a huge headache for the pilot.  Depending on the ambient conditions, this type of leak can sometimes be hard to detect.

Top 80 leaking redrive - factory defect

Opening up the redrive, it was obvious why it leaked.  The red arrows in the photo below point to where the factory technician missed the edge when applying the sealant and it wound up in the cavity where it was useless.  This is not the first time that I have discovered sealant in the wrong place in a new redrive.  Note: special bolts are needed to split the redrive case as well as a hydraulic press.  See the redrive rebuild page for how to do fix this problem.

leaking Top 80 redrive

B. Repairing redrive leaks

See the Top 80 redrive rebuild page or the Polini Thor redrive rebuild page on how to properly rebuild or repair a gear type redrive.

C. Top 80 fill plug relief valve issues

Before you take things apart, check to be sure the fill plug is not the problem. An easy way to check the valve relief pressure is to use a pop-off gauge.  The plug is actually a low pressure relief valve (1.5 psi) that uses a spring loaded ball pushing against an orifice.  The ball can become stuck or jammed over time preventing the relief of pressure buildup inside the redrive.  Pressure can increase enough to force oil out through the seals.  The increase in pressure can be caused by running the engine at full load (it heats the redrive up more) or going to high altitude.  The seals in the redrive are not pressure seals but ordinary shaft seals which is why they can leak under certain conditions.

It is easy to fix the jammed fill plug/relief valve.  Cut the tip off a wood toothpick and run it into the hole in the bottom of the plug pushing the ball back and forth.  You can tell right away if the ball is jammed.  Put some penetrating oil down the hole and then use air pressure to blow the penetrating oil through the entire relief valve.  Doing this a number of times will thoroughly clean out the valve.

It is a good idea to check that the valve is not jammed every time you change the redrive oil.

cleaning the gear box relief valve

If you are a heavy user of your Top 80, you might want to consider doing away with  the factory supplied plug.  Pilots who climb quickly to high altitudes or run at full power in hot climates may find that cleaning the relief valve/plug is insufficient to prevent oil leaks.  When the pressure in the redrive is greater than outside, the pressure will also tend to force the seal tighter on the shaft – this excess pressure will wear the seal out faster than normal.  This is because the OEM seals used are not pressure-type seals.  Miniplane uses this relief valve/plug so that the motor can be stored in many positions without loss of redrive oil.  The vast majority of users, however, keep their paramotor in an upright position.  If you ship your Miniplane, for example, you can use the original plug instead of the relief valve.

To prevent relative pressure build up in the redrive, use the plastic plug that was shipped with your Miniplane.  To make the plug a "breather" type, drill a tiny hole in the center of the plug with a #60 drill or the smallest bit you have.  The small hole will let the redrive breath but keep most dust and contaminants out.  The modified plug will keep the pressure the same inside and outside the redrive at all times.  The downside is that it is much easier to have an oil spill if you lay the engine down so that the top of the redrive slopes downhill.  It will slowly drain out and make a mess on your harness.  When you transport the engine, it is a good idea to put an unmodified plastic plug or the relief valve in the redrive in order to prevent spills.

cleaning the gear box relief valve