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Redrive leaks & fill plug issues

by Had Robinson

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

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 oil leaks

It could be a leak in the main engine seal but this is not as common as a redrive leak.  Engine oil is much thinner and tends to darken sooner than the heavy 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 two paragraphs down).  If there are no leaks in the redrive, the source of the leak is likely a bad main engine bearing seal.

If the oil from the leak is black and icky, it could be a leak from the exhaust manifold or from the cylinder head.  The oil gets black from having been burned (run through the engine).  Redrive leaks and leaks from the carburetor, the cylinder head, the engine bearing seals will always be clear.

Redrive oil leaks

If you notice light-colored (not black) oil around the base of your Miniplane engine, on the redrive or muffler, you probably have a leak in the redrive seal or, much less often, a leak in the redrive housing itself.

An easy way to check for leaks, and where they are, is to take a heat gun and heat up the redrive quickly.  This will pressurize the air inside it safely and heat up the oil.  The fill plug must be installed for this to work.  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.

This is a new redrive that came from the factory without any sealant between the case halves.  Unfortunately, the pilot was not a mechanic and thought either his fill or drain plug was leaking and attempted to tighten it more.  I pressure tested the redrive and oil came pouring out between the case halves.  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.

Top 80 leaking redrive

B. Repairing leaks

Mark Kubisch has a good method to repair the redrive, if needed.  When reassembling the case halves, be certain to tighten the screws in a "cross pattern" to the correct torque.  Do NOT fill the case with oil for at least 8 or more hours to allow the sealant to cure.  Use RTV "grey" type sealant.

C. Fill plug relief valve issues

Before you tear things apart (or send it off for service), check to be sure the fill plug is not the problem. An easy 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.  Spray some WD-40 down the hole and then use air pressure to blow the WD-40 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

D. A better fill plug

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 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 relief valve/plug.

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.  The small hole will let the redrive breath but keep dust and other contaminants out. This simple replacement 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 slopes downhill.  It will slowly drain out and make a mess of your harness.  When you transport the engine, it is a good idea to put an unmodified plastic plug in the redrive in order to prevent spills.

cleaning the gear box relief valve