Gunair Aviation

0-300 oil leaks

The pan gasket may be the problem but usually not.  Another pan leak that happens more than you might guess is the oil pan corrodes though at the low point (in this case the rear) of the oil pan.  This is because water settles to the bottom, where it picks up sulfur from used oil and becomes corrosive.

Copper crush washers tend to leak after being used more than once. They are found on the rear case and the fittings on the sump.

I majored an O-300D and was thrilled to see no leaks but later one developed at a pushrod tube boot (I had missed the groove with one spring clamp).   Sometimes all you need to do with those rubber boots is reposition the boot so that it is staggered with its mate. If the re-builder has failed to get the boots seated on the tubes and lifter covers so that the springs in are not properly seated over the grooves in the boots it is going to leak.   Take a good mirror and flashlight and check all that out.  You might be surprised that just by repositioning the boot and spring the leak will stop.

Check out the accessories drive pad seals for the Generator/Alternator and the vacuum pump.  These are easy to change when the engine is off the airframe and not so easy when it is buried next to a firewall. Check out the tach cable seal area.

Check out the covers that are not in use on rear of the accessories case. These have a gasket under them.  Check the torque and look for leaks. The oil filter housing may be loose or have a reused gasket on it.  Check out the oil temp bulb, it has a copper crush gasket also.  The rocker covers should be easy to spot it they are a problem.  The pushrod tube at the cylinder end may need to be re-swedged if they are the old type.  Check out the oil filler neck and cap.  The gasket at the base of the filler neck may be leaking due to low torque.  The cap usually has a gasket under it, make sure it is still there and check condition. 

One more fix if you have a leak on the crankcase split line. With TCM's recent approval you can attach a vacuum cleaner nozzle to the oil filler spout and create a vacuum inside the crankcase.   You can then, after cleaning the affected areas, take some high-quality cyanoacrylate glue (like aeromodelers' CA glue hot-fix, for example, or my favorite is West Systems Epoxy) and shoot it into the external areas of the engine where it leaks.  Be sure to read TCM's service letter first.   I have used this fix around studs, through bolts and crankase seams. It can save a huge amount of labor to stop a small but annoying seep.

Take a look at the breather tube.  It might be blow by.  If most of the oil is on the belly and not inside the engine cowling the odds of that being the problem just went up. 

Good luck,

Sam & Ed