Rotax A box oil loss

I am running a Rotax 277 w/ "a" gearbox and am loosing oil while in flight. I have relocated the vent line behind the cowl but did not help much. I have also replaced the crankcase seals thinking crankcase pressure might be the problem. Does anyone have any suddestions?
Thanks, Monte

It is common for oil to blow out of the vent tube, a good fix is installing a small fuel filter on the vent tube. In any case , external leak should be pretty obvious,
When the crank seal is worn it will suck the oil out of the gear box and creates a seemingly mysterious leak.
It is also common for the seal to leak, The front seal is very difficult to replace as the engine has to be completely disassembled and the crankshaft pushed out of the front crankcase halve which takes a special tool and great force to do, so I kinda wonder about your claim you did it. If there is no visible leak the seal is the only way it can go.

Just a thot......
The proper gearbox oil level is at the Bottom Screw. If filled to the Top screw, you WILL lose some excess oil.

I've seen a couple boxes overfilled this way.

Thanks George, the leak is definitely coming from the vent. i tried the filter but it still leaks. i did dissassemble the engine and replaced the seals. Next i will install a small pressure gauge on the vent line and monitor it while flying.
Why would you even need a vent if there was not any pressure?

The gearbox was filled to the bottom screw.
Thanks

Originally Posted by texas-buzzard

Why would you even need a vent if there was not any pressure?

As the engine runs, the gearbox oil heats up and the oil and air in the gearbox expand. If not vented, pressure builds up and possibly blows out the seals.

-Dana

Congress shall make no law....What part of NO didn't you understand?

Before i tried mounting the filter pointing down with the drain out the bottom of the aircraft, now i will point the filter up above the gearbox with the drain line above the filter. This should let the air escape while the oil remains in the filter during heat expansion and will drain back after cool down.
We will see this weekend. Thanks for all the replys.

Do a pressure leakdown check on your engine.The PTO end seal may be going out and crankcase vacuum sucking the gearbox oil in.
Steve

Thanks Steve, i did a pressure leakdown and the seals are holding. I even mounted an old ASI to the vent line and noticed a very small amount of pressure which might be forcing the oil out since the vent is directly over the main drive gear which brings oil up top. I relocated the vent to the protrusion where a bearing would mount if the gearbox had a 3.00-1 ratio. This is out of the gear oil flow and has worked great.

Monte,

You might also try a different gear oil. Many gear oils have an additive that makes them "stringy", so the oil doesn't spray off parts too quickly, similar to M/C chain oils. That might encourage the oil to creep out the breather.

Most common gear oils have multiple ratings on the bottle, like GL-4, GL-5, and GL-6, so they're supposed to be able to work in any transmission or rear end. But that means they have all the additives to meet all those specs.

As you may recall, I was a transmission mechanic for 15 years. We learned that for some applications, you want just a single rated gear oil. For example for VW beetles, anything higher than a GL-4 was too slippery. The syncros couldn't bite through the high film strength oil on the syncro hub of the gear, enough to accelerate the gear. Customers would bend the syncros (blocker rings) trying to force them into gear.

Few oil companies address this issue with their gear oils. I'd suggest using a straight GL-4 for your application. Amsoil and RedLine both offer straight GL-4, without the stringy additive. And GL-4 would be plenty slippery enough to lubricate the gears and bearings in your A-box. I don't know about other brands.

We learned that most brands of oils do multiple-labeling, where they put different labels on the same oil... particularly on motor oil... for example "new car oil" or "high mileage car oil", and even different viscosity combinations. They might manufacture a 5W-40 and sell it in different bottles marked 5W-15, 5W-20, 5W-30, 10W-30, 10W-40, and anything else that falls in the 5W-40 range.

That is probably true for some brands of gear oils. They might sell a GL-4, that's really the same as their GL-4, GL-5, GL-6. So I'd only use the Amsoil or RedLine brands straight GL-4, that we proved out.

Another suggestion would be to plumb the breather into a catch bottle. That would help you get a feel for how much is coming out, or if something else is going on. Check out the little catch bottle I added to LeRoy's Flightstar II on his engine breather. (It's strapped to the battery.)