The International Cessna 170 Association

Time for some forum diagnosis. When I fly for a while I am getting a very fine mist of oil on my windshield. I find some oil on the engine case at the front. I find oil on the inside of the left cowling. My thought is that I may have a prop shaft seal leaking. I have never had one previously so am not sure what to be looking for. Very little oil around the crankshaft itself behind the prop. Suggestions?

Here are a few suggestions. Don't fly so long. Paint your plane Red so it is so slow it won't get on the windshield.

Don't know what a leaking crank oil seal looks like. My experience is you found what you are looking for when you found the oil on your windshield.

Put the oil cap on, Frank??

A leaking front seal will put an oil mist on the windshield. I have had it happen on 3 Continental engines including my 170.

There is a Service Bulletin to sand the crank area where the seal rides on the crankshaft.
I have some of the 180 grit emory cloth if you need a small piece. The sanding actually tends to move the oil away from the seal.

My C-145-H has a split seal and it wasn't difficult replacing.

On the other 2 Continental engines I changed from Phillips 20W50 to Aeroshell straight weight and they stopped leaking.

DWood wrote:A leaking front seal will put an oil mist on the windshield. I have had it happen on 3 Continental engines including my 170.

There is a Service Bulletin

ContinentalSB-M76-4.pdf

to sand the crank area where the seal rides on the crankshaft.
I have some of the 180 grit emory cloth if you need a small piece. The sanding actually tends to move the oil away from the seal.

My C-145-H has a split seal and it wasn't difficult replacing.

On the other 2 Continental engines I changed from Phillips 20W50 to Aeroshell straight weight and they stopped leaking.

now this is just my opinion.
What's with sanding crank shafts? Do you have any idea what 180 grit will do to a crank shaft? Crack just looking to occur putting concentric scratches into the surface! I'd be looking at the crankcase breather tube. Inside and out including rubber hoses and clamps as well as winter flying tip threads within this forum that mention breather tubes icing up causing blow-by the front seal.

Had this happen to me on a C-85 and it was the crank seal. Mine popped and within a few minutes was so bad I couldn't see out the front. Turns out the breather froze up, building up enough crankcase pressure to blow oil out the seal. Now we run the breathers up over the top of the engine, and drill multiple relief holes down the length of the tube, not just a few inches from the bottom.

DEM

To locate an elusive oil leak,horoughly wash the engine down,then get a can of developer as is used for NDT dye check testing and spray the suspect area. After the developer drys, run the engine up and look for the source of the leak. It will show up through the white developer.
Back in February of 1951 I went up to Downsview Canada, to pick up the first De Haviland Beaver the USAF bought for service testing. That's the way the factory wechanics dried up the oil leaks on the second hand P&W R-985 engines they had to use building those airplanes.
Same thing on the Otters with R-1340s and Caribous with R-2000s. All built with out of production and previously used types of P&W engines.

minton wrote:

DWood wrote:A leaking front seal will put an oil mist on the windshield. I have had it happen on 3 Continental engines including my 170.

There is a Service Bulletin

ContinentalSB-M76-4.pdf

to sand the crank area where the seal rides on the crankshaft.
I have some of the 180 grit emory cloth if you need a small piece. The sanding actually tends to move the oil away from the seal.

My C-145-H has a split seal and it wasn't difficult replacing.

On the other 2 Continental engines I changed from Phillips 20W50 to Aeroshell straight weight and they stopped leaking.

now this is just my opinion.
What's with sanding crank shafts? Do you have any idea what 180 grit will do to a crank shaft? Crack just looking to occur putting concentric scratches into the surface! I'd be looking at the crankcase breather tube. Inside and out including rubber hoses and clamps as well as winter flying tip threads within this forum that mention breather tubes icing up causing blow-by the front seal.

This is a Continental recommended procedure. It involves using the abrasive paper to impart a "swirl" to move the oil back from the seal to the crankcase. In other words, the swirl marks spin the oil against the oil seal's wiper to help keep the oil inside the engine. There is no hazard to create any stress or cracks.

Paul, I wish it was as simple as putting the oil filler cap on to fix this problem However, leaving the oil filler cap off does not result in any oil on the windshield (or so I am told by those who have done it ) Guess its time to have my A&P check to confirm my suspicions

Frank-
It probably doesn't apply to your engine but if it has an IO-360 crankshaft, check the un-safetied plug in the crank bushing that blocks off where the supply of oil would go to a constant speed prop. Mine has backed out two times and the windshield is where it is first evident.

If it is a stock C-145 or O-300, check the "freeze"plug in the center of the propeller shaft. I had one come out and it "breathed" oil vapor out the front end.

You'd think TCM would secure those better when those cranks are used in O300's.

gahorn wrote:You'd think TCM would secure those better when those cranks are used in O300's.

Yes you would but TCM is ho-hum about it when one comes loose. There is no way to safety the plug and the only thing TCM uses to keep the plug from coming out is a crush washer and Loctite on the straight plug threads. There was no Loctite on my plug when it came out the first time, 10hr since NEW from TCM.

I know of one owner that "staked" the plug in the IO-360 crankshaft using a center punch after heard about Richards problem. This was on a TCM IO-360 conversion using a fixed pitch prop.

Richard & Dick, thanks for the reminder regarding the prop shaft plug. I doubt that my crank is an IO-360 crankshaft, but it is an O300-B shaft and, in theory, the plug could leak even though the two position lever is wired closed. I will pull the spinner and check that out prior to having my A&P take a look.