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John Gallard24/07/2008 10:22:00
51 forum posts
I have a OS46LA approx 20 hours use. Last session I noticed  black oil in the engine bay, on inspection I noticed a screw was missing from the crankcase backplate. I replaced this screw but the oily subsance is still apparent, I now notice that there appears to be fluid leaking from the prop spinner end of the crankcase. Since there is no seal, O ring or bearings on the end of the crankcase how is a seal created at this point? I have noticed with my OS engine and a couple of othersat the club that there is a small amount of end play on the crankshaft (one of them being a new engine) By the way the engine runs perfectly. Any comments please.
winchweight24/07/2008 10:55:00
2516 forum posts
67 photos

The backplate screws on these engines do come loose. I always open the backplate up and run a bead of silicon around it, refitting the plate and screws before it all cures. That usually stops leaks here. Because this is a plain bearing engine, they do tend to leak a little around the prop driver. there's not much you can do except try to minimise the amount of fluid inside at shut down. this can be done by runing until tanks dry, or by pinching the carb line and starving the engine to a stop. Always empty the fuel tank, and avoid storing the model nose down. The black oil is most likely from the exhaust pipe, as the oil doesn't usually get blacked until after combustion. So check for leaks at the manifold, check the exhaust pressure line isn't leaking. Otherwise, it's likely to be a bit of oil that has blown back over the model during aerobatics etc.

The play in the crank, provided it is a fore / aft play, not side to side, is normal. Most of it gets taken up by the prop being tightened, but not all.

Finally, I use the foam type of ear plugs that you roll with your fingers and stick into your ear as a plug for the exhaust and the carb inlet. This keeps most of the reidual leaks at bay and keeps the car and garage clean.

Eric Bray24/07/2008 13:17:00
6600 forum posts
2 photos
Black oil in and around the engine can also be a sign of bearing wear, (the black is metal dust!). Ensure that your prop AND spinner are balanced, to reduce oscillatory stresses when the engine is running! Once a plain bearing has worn oversize, there is nothing you can do about it, without going to significant expense. Fore and aft movement of the prop-shaft is normal, it is called end-float! Side to side, (rattling) is naughty, and a sign of wear- approaching worn out!
John Gallard24/07/2008 17:20:00
51 forum posts
Thanks for response, I'll check all points mentioned
Allan Jordan25/07/2008 13:38:00
495 forum posts
 Hi John, Much as I hate to say it, I think you have a real problem with this engine. It needs sorting out.
Mark Lubbock30/07/2008 11:24:00
313 forum posts
13 photos

Hi John,

If your engine is still running OK, I wouldn't worry too much. Most plain/bushed bearing engines leak to some extent, particularly as they get older-there always has to be a bit, as this is the only way oil gets to the front. Plain bearings normally wear to a slight bellmouth shape, but the contact areas remain quite large until really excessive wear develops. The seal is just a result of these close tolerances & long contact areas, coupled with relatively thick, oily fuel-the main reason model fuel has much more oil content than full size 2 strokes. Don't worry at all about the fore & aft movement unless it is excessive (>.5mm perhaps?), but check that the thin steel shim washer is still in place-this acts as a thrust washer when/if you use a starter & if this is missing the prop driver will rub the crankcase & give the black residue you describe. also the crankpin can rub the backplate-check this for circular marks inside.When the engine is running in a normal tractor installation, the crank is pulled forward anyway, taking up any end float. You could fit another washer, but always maintain a 'little wiggle'.

Wear on the bearing can be ascertained by trying to move the crankshaft up & down, you would not normally expect anymore than a very slight movement, but again, if the engine is running OK It would not be worth doing anything about it.

Hope this helps.

John Gallard30/07/2008 21:36:00
51 forum posts

Hi Mark

Thanks for your comments, engine does run fine and a couple of guys at the club suggest the same ie a little leakage at the prop end and nothing to worry about at this stage



Engine Doctor03/08/2008 11:01:00
2341 forum posts
28 photos

Hi John . As your club mates say , the small amount of back and forth play and small leakage is nothing to worry about and is perfectly normal on these plain bearing engines. When the plain /main bearing finally wears beyond its usable tolerance you may notice that fuel actually spits out whilst the engine is running ; this will also coincide with a fall off in performance . The use of good fuel and properly balanced props will keep your motor running for  years. I have just retired an OS 35 FP from my club trainer ,used to buddy newcomers. Its been flying that model for about five years and was still running fine ,just wanted a bit more power. A quick check to see if the main bearing is ok  is to hold the prop in line with the cylinder and test for movement back and forth on the prop tips . This should be done when the motor has just been run as thick oil will mask the play. Any noticable play will be detected . Only worry about this play though when the performace drops off. Black oil can appear at the front of the main bearing if the thin steel thrust washer has been lost/removed when the prop driver has been removed. Thjis allows alloy to rub on alloy and a resulting black mess will be created (aluminium particlesin oil)



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