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OS 55AX Engine - crankshaft end float

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GrahamWh28/09/2014 23:00:51
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356 forum posts
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I have an OS 55AX which has survived a nose dive crash in a Blackhorse Renegade and am near to completing the repairs to the airframe.

The engine has been cleaned out, and turns okay by hand but I have noticed about 1mm of play when I pull and push the prop shaft forward-backward (no sideways movement). Does anyone know if these engines have this amount of end float as I can't remember what it was like before the crash which was several months ago?

Cheers, Graham

cymaz28/09/2014 23:05:46
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If you push the crankshaft back does it still spin freely without any grinding? If not should be fine. If you are still worried then heat up the case and see if the crank can be pushed forward any further on the bearings. Use the same technique if you were replacing a bearing set.

GrahamWh28/09/2014 23:18:07
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356 forum posts
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When pushed backwards, there is a slight grinding, but not if pulled forwards, as the prop will be. Worth taking engine back off and opening it up do you think?

Martin Harris29/09/2014 01:27:59
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End float is not a sign of damage on your engine - the only positive location is in the forward direction as the crankshaft is a close but not interference fit in the main bearing. An engine with a taper fit driver will usually show little or no detectable end float but your driver locates on a stepped face. The grinding is likely to be light contact between the crankpin/conrod and the backplate.

However, a hard impact can put microscopic indentations onto the bearing balls and tracks which will lead to reduced bearing life so I'd keep an eye (and ear) out for early signs of bearing failure - any roughness or play developing will need investigation...

Edited By Martin Harris on 29/09/2014 01:28:32

GrahamWh29/09/2014 11:11:05
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356 forum posts
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Thank you Martin. I'll keep an eye on it and I have ordered a new set for when it eventually wears out from RC bearings - a lot cheaper than OS!

Engine Doctor05/10/2014 10:46:49
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2591 forum posts
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Hi Change the bearing before they fail or you could end up with a chewed up engine . Have you had the prop driver off of the engine ? ther is usually a shim in between the driver and the front bearing on most engines . Look at explode view in link . The shim /spacer is part no 10. If this is missing then the crank pin will hit the back plate a nd chew off metal fillings that can damage the other internals .

Edited By Engine Doctor on 05/10/2014 10:47:40

GrahamWh10/10/2014 16:54:31
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356 forum posts
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I have followed your advice ED, bearings replaced with a SS one at the rear and normal one at the front (with the inner shield removed as per the original for lubrication) all for £6 plus postage. Much quieter. Fired it up this pm and working smoothly. With a slightly rich mixture, it is turning a 13x5 MA K series prop at 2,400 to 9,600 (9,900 leaned out). Thanks.

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