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Prop shaft wobble

Old 11-25-2019, 02:40 AM
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EF
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Default Prop shaft wobble

I have a 26cc “Zenoah copy” - a QJ engine brand new never run.

I noticed after installing it on a test bench and securing the prop, that when I turn it the front end of the prop shaft seems to wobble off center.
I’d say the wobble is about 1/16”.

This is my first gas engine and I have no idea if this is common or how critical is it.

I have about 45 years experience with glow engines and have never come across this kind of thing.

Would appreciate advice from the more experienced...
TIA
Old 11-25-2019, 05:08 AM
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ahicks
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Define "wobble" please. I'm not sure if you are talking about sloppy bearings, what appears to be a bent crank, or something else.
Old 11-25-2019, 05:20 AM
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EF
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Bearings are new, engine is new, it’s as if the shaft is slightly bent from the factory.

Maybe I should have written ‘crooked shaft’...

Last edited by EF; 11-25-2019 at 05:47 AM.
Old 11-25-2019, 08:43 AM
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Excessive prop shaft or hub wobble / runout isn't normal and should be corrected. Some (few) engine types will have a bit of endplay and if the design permits, this can be normal.

I check all new engines with a dial indicator and prefer to see less than .003" runout in any radial or axial direction. Measurements are taken on the tip and root of the prop shaft in front of the prop hub as well as the face and side of the prop hub. Most quality engines will show runout measurements in the .001" - .002" range or less! Lower cost engines have higher tolerances but even these will have prop hub measurements within tolerable limits. Engines that retain the prop hub with a long shouldered nut (the type where the nut actually forms the part that projects through the prop and isn't threaded) often fail my tests with excessive runout at the tip. I keep an assortment of new / used nuts on hand and usually correct this problem by selective fitting a different nut. Runout at the tip of these shafts is a design problem but not one you will see on any Zenoah engine (or copy) as these engines don't use this type of hub retention.

Some will say that Chinese engines are worse than others for runout but I've found it depends on the Chinese engine brand and the design of particular engines. Even high quality engines such as DA and 3W have had problems over the years (not common) so no engine brand is completely immune to runout problems. Any engine that suffers a sudden prop strike or one that shatters a prop should be checked.

Another problem that twin cylinder engines and engines with pressed together crankshafts can have is crank twist. In this case, the crank becomes misaligned between its various segments and the prop shaft / hub can wobble quite a bit in severe cases. All Zenoah engines can suffer from this problem though it is usually caused by a severe prop strike or crash. In this case, the wobble / runout is a symptom of a greater problem within the engine. Most newer single cylinder engines use cantilever crankshafts (all made from one piece of steel) and don't exhibit this problem.
Old 11-25-2019, 08:58 AM
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EF
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Thanks for your detailed and informative answer.
I guess it’s not quite a quality engine...

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