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Old 07-17-2007, 07:08 PM   #1
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I have less than 1 gallon old Four Star 40 with a OS 46LA mounted on a hard engine mount that came with the plane. The problem is that it vibrates the plane allot especially at low idle when I can hear it the most. I've tried balancing the prop and the spinner, checked the whole plane/engine over for anything loose but everything is fine. I noticed that the engine has about 1/64th" play when you move the crankshaft in and out. So my question is, is it OK to have that kind of play? And is there anything else that could cause this?
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Old 07-17-2007, 09:34 PM   #2
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That sounds ok. You have a bushed engine versus a ball bearing engine.
Unfortunately, if you want the smoothness, you will probably have to upgrade.
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Old 07-17-2007, 10:49 PM   #3
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One part of a check that I do is check for a bent shaft. Does not need much but that will also cause the problem. The problem at idle seems the worse but you hide it with higher rpm when the vibration frequency is closer together.
Also the 1/64th end play is okay and I would be more worried with none as it could be wearing on your back plate.
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Old 07-17-2007, 10:54 PM   #4
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I have heard a few LT 40s and 4* 40s vibrate with 40 LAs on them. It seems to be typical of that engine : Don't know why exactly. The first time I heard it, it was so noticable I ask the owner if there was something wrong with the motor.

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Old 07-18-2007, 11:06 AM   #5
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i also have an os 46la and i also found it vibrates alot at low rpm never done any dmg and cleans up right away as rpm increases I wouldn't worry about it
we are dealing with single cylinder engines, there gonna rock a little nothing to counter balance like bigger engines would have
when things get worse as rpm increases then i'd be worried or if it sounds bad while idling

and i have the same play in the crank
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Old 07-18-2007, 11:43 AM   #6
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just a little warning about plain bearing engines.

The amount of shaft end play is determined by two things. The prop driver and maybe a thrust washer rubbing against the crankcase when pushing the shaft back and a shoulder on the shaft rubbing against the inside of the crankcaes when the prop is pulling on the shaft.

Use of an electric starter on a plain bearing engine is not advisable. Eventually the front of the case will wear and the crankpin will gouge into the case backplate.

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Old 07-19-2007, 09:43 PM   #7
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I had the plane inspected at the place I bought it, was told that there was nothing wrong with it. I guess I'll fly it the way it is. Thanks guys for your input.
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Old 07-20-2007, 11:53 PM   #8
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I fly diesels, so i would likely had never noticed the vibration! :P
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Old 08-29-2007, 01:59 PM   #9
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My 40 LA vibrates like that too and has similar play.

I blade and hub balance my props to perfection with a magnetic balancer to no avail. It resonates right through the airframe and sounds like hell.

It has at least 20 hours of flying time on it and nothing has ever gone wrong with it. It is tight in its mount, starts and runs flawlessly in typical OS fashion, sips fuel and there's been no discernable change in clearances.
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Old 08-29-2007, 02:32 PM   #10
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I have a .46 LA on an Eindecker E1 and the plane vibrates at low rpm also. Throttle up and it goes away. There is also endplay in and out on the crank. The motor starts and runs very good and has less than 1 gallon of fuel through it so I think it is just the nature of these engines.


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