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wakataka
Joined: 11 Aug 2012
Posts: 48
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 Posted: Wed Jul 09, 2014 9:01 pm Post subject: Uncaged bearing - the dreaded "tink-tink" sound |
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I thought I would pass along a little tip that might save someone some grief. I've got a 377 on my 1985 Firestar with an unknown number of hours. One thing I always do on preflight is slowly turn the engine over and listen for any unusual sounds. On my last preflight, I heard a slight "tink tink" sound that was not associated with any one spot on the rotation of the engine, and irregular in tempo. It's not very loud and you might easily think nothing of it. Fortunately, I had heard this sound before on a snowmobile engine. It's the sound of the balls in one of the main bearings falling down the back side of the race after the plastic bearing cage has gone away. When the cage is gone, all the balls drop to the bottom of the bearing. Then when the crank is turned slowly, they climb up to the top of the bearing and then fall one by one back down again. Each time a ball comes over the top and falls, it makes a little tink sound when it lands on it's neighbors hanging out down in the bottom.
The engine will still run, but a catastrophic and expensive failure is in your near future if you continue to run with an uncaged bearing. I'm not sure what causes this. In the engine in question it was the bearing on the mag end. On the snowmobile, it was the PTO bearing and one of the center bearings. The crank was slightly bent on the airplane engine. I'm thinking that could have been a symptom of running with a damaged bearing or it could be the cause of the bearing's demise.
The bearings in the gearbox have steel cages. I've always wondered why they use bearings with plastic cages inside the engine.
So pay attention to those little things during preflight and avoid the big things later on.
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olendorf
Joined: 06 Jul 2006
Posts: 140
Location: Schenectady, NY USA
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| Posted: Thu Jul 10, 2014 5:53 am Post subject: Re: Uncaged bearing - the dreaded "tink-tink" soun |
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Sorry about your engine. Please document your tear down and rebuild if you do one. Some of us guys are still running Rotax 2 strokes and would be very interested in watching along.
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Scott Olendorf
Original Firestar, Rotax 447, Powerfin prop
Schenectady, NY
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Ducati SS
Joined: 15 Oct 2009
Posts: 93
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| Posted: Sat Jul 12, 2014 9:31 am Post subject: Re: Uncaged bearing - the dreaded "tink-tink" soun |
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This type of failure occurs with metal retainers as well and often leads to a top end melt down. Certainly there are numerous potential causes for most mechanical failures but I believe one of the leading causes for this type of bearing failure is rust. I remember one engine in particular ware the retainer was broken, ground and mutilated almost beyond recognition. Customer brought the sled in with a melted piston, piston melted because PTO seal was leaking, PTO seal leaked because crank wobbled, crank wobbled because bearing failed, when I examined what was left of the bearing I found rust. That engine was by no means the only one I found corrosion in. 2 strokes are rather difficult to adequately protect the lower end during storage. First unlike a 4 stroke a 2 stroke draws raw fuel into the crankcase on shut down when we kill spark but the engine spins down drawing fuel thru the idle circuit. especially bad if the fuel contains ethanol. Oil Poured in the spark plug hole will not necessarily find its way to the crank bearings and almost certainly will not reach the lower rod bearing. Fogging is an option but I don't like the stress on the engine as fogging oil forces a stall. Also on a 2 stroke if the piston is any ware near the top of its stroke at rest there will be a direct route for moist air to enter the crankcase thru the open intake port. This was a particularly frustrating aspect of being a snowmobile mechanic as almost no one would bother to properly store their machines. The failure occurs so long after the machine is put back in service that it is almost impossible to convince the customer of the true cause.
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