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Engine basics => Engine set up tips => Topic started by: Terry Caron on June 26, 2014, 09:45:34 PM
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Hi all -
I'm replacing the crankcase on a used .46 LA due to worn mainbearing - is a break-in procedure necessary?
Thanks.
Terry
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Yes, because the parts have been taken apart and put together again they will not be mated together, so a run in should be done. Following the original break in procedure of running it slightly richer than normal for a few tanks of fuel should be all you need. The manual recommends 6 flights starting from somewhat rich to the correct setting. It won't take long at all.
Abbreviating the procedure somewhat may be acceptable if the first couple tanks act as expected. (so ground run + 2-3 progressively leaner runs instead of 6 runs)
http://www.osengines.com/manuals/index.html
Phil
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Hi all -
I'm replacing the crankcase on a used .46 LA due to worn mainbearing - is a break-in procedure necessary?
Thanks.
Terry
Probably, to the extent break-in is "required" on any of these engines. Actually I would be quite interested in your results. I think almost all of the high fuel consumption at the beginning is due to the front bearing being too tight, and rapidly opening up when you first start running it.
I would be particularly interested in any observations you might have on how warm the bearing area gets. When I started running my 25LA, the venturi and front of the case got very hot, and the fuel consumption was absurdly high. Over the next 20 runs or so, this went away, the venturi and bearing area stopped getting hot, and the fuel consumption went down dramatically. The first 2-3 runs were particularly notable, but the engine was going richer and richer during flight over maybe 20-30 flights. It's steady as a rock now at about 50 flights.
Of course you probably don't really need to break it in to avoid damage, it's safe enough to fly as long as you are aware of the issues.
Brett
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Well, I've run it on the bench for 5 ea. 2 oz tanks, starting rich 2-stroke on a 9-5 APC then leaning next 2 tanks, last 2 on the flight prop (12 1/4x 3 1/2 APC) with last tank at normal take-off rpm (9.8K), which it held just fine.
No excessive heat generated.
However......
there's sideplay on the shaft as in the old crankcase.
Unfortunately, I didn't think to measure the shaft and bearings on each when I had it apart.
The old bearings measure .4736" I.D. - anyone know what .46LA factory spec is?
Terry
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Maybe it was your crankshaft that was worn? Or both crankcase and crank? I usually break things before they wear out, but I'm pretty sure that if there was grit involved, both case and crank would have material removed.
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I didn't examine the crank closely, but the case bearings aren't scored so grit wasn't involved.
And wear on a steel shaft is way less likely than on the bushings.
Guess I'll take it back apart and measure everything.
Terry
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The shaft front measures .4714", rear .470" with a few 10-thou of ovality.
The new case bearings measure .473".
Not much help without knowing factory specs though.
I'm thinking maybe it's just whatcha get from O.S.
My .20FP and .25 FP have discernible sideplay too.
Oh well - they run well enough right now.
Terry
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if you call hobbico support, they're really good about getting you the specs. in the situations I've had with excessive crank play, it was the crank worn. i thought it would be the other way around.
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I have had several LA series engines that were tight in the crank / crankcase dept. Most of them wore (er I mean ran in) pretty fast. However one particular engine was still a bit tight when the P/L had worn out. It was fine after a little running on a new P/L. That is definitely a very rare case indeed.
Yes you would think that the sleeve bearing would wear out first. However this is not the case. Any nasty hard bits get embedded in the softer bearing material and this acts as a lap on the shaft. Thus it is usually the shaft that wears, especially if the engine has been lawn darted.
If I remember the storey correctly, the camshafts on the Merlin engine ran straight in the block material, i.e. there were no camshaft bearings as such. I think the engines could be rebuilt four or five times before the block was scrap due to too much clearance in the camshaft holes.
Yes it is counterintuitive, before you realise what is going on.
Andrew.