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HomeAircraftHelicopterEngines Plugs Mufflers Fuel › Connecting rod or crankshaft pin, or both?
04-30-2010 01:31 AM  8 years agoPost 1
n808

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Seattle, WA - USA

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I see that there is noticeable slop between the connecting rod and crankshaft pin on my OS50, probably messing up the timing a little. It still runs very good though. Is there a way to know if I need to replace one, or the other, or both, before ordering both rod and crankshaft?

Karl.

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04-30-2010 02:02 AM  8 years agoPost 2
TMoore

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Cookeville, TN

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Yep.

You measure the clearance.

TM

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04-30-2010 02:25 AM  8 years agoPost 3
n808

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Scroll down for the answer...

Which I don't understand, still in the nitro n00b category. Clearance where?

I don't have parts to compare with to know what the ID of the conrod or the OD of the crankshaft pin should be when new.

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04-30-2010 02:34 AM  8 years agoPost 4
TMoore

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Cookeville, TN

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The clearance on the big end of the conrod should be around .002" of an inch. This means you measure the inside diameter of the rod and the outside diameter of the crankpin and the difference is the clearance.

TM

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04-30-2010 02:53 AM  8 years agoPost 5
Gernejr

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Miami, Fl

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TMoore Where do you find spec on motors? I looks for spec on YS91sr .Thanks

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04-30-2010 02:57 AM  8 years agoPost 6
n808

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Seattle, WA - USA

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Thanks. That's good to know for reference. First I will need a more accurate digital caliper. Then if the clearance is too big, I still have to figure out if the wear is mostly on the conrod or the crankshaft pin, which was the original question.. I guess it's likely both need replacement at that point (about $64). I'll wait until I pull the engine next and check again then, at which point I likely need a new ring and/or bearing as well, maybe even sleeve. And then I might as well get a new OS or YS 50. They're as low as $160 these days.

EDIT: Actually, I have a brand new YS50 ST I got a great deal on, just waiting for the day when one of my OS50's is worn out.. or sooner for that matter I love how the 56 craze has made the 50's so cheap..

Karl.

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04-30-2010 03:06 AM  8 years agoPost 7
Gernejr

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Miami, Fl

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How many gallons form fuel has that engine seen? How many times has it overheated/ lack of oil? If the piston still looks good then the rod and crank are fine. First to go PISTON. If the liner does not have grooves then you can clean engine ,lghtly hone the liner ,install new piston ring ,install new rear bearing maybe front bearing too. Then go burn another 20 gallons...

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04-30-2010 03:07 AM  8 years agoPost 8
TMoore

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Cookeville, TN

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TMoore Where do you find spec on motors? I looks for spec on YS91sr .Thanks
The manufacturers don't typically publish specs as far as dimensional tolerances are concerned. The .002" number is one that I've had in my head for a long time mainly for 60-91 heli engines. The clearances for Ducted fan engines typically run a little larger than that. For the 50's and smaller the clearance is usually reduced.

TM

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04-30-2010 03:20 AM  8 years agoPost 9
n808

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Seattle, WA - USA

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How many gallons form fuel has that engine seen? How many times has it overheated/ lack of oil? If the piston still looks good then the rod and crank are fine. First to go PISTON. If the liner does not have grooves then you can clean engine ,lghtly hone the liner ,install new piston ring ,install new rear bearing maybe front bearing too. Then go burn another 20 gallons...
Around 300 flights, so if I reckon about 12 flights/gallon, 25 gallons. I can't remember any serious lean episodes on that one. Piston still looks good.

That's good advice, Gernejr! I will keep that in mind. Bearings are almost new and ring is good for now, as is the piston. I just opened the back since I had it out to change a broken clutch. I haven't really checked the conrod/crankshaft clearance before, and what I am seeing might well be within spec...

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04-30-2010 03:21 PM  8 years agoPost 10
bbaxter

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Central Illinois

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Basically, if you see actual VISIBLE movement of the crankpin in the lower-end of the connecting rod, it should be replaced. Mostly, one sees oil squeeze, which is many times misinterpreted to be actual movement. 0.003" is usually the clearance for a brand-new engine. It can get as loose as 0.007"-0.010" before it's too bad to use.

High-RPM engines, such as heli, ducted-fan, and speed engines usually need the larger clearance so that enough oil can get between the rod and crank while the engine's running. More oil is also highly recommended.

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