OS 81 Alpha - snapped connecting rod - RC Groups
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Dec 04, 2017, 09:18 AM
Master re-kitter
SRQFlyer's Avatar
Discussion

OS 81 Alpha - snapped connecting rod


I was asked to repair this engine by a very knowledgeable fellow club member. The engine is not his and I will have to get more base information regarding circumstances, fuel, load, etc. He was told that "it just stopped" in the air. He had removed the rear cover to do a preliminary inspection. The photos show it as I received it. I'm not sure if he wiped out any oil residue or not - looks pretty dry to me.

When I removed the bottom end piece of the rod, the bushing slid out! This, of course, would allow misalignment of the oil holes. I'm not sure if this is cause or effect. Anyway, I'll get more pictures of the rod fracture and the crank pin once I get the engine apart. I post more info and pictures as I get a chance.

Let the conjecture begin!

Jim
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Dec 04, 2017, 09:40 AM
the great Gassif´er
no rod puncturing the case, or getting entangled with the crank?
Dec 04, 2017, 09:45 AM
Registered User
earlwb's Avatar
I am suspecting that someone tried using RC car engine fuel in the airplane engine. RC car fuel can have as little as 8% oil in it. I have seen people do that before. They go to those hobby shops that only cater to the kids and their RC cars and that is the only fuel that they see for sale. Also it tends to be cheaper than the regular fuel as they use less oil of course.

Now it could be where the fuel is normal, but the owner had run the engine too lean. It may have adjusted ok on the ground, but leaned out in the air on him. But it wasn't lean enough to seize the engine up. There may still be some galling on the cylinder and piston though.
Dec 04, 2017, 10:10 AM
Master re-kitter
SRQFlyer's Avatar
Quote:
Originally Posted by Brutus1967
no rod puncturing the case, or getting entangled with the crank?
The top half of the rod is hanging down from the piston. No case puncture. I'm calling the fellow club member now to get more details. Will advise.
Jim
Dec 04, 2017, 10:51 AM
Registered User
I TOBOR's Avatar
You can see that the rod end has been wiping on the inside of the crankcase and even shows wear on the rod and inside the crankcase. The bushing looks mighty loose too.

Another case of over revving, maybe.
Dec 04, 2017, 10:54 AM
the great Gassif´er
Quote:
Originally Posted by SRQFlyer
The top half of the rod is hanging down from the piston. No case puncture. I'm calling the fellow club member now to get more details. Will advise.
Jim
He's a lucky fellow then: probably not too difficult to get going again....
Dec 04, 2017, 10:56 AM
Registered User
I TOBOR's Avatar
Sport & Aerobatic
Size(DxP)
13x7-9, 14x6-7

Prop recommendations, should keep the rpm in the safe zone.
Dec 04, 2017, 11:06 AM
Master re-kitter
SRQFlyer's Avatar

more pictures


I talked to the intermediary and he said the owner was a careful, long-time pilot. No lean running. Still awaiting answer about the fuel and wiping the case out.

ouch - look at the gouge in the case!

More pictures of the ends of the rod pieces. The last two show how I think it went together. This engine may have been running with the rod broken part way through??

The big end is very sloppy on the crank pin, which doesn't look to have been overheated (at least to me). The outside of the big end wasn't rubbing on the bottom of the case, I don't think.
Dec 04, 2017, 11:17 AM
Master re-kitter
SRQFlyer's Avatar
Quote:
Originally Posted by Brutus1967
He's a lucky fellow then: probably not too difficult to get going again....
Yes - looks like a rod and bearings are all that is needed!
Dec 04, 2017, 01:00 PM
Registered User
That crank looks like it is scored and also looks like the bushing seized on the pin then released when it cooled down
Dec 04, 2017, 03:21 PM
Registered User
I TOBOR's Avatar
It doesn't look loose in picture # 11. Great mystery eh.
Dec 04, 2017, 03:51 PM
Registered User
Cougar429's Avatar
Think you lucked out there. Usually broken conrod takes the case with it, (I have a Gemini 160T with that right now).

The fact it broke at the very end of the radius suggests either a manufacturing problem or possible material failure.....until you add in the loose bushing in the big end.

I would go with the lack of lube or overheating turning the bushing in the rod until completely closed. Once at that point it likely twisted the big end off the rod.

I ran into something similar with my first 4-stroke, a Saito 91, (for some reason one of the few with no bushing). Had nearly a dozen trouble free flights till the cowl was installed. First flight locked the big end to the pin. Landed and as the engine cooled was able to get it to turn, although not too easily. New rod and solving the cooling issue had well over 300 subsequent flights before selling the airframe and engine.

That was on 17% lube, so perhaps not directly this owners problem. Can you get info on installation?
Dec 04, 2017, 04:35 PM
Registered User
I TOBOR's Avatar
C, I don't think that the 91-s ever had bushed rods, this is a test of an early and the tester mentions that the rod is not bushed. I'm not sure who
the tester was. He loved the engine though.
Dec 04, 2017, 04:40 PM
Registered User
Quote:
Originally Posted by I TOBOR
C, I don't think that the 91-s ever had bushed rods, this is a test of an early and the tester mentions that the rod is not bushed. I'm not sure who
the tester was. He loved the engine though.
Yes the 91-s has bushes rods. I just put one back together that I replaced the bearings in
Dec 04, 2017, 04:41 PM
Master re-kitter
SRQFlyer's Avatar
Quote:
Originally Posted by BillyP.
That crank looks like it is scored and also looks like the bushing seized on the pin then released when it cooled down
Upon closer inspection, there are some very faint score marks on the crank pin and it is slightly "blue". I wonder if I should polish them out vs. the expense of a new crank for an obsolete engine ??


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