flhyr

Guys, How do you know if you need a new connecting rod? (aside from its broken)

I recently replaced the bearings in my OS 46 SF ABC and got it running like a banshee, but I noticed that there was some play in the bushing between the connecting rod and the crankshaft, and when it runs it seems like you can here it clicking sometimes, mostly during idle and midrange.

It makes a sound that reminds me of a landing gear wheel sliding back and forth against the wheel collar.

The engine is about 18 years old, and has many hours on it (hence the new bearings) but still has crazy power.

What do you think?

NM2K

My Feedback: (14)

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Don't be misled by how the engine sounds when not running. I have seen a few times over the years when the connecting rod bushings would not fill with oil and would then permit the engine to sound clicky when flipping by hand. After running the engine again, the clickiness went away. All I could assume was that the bearing picked up its oil film again and filled the space with oil.

I've seen people send brand new engines back for warranty work because of this same clickiness. The engines were returned with a note explaining that everything was okay. Many times the clickiness was gone. Or went away after the next running. I think we are hyper sensitive to things some time.

However, if you feel your engine is making an unusual sound that when the engine is running, you may want to ensure that your new bearings are properly seated.

If your connecting rod is egged out at either end, you should see some debris in the exhaust oil. Have you checked it for being bent?

flhyr

Thanks Ed,

In fact, one of the reasons I changed the bearings was because I was starting to get dark stuff out the muffler, and I was able to wiggle the prop tips by hand. I'm pretty sure that the bearings are seated well since before putting the piston/connecting rod in the crankshaft would rotate freely and the weight would always fall to the 6 O'Clock position on its own without any help. I haven't run it much since the bearings were replaced last night, but I'll keep my eye out.

Although it does sound clicky even after being run, but by the same token the connecting rod was still really hard to get on and off the crankshaft due to the tolerances which probably means its ok, right?

delman

You might make sure you put the rod back in place with the beveled side of the bushing to the front of the engine.

djlyon

If you don't have the clickeness in that rod that you are talking about it won't last long You need a fair amount of play there for the oil to get in there and work

PropSpinner

My Feedback: (15)

I read somewhere on RCU that if you just see oil squeeze out of the bushing you are OK but if you see the crank move in the rod you need to replace the rod.

Not24

My Feedback: (1)

My OS 1.08 made that sound from day one. Upon disassembly, it was noted that the rod had lots of clearance on the crank. Putting a pump on the engine allowed me to richen up the midrange mixture, and the sound went away. It only makes that sound when it's too lean. It was lean in flight before I installed the pump, but all is well now. I agree that if the tolerance were any tighter it would be impossible to assemble the engine.

Rcpilot

My Feedback: (78)

18 yr old engine with lots of hours on it? For $6 or $8 I'd replace the rod.

DarZeelon

Flhyr,


When the parts are assembled dry, the rod bearing clearance should be visible.

The oil must be allowed to flow in freely, without any pressure.

If the crank-pin diameter is 6 mm, with oil fed by a pump, you would want about 0.01 mm of oil clearance.
For a free flowing oil/fuel mix, it must be much larger, since there is nothing to compel the oil to come in.
So the clearance should be about 0.02-0.03 mm.

This is visible to the naked eye.

If the piston is lodged at TDC (it should catch there, if your ABN piston/sleeve set is in good condition...), you should be able to rock the prop about 10-15 degrees, perhaps even more, without the piston moving.
This demonstrates the accumulative clearances of the 'rod on crank-pin', 'rod on wrist-pin' and 'wrist-pin in piston bosses'.

In a properly running two-stroke engine (not when it is four-stroking), the con-rod is always under compression, so this clearance will not be apparent.

TimC

Dar, if I remember my plain bearing theory right, a shaft spinning in a plain bearing will draw oil in from the ends to the center where it will exit through the provided hole. This would seem to apply at least to the big end and probably apply to a shaft rocking in a plain bearing.

DarZeelon

Tim,


I am not aware of this logical theory and if it is true, it can only apply to the lower/big end.
It can only be by pure chance that a drop, or two of oil reaches the top/small end bearing of the con-rod.

The theory I am aware of is that while the shaft is spinning within a bearing, it causes the thickness of the oil-film around it to be nearly uniform, despite the presence of forces that 'try' to push it in a particular direction.

This theory does not work for the top end either, so it needs plenty of oil to be splashed around, if it is ever going to survive for long.
It is luckily not spinning, but only reciprocating about 30 degrees, from left, to right and back and its average rotational speed is about 1/6 that of the lower end and it comes to a complete stand-still, twice in every revolution.

You can never rely on any film thickness theory in that spot.

flhyr

I think everyone here is right, an 18 year old engine isn't gonna be hurt by putting a new connecting rod in it, and the one it has probably okay as most the things in the posts that described a good rod apply to my engine, plus like i said, in my US 40 running it on the ground, if you hold it verticle it will nearly hover the 6.25 lb airframe with a 10X6 MA.

I tell you what, they just don't make the ABC engines like they used to... true chrome plating on this one and the piston is still sticky at TDC after 18 years (give or take a few) and still a reliable idle.

DarZeelon

Flhyr,


Are you sure it is a true chromium ABC?

Engines from this generation, made by OS, were labeled ABC, although they were really ABN.
They said the 'C' stands for composite...

The ABN sleeve has the coating all around it, including the outside, since the nickel doesn't actually stick to the brass.
So a complete continuity is required. The outside has a molted appearance and the inside is shiny.

The true ABC sleeve has the chromium plating on the inner surface of the sleeve only and sometimes in the ports. The outside surface is bare brass.

See photos.


The fact that this engine can hold your 6.25 lb. model in a vertical attitude, with a notoriously inefficient prop, doesn't tells us much about its power, or its construction.

Putting in a new connecting-rod, to replace a good, running unit, is to introduce an unknown into the equation.
It is also a waste of money.

...If it's good, don't replace it!

flhyr

I'm pretty sure its true abc.

The sleeve looks just like the picture on the right with a brass color on the outer surface, but the chrome on the inner surface and ports. This engine was bought right after the FSR series went away I think, but not sure.

Your right about changing the connecting rod on a good running engine, I'm just gonna keep my eye on it for any signs its getting worse, other than that I'm just gonna run it.

DarZeelon

Flyhr,


If it looks like the right side photo, it is apparently a real ABC; one of the first SF production runs.

They lasted as long as the FSR series engines, while the later SFs were ABN and were the first OSs to peel their sleeves.


Make sure you use plenty of Castor oil in your fuel, with 20% total oil and 5-10% nitro and it will keep you happy for a long time.

...And put an APC, a Graupner, or a Bolly up front. You will see the flight performance improvement with your bare eyes.

flhyr

Dar, In the past all I used was cool power 10%, and had no problems whatsoever... why is everyone so down on synthetic oil?

w8ye

My Feedback: (16)

A lot of people I know use Cool Power only. They seem to get along just fine.

Enjoy,

Jim

DarZeelon

Flyhr,


It is partially from experience.

Also, even Morgan, the manufacturer of Cruel Power recommends their Omega line of fuels, that are 30/70 Castor/synthetic, for ring-less engines (ABC, ABN).

Klotz rate their own Techniplate synthetic as inferior to Castor (their BeNOL) in heat resistance (100 degrees lower flash-point) and in wear protection (scuff resistance).

The [link=http://www.osengines.com/faq/product-faq.html#q2]OS Q&A also recommends[/link] that you to use fuel with a moderate percentage of Castor oil.


So why take any chances with American all-synthetic lubricants?

Just use some Castor in the fuel and bring along some cleaner spray and a rag, to clean your plane after you're done flying.

misterpanda

Like the old saying goes: "if it aint broke, don't fix it"

w8ye

My Feedback: (16)

Off topic...

Windex causes the corners of your Monocoat or Ultracoat trim to loose their adhesive over time. But that's another story.

Enjoy,

Jim