mtntopgeo

My Feedback: (12)

As of this morning, there is one less operable .074 O.K. Cub on the planet. (I know, "no big loss"). Bought this thing when it was used, but in good shape. It was mounted on a 7 OZ EPP foam mustang. I got 'bout 30 flights outta her, when this happened. Upper part of the small end of the con rod decided to part company from the rest of the rod. No sign of lube failure. (Running 33% nitro with 25% oil, mostly castor) Prop is a 4.75x4, & Rpms on the ground are 14.7K. Doesn't sound like it unloads a bunch in the air. The debris stuck to the big end (see the pic) is what's left of the small end. It got wedged between the big end & the case. Made for a rather abrupt stop. & spit off the prop in the process. (I actuall found the prop, screw, & washer out on the runway.) Question is, where (PRMs) do things normally fail? I read that guys with modified Cubs (hogged out support posts, higher compr ratio, & turbo head/plug, are running at 17K.!!! Maybe mine was just tired & old (like its present owner.) If I were to put another cub on this plane, or repair this one, what would be a proper metat to build a rod outta? 6061? ................................ George K.

noveldoc

My Feedback: (2)

Methinks your prop was a bit small. I usually fly a 6" on an 049 and 7" or so on your size.

Tom

Mr Cox

It is a little hard to tell from the picture alone, but it looks to me like it may have overheated in the small end rather than having just been over-reved.
It is unbushed aluminium, so full castor would be the better. Perhaps also less nitro, but the oil is the most important part.

combatpigg

My Feedback: (3)

I've made unbushed rods out of 6061 T5 hardness. Make sure it is T5 or T6. Obviously the rod will be more prone to failure, not last very long if it's over worked. I even had rods made like this work in FOX.35 powered sport racers. The metal can't be allowed to get hotter than it would normally get in a running engine while you are machining it.

BMatthews

With that prop you were just expecting too much from that engine. They just are not made to run that fast. Likely the engine was holding back the prop rather than the other way around. A 6x4 would have been a more appropriate prop for the Cub. They were just a general sport engine after all.

gcb

Another thing that might have happened is that ANY engine that has been sitting around for years is prone to have the conrod frozen to the wrist pin either by corrosion or castor varnish. It is a good idea to ensure that it is free before running the engine. If it is not, the wrist pin ends become the pivot and can cause excessive wear..or worse.

I understand that conrods on the diesel versions were made stronger. Don't know this for a fact.

George

mtntopgeo

My Feedback: (12)

Tom & Bruce; Yeah, the prop is a bit small, but did you notice the RPMs? 14.7K just doesn't sound realistic, as far as being "too much". Best "all-'round prop for this plane/engine combo appeared to be a 5.7x3; (about 14K). ..... When I was a kid, always ran a 6x3, or 5x4. Of course back then, (60+ years ago) props were wider & thicker than they are now.

Mr Cox; Pic shows a bit of a gold tint on the rod with a dark area near the small end. In reality, the rod is all light alum. color. I did worry about not using "full castor" fuel, & when I realized that the engine had seized, I was sure that it was a lube failure, but I expected a piston/cylinder problem.

CP, thanks on the tip for the alloy. I had no idea of the different suffixes.

George, Pretty hard to tell "after the fact", but the area of the wrist pin, where the rod rides looks normal. (clean, oily, & shiney). Towards each end of the wrist pin, (areas that ride in the piston) was a good deposit of varnish. Pin was very snug in the piston, had to be pushed out. I doubt that the pin was full floating even when the piston warmed up. Something to look at before I fire up the next relic.

Thanks for your inputs, guys. ........................... George K.

RocketRob

My Feedback: (1)

7075 or at minimum 2024t3 for a rod

controlliner

Fox uses 2024 T3 on their unbushed rods.

mikegordon10

I have broken at least a dozen Norvel rods over the past 3 or 4 years but this is on .061 AME's turning 4.5 X 2.6 props at a little over thirty grand. I just figure it comes with the territory. This is on Sig's 30% Competition fuel with 2 oz of castor oil added to each quart. I'm never happy when it happens but not surprised either. Last race I broke 2 before the race ever started!

BMatthews

Well, the trouble is that the Cub isn't a fancy breathing engine. It'll only spin so fast breathing through the little hole in the crankshaft and the little flutes up the support legs for the cylinder. So you can prop it down all you want but it's starved for food and air so it just won't spin faster. The RPM you got wasn't the limit produced by the prop. It was more produced by this lack of breathing. I'm pretty sure that with that dinky prop you were allowing it to effictively do a shaft run so the piston just beat away at the conrod until it was no more.

mtntopgeo

My Feedback: (12)

OK, think that I'm starting to see the picture now (it ain't clear, but I'm starting to see it.) Cleaning up another .074 cub now. Will try a 6x4, a tad less nitro, & a bit higher % castor. I'll tare it down after another 30 flights (if it last that long) & report back. .............. George K.

Japanman

George:
If you can get your hands on some, 2024 or 7075 (or pretty much any of the 7xxx alloys) will make a better rod than 6061 or any of the 6xxx alloys, but as CP says, even 6061 is good enough.
I have found that 2024 works well as an unbushed rod material in some pretty revvy highly stressed engines: some people think it is the copper in the alloy (copper being the main element in bearing bronze.)
I can`t imagine the stresses at 15Kish are going to blow out the rod like you show with everything being right: i would tend towards the idea that the little end was gummed up and seized; could be a casting defect; could be junk alloy of the rod... At any rate you making a new rod is going to make it a lot better than new.

Stefan

mtntopgeo

My Feedback: (12)

Stefan, Of all that has been said on this topic, I lean more towards the "casting defect/junk alloy" than the other reasons given. I see where Bruce is coming from, 'bout a "shaft run", but where things get cloudy on that theory is that we're not powering the crank from an outside 15,000 RPM source. It's obvious that at least a few molecules of fuel & air are reaching the combustion chamber & are providing power. Having trouble seeing where there can be any rod stretch/distortion, or any unusual rod/piston interactions under those conditions, especially at such modest RPMs. However, I'm lost enough, that I still intend to be conservative with the current engine. Once I have the original engine repaired, I'll go back to using a prop that the plane/engine likes, & see what hapens. .......... Found a chunk of 7075 in a pile of my stuff. Now I gotta grab the ball gauges, find the milling vise, & fire up the Logan. ................... George K.