acceptable rod bush clearance on new .61fs ?

Glow/Nitro and gas engines, propellers, fuel and other stuff associated with making lots of noise and thrust.

acceptable rod bush clearance on new .61fs ?

Postby AE25 » Sun Oct 10, 2010 11:36 pm

recently bought an ASP61fs. everything looks good, pulled all covers off for internal inspection, no swarfe, looks in good order etc.
One potential issue i've discovered is the amount of backlash in the rod/crank bush. at TDC i can get about 30 degrees of crank movement with no travel of the piston. this is all backlash in the rod assembly. I know if this was an engine for my real car, it would kill itself very quickly through oil loss on the main bearing journal. The engine has a clatter sound when flicked over by hand. I've not yet run the engine.
is this normal on a model nitro engine?
-Mark
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Re: acceptable rod bush clearance on new .61fs ?

Postby jeffie8696 » Mon Oct 11, 2010 4:51 am

Although I would have to see it for certain I would say it doesnt sound terrible. It is usual for the engine when at TDC to have a little slop. When the engine warms and the metal expands it will probably diminish. Another reason I like to use fuel with 18% or more(preferably more) of oil.
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Re: acceptable rod bush clearance on new .61fs ?

Postby AE25 » Mon Oct 11, 2010 9:04 am

oop i meant 'big end' in first post.
i've read of more than one person say they've had a rod failure on this particular engine. could be wrong fuel or setup, who knows. but general reviews say they're reliable.
just seems to be excessive bearing clearance even for a non pressure oiling system.
bit hard finding what is 'acceptable'. argg
-Mark
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Re: acceptable rod bush clearance on new .61fs ?

Postby AE25 » Mon Oct 11, 2010 9:08 am

oh also, aluminium con rod would have higher expansion rate than the steel crank.. so can't see it getting any better with heat?
-Mark
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Re: acceptable rod bush clearance on new .61fs ?

Postby jeffie8696 » Mon Oct 11, 2010 8:23 pm

If it is much of a concern a replacement rod is very reasonable from Justengines.
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Re: acceptable rod bush clearance on new .61fs ?

Postby RCModelReviews » Mon Oct 11, 2010 10:27 pm

I wouldn't worry too much -- the clearances have to be much higher in a non-pressure lube system like the ones we run in our model engines than in a pressure-fed bearing such as you'd have on a car engine.

Unless the engine starts making noticeable clunking noises when running it's not going to be an issue.
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Re: acceptable rod bush clearance on new .61fs ?

Postby AE25 » Sat Oct 16, 2010 4:49 am

coolies. ran the first tank today and it runs well. even on the factory recommended rich needle setting of 2.5 turns it was pulling around 9000rpm at full throttle and ticking over nicely at idle.
no noticeable funny noises when running. only a ticking can be heard from backlash in the cam when slowly turning the prop by hand.
-Mark
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Re: acceptable rod bush clearance on new .61fs ?

Postby RCModelReviews » Sat Oct 16, 2010 8:05 pm

Sounds good! Keep us informed.

I've only seen one other ASP61 running and although it wasn't a powerhouse (compared to a far more expensive Saito), it ran smoothly, throttled well and seemed like a very nice little engine.
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Re: acceptable rod bush clearance on new .61fs ?

Postby TMKits » Fri Nov 26, 2010 12:45 pm

Although not your engine, some info on rod clearance from Jett engines FAQ:
Question:
My rod is making a clicking sound. I fear it is too loose and about to break. What should I do?
Answer:
If your engine is new do not be concerned. The parts are inspected during manufacturing and during assembly. Your engine has also been test run just prior to shipping. Everything is OK.

Now, what is the reason for the clicking? Jett rods have about .003-. 004 (.07-. 1mm) rod-to-crankshaft clearance. This is approximately twice that of an O.S. This is why you see the extra motion. This is not a problem and here is why: During high-speed operation the engine loads the rod in one direction only—the piston pushing down on the crankshaft. This means that the rod is almost always touching at the top of the shaft, leaving a gap at the bottom. The gap is where the lubricant is stored and the more the better. During idle, you may hear the click of the rod moving back and forth on the shaft. The loads are very low at this speed and do not cause a problem.

We have measured rods after years of hard use and find the wear almost zero. In fact, we often say, “rods don’t wear”. Years of pounding are more likely to fatigue the rod body itself, and for this reason it is reasonable to replace them every few years. In racing, once a year would not be unreasonable. Always replace your rod after a shaft run (no propeller).

Finally, do not measure rod play at top dead center. This magnifies the clearance. Place the piston about one-half way between the top of the exhaust and top dead center. You will find a big difference.
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