Resonance!

[Kiwi303]

An interesting titbit about manufacturing pistons is the good ones are not machined round like on a lathe but oval like a camshaft lobe.

 

Look at the underside and you will see where the pin travels from side to side has significantly more meat to the forging/casting while the skirts to the sides where the pins are not are much thinner.

 

A good piston manufacturer grinds them oval so the wrist pin sides are ever so slightly narrower than the bare skirt sides. That extra volume of aluminium means those sides expand slightly more, so a round lathed piston would expand to become oval, while a cam ground oval piston expands to become round in the piston bore.

 

 

[Jabiru Phil]

Yes Peter.

 

Mine has the same characteristics that you describe.

 

I first noticed and was very concerned when I turned the prop after refuelling at Narromine from Caloundra to Loxton about 6 years ago. I rang my LAME, an expert on Jab engines and he said " totally normal"

 

Over 500 hours since and still very stiff after taxi back to hangar.

 

I checked another 230 here recently after landing with Similar tendencies.

 

Both about same hours and Definately no fretting evident.

 

More likely as mentioned, piston expansion.

 

Not a problem as far as I am concerned, motor performs to spec.

 

PHIL.

 

 

[jetjr]

Yes its common

 

Couple of points i see,

 

things grow in all directions with heat not just expand. They also cool diferently too BUT the heating and cooling happen from different sources so its possible that the cank is hot and expanded whilst alloy cools faster from outside of engine creatinga different mix of hot and cool parts to what happens in heating cycle.

 

Next pistons arent very tight in cylinders, thats what the rings are for, if pistons are grabbing on cylinder itself, you have big issues

 

 

[facthunter]

A lot of thinking going on.

 

If the crank gets tight the end float will be stiff to achieve. If it's still free then there is clearance and not much distortion happening..

 

Pistons... The material DOES expand more than the cylinder. Aluminium alloy more than steel.

 

Also heat flows from hot to colder so the piston will always be hotter than the cylinder when the engine is delivering power.

 

It will also be hotter when first shut down but since no heat is being applied to anything, then it will fairly quickly tend to equalise or close to . any tightness due to close piston fit would rapidly dissipate in a matter of minutes. If it was so tight a fit that it didn't, the engine would fail on climb out initially when a lot of heat has to transfer. Also, if the pistons had a bit less clearance when new, such that they did what some think they are doing there would be more evidence of failure and scuffing/burnishing on piston skirts in new engines.

 

Also if the fits were that critical engines with a few hundred hours on them would not exhibit the same effects, as the tight areas would wear a bit, enough to change the feel. Nev

 

 

[Bruce Tuncks]

Same for me Peter and I agree with your analysis, although I haven't noticed the "running hot" bit. I'll have a look at that soon.

 

I do recollect an item about Jabiru increasing the piston clearance in recent times.

 

 

[jetjr]

The new Gen Jabiru engine with alloy barrels is very free turning, not sure what that means as bottom end is the same.

 

CAE engines are also much easier to turn both hot and cold - not much difference.

 

 

[geoffreywh]

A 400hr engine that I wrote about was EXTREMELY stiff when hot. Probably would take a 10kg weight hung on a prop tip to move it. Since reporting that I did some investigation. Firstly Jabiru tell me that they no longer use loctite on thru' bolts (Hurrah!) Then I loosened the thru' bolts and retorqued to 30 ft lb, (some improvement in the prop feel. ) Then to 20 ft lb. Beautiful and end float now present. . Result? I propose Bulk engine strip and lessen main bearing shell "pinch" ...Note if you get hold of a prop tip and move it gently , i mean really gently, you should be able to feel (or hear) a very slight "loose " patch, That would be pistons rocking in the bore ( or a big end bearing gone!) If that very small movement is absent then the crank is being held tight.