Another Pandoria's box

Tom-D

Tom-D

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Tom-D
Yesterday I went to help a 170-B owner replace two cylinders on his 0-300. The engine has been running fine, good oil pressure, good power, smooth operation all around. Recently started to exhibit a little morning sickness, #2 & #5 show a drop in compression since last annual. These cylinders are Superior Millennium with about 900 Hours since new. The Owner wanted them pulled to see what could be done to restore the engine to good health.
We got #2 off and found the piston pin was frozen in the piston, by heating the piston we removed the piston pin from the piston just to find the piston had started to gall metal to the pin. The decision was made simply to replace #2 with the spare new cylinder, So we move over to #5. and find the same thing happening.
Now for the kicker, we can now see the cam and lifters behind these two cylinders, the forward cam lobe that runs the exhaust valve for #6 is near gone, and the lifter face is eroded really bad.
The rods on both #2 & #5 can be wobbled about 3-4 degrees sideways showing the rod bearing are excessively worn.
The owner made the decision to overhaul the 0-300-A. Log books show 1750 +- since last major.

you just never know what you will find when you open Pandora's box. :)
 
Tom: how does ~1750 time since overhaul compare with your experience with O-300's?
How many years ago was this overhaul?
 
Tom-D said:
Yesterday I went to help a 170-B owner replace two cylinders on his 0-300. The engine has been running fine, good oil pressure, good power, smooth operation all around. Recently started to exhibit a little morning sickness, #2 & #5 show a drop in compression since last annual. These cylinders are Superior Millennium with about 900 Hours since new. The Owner wanted them pulled to see what could be done to restore the engine to good health.
We got #2 off and found the piston pin was frozen in the piston, by heating the piston we removed the piston pin from the piston just to find the piston had started to gall metal to the pin. The decision was made simply to replace #2 with the spare new cylinder, So we move over to #5. and find the same thing happening.
Now for the kicker, we can now see the cam and lifters behind these two cylinders, the forward cam lobe that runs the exhaust valve for #6 is near gone, and the lifter face is eroded really bad.
The rods on both #2 & #5 can be wobbled about 3-4 degrees sideways showing the rod bearing are excessively worn.
The owner made the decision to overhaul the 0-300-A. Log books show 1750 +- since last major.

you just never know what you will find when you open Pandora's box. :)
Click to expand...

I'm only asking... why replace the cylinder and not the piston?
 
GRG55 said:
Tom: how does ~1750 time since overhaul compare with your experience with O-300's?
How many years ago was this overhaul?
Click to expand...
Pretty typical.
 
Hmmmmm. Sounds like my situation with the A65 two annuals ago. Had two cylinders fail compression check. Later learned all four were cracked and non-repairable. Also discovered pitted main bearings and badly worn cam followers. Thankfully the crankshaft and cam were ok. Wish I coud drop a Rotax in the old girl.
 
N659HB said:
Hmmmmm. Sounds like my situation with the A65 two annuals ago. Had two cylinders fail compression check. Later learned all four were cracked and non-repairable. Also discovered pitted main bearings and badly worn cam followers. Thankfully the crankshaft and cam were ok. Wish I coud drop a Rotax in the old girl.
Click to expand...
That would be like putting a diesel in an vintage steam locomotive.
 
On these engines, are the connecting rods from opposing cylinders on the same crank journal, or are they separate?
 
so....what did his mechanic think?

btw....is heating a piston an approved method to remove the pin? Not my preferred method.
 

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455 Bravo Uniform said:
On these engines, are the connecting rods from opposing cylinders on the same crank journal, or are they separate?
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Every flat aircraft engine I’ve overhauled has separate journals for individual cylinders. Look at bore size and spacing, a common journal isn’t likely to work.
 
mondtster said:
Every flat aircraft engine I’ve overhauled has separate journals for individual cylinders. Look at bore size and spacing, a common journal isn’t likely to work.
Click to expand...

Thank you; I figured the cylinders were offset slightly so as to allow a common journal. (I guess all I needed to do was google a pic of a crank.)

I’d love to buy a junk engine with unusable core and disassemble it myself just to learn.
 
That sounds like the same thing we found with the O-470 in my C-180. The overhaul is nearly done now so hopefully we will be flying again soon.
 
Checkout_my_Six said:
so....what did his mechanic think?

btw....is heating a piston an approved method to remove the pin? Not my preferred method.
Click to expand...
It's nice to use those tools, but you'll never get that tool on #3or4 on the 0-300. How hot do you believe we heated the piston? as hot as it gets when running?
Heating slightly hotter that you can hold, and the pin slides right out with your fingers (most times)
 
455 Bravo Uniform said:
On these engines, are the connecting rods from opposing cylinders on the same crank journal, or are they separate?
Click to expand...
An 0-300 crank.. each cylinder has its own crank pin.
 

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Fearless Tower said:
If the engine had 1750 since major, why was it topped at ~850? That sounds more like a big bore Continental.
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I was told that it had 1750 turns out to be a 1100 since a G&N overhaul. and not a lot of detail about what was done then.
 
Tom-D said:
An 0-300 crank.. each cylinder has its own crank pin.
Click to expand...

Thanks.

From the pic:
-are the journals filleted? Can’t tell.
-on the rearmost journal, is that a removable balance weight?
 
455 Bravo Uniform said:
Thanks.

From the pic:
-are the journals filleted? Can’t tell.
-on the rearmost journal, is that a removable balance weight?
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Filleted ?? not understanding the term.

Yes the crank has dynamic balance weights to dampen the 5th order vibrations, as per the Overhaul manual. And yes they can be removed and the bushings replaced.
 
Last edited:
Sorry. I come from the automotive side, building Buick 455s and turbo V6s. The beefy v6 cranks have factory filleting (pressure rolled radius at the 90 degree at the jounal- looks “under cut”- for stress relief and strengthening). Couldn’t tell from the pic, but almost looked like it.

Fillets:
http://www.gnttype.org/techarea/pictureguides/cranks/crankguide.html
 
is this a better picture?

The crank pin is actually radius to center the rods.
 

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455 Bravo Uniform said:
Sorry. I come from the automotive side, building Buick 455s and turbo V6s. The beefy v6 cranks have factory filleting (pressure rolled radius at the 90 degree at the jounal- looks “under cut”- for stress relief and strengthening). Couldn’t tell from the pic, but almost looked like it.

Fillets:
http://www.gnttype.org/techarea/pictureguides/cranks/crankguide.html
Click to expand...
Unlike most automotive cranks, aircraft cranks generally are radiused at the pins. Helps the crank take the bending stresses without breaking, as well as keeps the rod ends from sliding around.

Sent from my LG-LS997 using Tapatalk
 
zodiacflyguy said:
Unlike most automotive cranks, aircraft cranks generally are radiused at the pins. Helps the crank take the bending stresses without breaking, as well as keeps the rod ends from sliding around.

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Click to expand...

Interesting. In automotive I’m used to seeing 2 rods on one crank journal and the clearance between the rods and crank side play is important. Any radius on the crank journal and the rod would lockup instantly.

Anyone got a closeup of the big end of the rod side (with an installed bearing)?

C0B92A8B-7FB3-412D-A587-8111E9743EC4.jpeg
 
455 Bravo Uniform said:
Interesting. In automotive I’m used to seeing 2 rods on one crank journal and the clearance between the rods and crank side play is important. Any radius on the crank journal and the rod would lockup instantly.

Anyone got a closeup of the big end of the rod side (with an installed bearing)?

View attachment 61718
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You can do that in auto engines because the crank is not subjected to bending due to having a bigass propeller bolted to the end. As the aircraft pitches up or down the prop resists this movement, and attempts to keep moving within the orginal plane of rotation. This induces a bending moment on the crank. Auto conversions have dealt with this stress in a few ways. On some you can grind the crank with a slight radius. On others the tolerances are too tight and all you can really do is nitride the crank and/or derate the engine. If you are not really familiar with aircraft engines the loose tolerances as compared with auto engines can be a bit surprising, but remember this: an aircraft engine can be started and after a halfassed warmup, go full throttle for several minutes and then thottled back to 70-80% for hours. And they last for 20 years or more this way. The closest auto comparison to that usage is probably NASCAR, but their engines are rebuilt constantly.

Sent from my LG-LS997 using Tapatalk
 
455 Bravo Uniform said:
Interesting. In automotive I’m used to seeing 2 rods on one crank journal and the clearance between the rods and crank side play is important. Any radius on the crank journal and the rod would lockup instantly.

Anyone got a closeup of the big end of the rod side (with an installed bearing)?

View attachment 61718
Click to expand...
http://2.bp.blogspot.com/-cuY3D9LEj...ing+rod+between+the+piston+and+crankshaft.jpg

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This is a picture of a cutaway engine (training aid) but you get the idea.
 
455 Bravo Uniform said:
Are the rod bearings radiused on the edges?
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From what I remember, kinda. Chamfer may be a more useful term. Like I said before, all these parts arent necessarily a tight fit, and if you are real finicky about .0005 tolerances, it will bother you. There are a few places like this, but mainly when assembling things like planetary gears onto bearings, and a few other critical places. Most everything else (in general aviation especially) is in the neighberhood of .015 to .030.

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zodiacflyguy said:
From what I remember, kinda. Chamfer may be a more useful term. Like I said before, all these parts arent necessarily a tight fit, and if you are real finicky about .0005 tolerances, it will bother you. There are a few places like this, but mainly when assembling things like planetary gears onto bearings, and a few other critical places. Most everything else (in general aviation especially) is in the neighberhood of .015 to .030.

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Click to expand...
The piston pin is a tight fit as is the rocker shaft, piston to cylinder is .038" the oil clearance on the crank is best at .002" (0-200/0-300)But like you say, nothing else is very tight.
 
Tom-D said:
The piston pin is a tight fit as is the rocker shaft, piston to cylinder is .038" the oil clearance on the crank is best at .002" (0-200/0-300)But like you say, nothing else is very tight.
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I knew it was something like that, I consciously choose to not remember tolerances verbatim. Forces me to look **** up per job. Sometimes some random engineer has an idea and stuff just gets changed....

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zodiacflyguy said:
. Forces me to look **** up per job. Sometimes some random engineer has an idea and stuff just gets changed....
Click to expand...

I send all the machine work out, that means every thing is suspect when it comes back. (people make mistakes, machines aren't always calibrated)
Every measurement is checked, every bearing is plas-ta-gauged for clearance, every ring is checked for end gap, every cylinder is Leak checked, every torque is verified by two people, the torque wrench is cal-ed prior to the build. I got burned on this once, never again, then I found the real cause was 4 bogus bolts supplied by the owner was the cause of the accident.
So now we add "where did these parts come from"?
 
zodiacflyguy said:
You can do that in auto engines because the crank is not subjected to bending due to having a bigass propeller bolted to the end. As the aircraft pitches up or down the prop resists this movement, and attempts to keep moving within the orginal plane of rotation. This induces a bending moment on the crank. Auto conversions have dealt with this stress in a few ways. On some you can grind the crank with a slight radius. On others the tolerances are too tight and all you can really do is nitride the crank and/or derate the engine. If you are not really familiar with aircraft engines the loose tolerances as compared with auto engines can be a bit surprising, but remember this: an aircraft engine can be started and after a halfassed warmup, go full throttle for several minutes and then thottled back to 70-80% for hours. And they last for 20 years or more this way. The closest auto comparison to that usage is probably NASCAR, but their engines are rebuilt constantly.
Click to expand...

Would seem like the ideal solution would be a geared prop. Would take the flying forces off the crank and into the custom-made gearbox designed for the forces. Isolates the combustion forces from the flying forces, allowing each side to be designed for their part. Yet...most auto conversions are still direct drive and gearboxes, whether on aero engines or conversions, have a bad reputation for reliability. Also, allows for a higher RPM out of the engine, which gets an auto engine closer the ideal RPM for their more oversquare geometries. Why is that?
 
so....what kind of horse power are they turning at those "ideal RPMs"?
 
Checkout_my_Six said:
so....what kind of horse power are they turning at those "ideal RPMs"?
Click to expand...

Well, assuming you're aiming for the lowest lb. fuel per HP, most auto engines like around 4500 RPM or so. Taking the 3L V6 in my car as an example, it's making 275 SAE net HP at that RPM. Being supercharged and intercooled, it's not ideal as an aviation engine, however.

I guess several Chevy 350 engines are common conversions. They turn about the same...275 HP at 4500 RPM. Maybe you turn the big V8 slower at 4000 and get 250 HP instead. They're not directly comparable numbers, since the SAE net for cars is different than how the HP numbers on planes are measured. That said, 250 HP compares pretty favorably to getting 200 HP out of an IO-360, which is about the same size.
 
HP is proportional to RPM, increase it for more HP until a mechanical limitation, call it redline (valve float, rod strength, etc). Auto engines are typically rated their HP at redline. No one ideal RPM for car engines, unless you define ideal for what.
 
455 Bravo Uniform said:
HP is proportional to RPM, increase it for more HP until a mechanical limitation, call it redline (valve float, rod strength, etc). Auto engines are typically rated their HP at redline. No one ideal RPM for car engines, unless you define ideal for what.
Click to expand...

Certainly, and I was running with the assumption BSFC was the goal.
 
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