How much is too much?

Tom-D

Tom-D

Taxi to Parking
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Tom-D
The C-150 /0-200 prop has too much rotational free play. you can move prop tip 1" up and down and not feel resistance.
 
I guess no one knows, I sure don't.
 
If I understand this the resistance would be from the piston moving in cylinder.

I would think there would be more free play at TDC/BDC.

LOTS of clearance at big end of Conn Rod might do that.

Seem to recall a a TCM AD that addressed just that.

Is this on an engine that is operational?

This could be one of those items that you can be around forever but not”see”.
 
Like the man says, free play is highest at TDC/BDC. In a four cylinder when two cylinders are at TDC the other two will be at BDC. One of the TDCs will be at the end of the compression stroke. Is this where you are seeing the 1" free play? If so, suggest you rotate the prop 90 degrees so the cylinders are at mid stroke. The free play can then be approximated as (big end clearance) X (ratio of propeller diameter : piston stroke). For example .006" X (70"/4") = 0.105". I'm not smart enough to estimate free play at TDC/BDC. Never noticed any free play on my plane but then it is a six so only two cylinders will be at TDC/BDC at any time.
 
I neglected to account for the piston ring side clearance. Service limit for the oil control ring is .006"; for calculations this can be added to the big end clearance. This doubles the above to 0.21", still much less than 1".
 
It doesn't change at any position.

this was during a pre-buy.

An inspector at the club, says it is too much.
 
flyingbrit said:
Like the man says, free play is highest at TDC/BDC. In a four cylinder when two cylinders are at TDC the other two will be at BDC. One of the TDCs will be at the end of the compression stroke. Is this where you are seeing the 1" free play? If so, suggest you rotate the prop 90 degrees so the cylinders are at mid stroke. The free play can then be approximated as (big end clearance) X (ratio of propeller diameter : piston stroke). For example .006" X (70"/4") = 0.105". I'm not smart enough to estimate free play at TDC/BDC. Never noticed any free play on my plane but then it is a six so only two cylinders will be at TDC/BDC at any time.
Click to expand...
What difference does make where the rotation group is ? wouldn't it be same no matter where it is?
 
Magman said:
Is this on an engine that is operational?

YES

This could be one of those items that you can be around forever but not”see”.
Click to expand...

This is true, I have checked the G0-300, because of the reduction gears that have a set limit.
but the 0-200 ?
 
flyingbrit said:
Like the man says, free play is highest at TDC/BDC. In a four cylinder when two cylinders are at TDC the other two will be at BDC. One of the TDCs will be at the end of the compression stroke. Is this where you are seeing the 1" free play? If so, suggest you rotate the prop 90 degrees so the cylinders are at mid stroke. The free play can then be approximated as (big end clearance) X (ratio of propeller diameter : piston stroke). For example .006" X (70"/4") = 0.105". I'm not smart enough to estimate free play at TDC/BDC. Never noticed any free play on my plane but then it is a six so only two cylinders will be at TDC/BDC at any time.
Click to expand...
Does it matter how many cylinders the engine has? wouldn't this check measure the cylinder that has the least ring to piston side clearance?
would you know which cylinder that was?
 
Tom-D said:
It doesn't change at any position.

this was during a pre-buy.

An inspector at the club, says it is too much.
Click to expand...

Find a limit in writing and we have a fault. Until then we do not have a fault.
 
Magman said:
LOTS of clearance at big end of Conn Rod might do that.
Click to expand...
That would be scary :)

the rod bearing clearance is .002" loose. I try to set it to .0015".

long time ago I measured a worn out bearing, it was.0035".
 
Bacho said:
Find a limit in writing and we have a fault. Until then we do not have a fault.
Click to expand...
That is what we are trying to determine, the general condition of the engine is a consideration
 
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Tom-D said:
I have checked the G0-300, because of the reduction gears that have a set limit.
but the 0-200 ?
Click to expand...
Well, the spec for the GO-300 would be due to backlash in the gears.
But an O-200 is direct drive. So, where would "free play" come from? Between the prop and the crank? Any would be too much. Bearings? You have a few thousandths of clearance which should translate to hundredths of an inch at the prop - if there was enough to move the prop 1", you have a problem that should be obvious as soon as you start the engine. Flange is integral to the crank on an O-200 - correct?
 
Capt. Geoffrey Thorpe said:
Well, the spec for the GO-300 would be due to backlash in the gears.
But an O-200 is direct drive. So, where would "free play" come from? Between the prop and the crank? Any would be too much. Bearings? You have a few thousandths of clearance which should translate to hundredths of an inch at the prop - if there was enough to move the prop 1", you have a problem that should be obvious as soon as you start the engine. Flange is integral to the crank on an O-200 - correct?
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we don't know, that is the scary part.
 
Theoretically 1 inch of double amplitude free play at the prop tip (assuming a 76 inch prop diameter) with all 4 cylinders of a direct drive engine approximately at mid stroke would suggest a total clearance of the rod big end plus little end plus ring gap of .012 inch.

This ignores main bearing clearance, there is no ring motion in the cylinder, and the piston isn't rattling in the bore. All of these will add to the apparent backlash at the prop tip.

Still seems way more than should be. Are you sure it is a full 1 inch D.A.?
 
nrpetersen said:
Theoretically 1 inch of double amplitude free play at the prop tip (assuming a 76 inch prop diameter) with all 4 cylinders of a direct drive engine approximately at mid stroke would suggest a total clearance of the rod big end plus little end plus ring gap of .012 inch.

This ignores main bearing clearance, there is no ring motion in the cylinder, and the piston isn't rattling in the bore. All of these will add to the apparent backlash at the prop tip.

Still seems way more than should be. Are you sure it is a full 1 inch D.A.?
Click to expand...
Thanks for the info, That is what we thought too.
But it was running and in annualed by an A&P-IA 6 months ago.
I was contacted by another IA, as a consultant, I assumed they knew
 
Tom-D said:
What difference does make where the rotation group is ? wouldn't it be same no matter where it is?
Click to expand...
At TDC/BDC the crank pin is moving perpendicular (sideways) to the piston/rod so the crank will move a relatively large distance before the piston moves noticeably. This is why you find TDC by stopping the piston short of TDC, measuring the angles in both directions, and splitting the difference. You know this procedure quite well as you have described it in this forum.
In the real world my calculated free play may not be noticeable due to the oil film in the big end and oil/carbon on the piston rings. AFAIK it is not a standard test for direct drive propellers.
However 1" does sound alarming. Have you removed the spinner and checked the integrity of the prop mounting bolts?
 
flyingbrit said:
At TDC/BDC the crank pin is moving perpendicular (sideways) to the piston/rod so the crank will move a relatively large distance before the piston moves noticeably. This is why you find TDC by stopping the piston short of TDC, measuring the angles in both directions, and splitting the difference. You know this procedure quite well as you have described it in this forum.
In the real world my calculated free play may not be noticeable due to the oil film in the big end and oil/carbon on the piston rings. AFAIK it is not a standard test for direct drive propellers.
However 1" does sound alarming. Have you removed the spinner and checked the integrity of the prop mounting bolts?
Click to expand...
As a crank pin moves off the 90 degree mark, another moves to take its place the amount that measurement changes will decrease as the number of crank pins increases
this 1 inch measurement never changes.

and yes it is tight.
 
nrpetersen said:
Theoretically 1 inch of double amplitude free play at the prop tip (assuming a 76 inch prop diameter) with all 4 cylinders of a direct drive engine approximately at mid stroke would suggest a total clearance of the rod big end plus little end plus ring gap of .012 inch.
Click to expand...
Hmmm...your calculations show much more free play than mine. Care to show your math (I'm sounding like my 8th grade science teacher!) or indicate where mine is wrong?
 
The assumed prop diameter should have been 70 inches, the stroke is 3.625 inches. Assuming no clearance elsewhere (such as main bearings etc), 1 inch at the prop tip equals (3.625/70) X (clearance). Therefore clearance = .051 double amplitude, which is a correction to my original calculations. On reflection I don't think there is a double vs single amplitude correction that needs to be applied. Originally I thought so.

That much clearance seems impossible unless a bearing shell has been left out (whee!). That's why I questioned the original 1" D.A. observation. Tom clearly says the backlash is present even at mid stroke. Otherwise it would be a (1-Cos) function which would be much more reasonable.

How do you get less free play when all cylinders are at mid-stroke? or are you looking at stroke end?
 
I suppose it could happen that all four big end rod cap bearings could have been left out! It would probably run but no oil pressure & would make a helluva noise!

Tom - If you get to see the insides of this engine you have to tell us what happened!
 
nrpetersen said:
The assumed prop diameter should have been 70 inches, the stroke is 3.625 inches. Assuming no clearance elsewhere (such as main bearings etc), 1 inch at the prop tip equals (3.625/70) X (clearance). Therefore clearance = .051 double amplitude, which is a correction to my original calculations. On reflection I don't think there is a double vs single amplitude correction that needs to be applied. Originally I thought so.

That much clearance seems impossible unless a bearing shell has been left out (whee!). That's why I questioned the original 1" D.A. observation. Tom clearly says the backlash is present even at mid stroke. Otherwise it would be a (1-Cos) function which would be much more reasonable.

How do you get less free play when all cylinders are at mid-stroke? or are you looking at stroke end?
Click to expand...
In any reasoning, It is my opinion this aircraft is not be flown..the A&Ps agree.

what the owner does with it is not my concern.

Thanks for the in put :)
 
nrpetersen said:
I suppose it could happen that all four big end rod cap bearings could have been left out! It would probably run but no oil pressure & would make a helluva noise!

Tom - If you get to see the insides of this engine you have to tell us what happened!
Click to expand...
I'm no longer in the engine rebuild business, I will probably never see the 150-/0-200 again.
 
The C-150-D was flown by a high time CFI with lots of 150 time on landing power reduced had a shudder in the engine that he had never had before on a 150. The end play on the prop for pull and push about 1/8" and a rotation of the prop tip about 1" as already talked about not sure exact as I did not put a dial indicator on it. This was a pre buy the buyer only wanted a 1 hour inspection of the plane and one hour on the logs. Note over the years this plane has sat for years at a time with no use several times, it does have a nice paint job.
 
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That noise was the crank trying to get out... :)

Good ol Bob. anyone who can fly a F-8 can fly anything. :) our acquaintance goes way back. (circa 1975)

It would fun to put a dial indicator on it and see really how many degrees it shows.
 
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when it's overhaul they will save the data tag. :)
 
AD 93-11-03 pertains to similar.
 
Tom-D said:
As a crank pin moves off the 90 degree mark, another moves to take its place the amount that measurement changes will decrease as the number of crank pins increases
this 1 inch measurement never changes.

and yes it is tight.
Click to expand...
In a four-cylinder opposed engine all the crankpins are at 90 degrees at the same time.

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