4.2 lost compression
11-01-2015, 02:35 PM
#1
4.2 lost compression
I have a Fox (looks like a Brison) 4.2 that on the last flight lost most of it's compression at TDC.
I first checked the plug and all bolts.
Today I pulled the head off and rings look fine and cylinder wall is slick all around.
In the bottom of the case I did find with my finger a couple of flat microscopic 1/8" long pieces of metal???
Would this be chrome liner??
Is it possible to fail like this from going from high compression to almost nothing?
Engine has five-six gallons of fuel with no problems.
I am running 40-1.
Thanks
Jim
I first checked the plug and all bolts.
Today I pulled the head off and rings look fine and cylinder wall is slick all around.
In the bottom of the case I did find with my finger a couple of flat microscopic 1/8" long pieces of metal???
Would this be chrome liner??
Is it possible to fail like this from going from high compression to almost nothing?
Engine has five-six gallons of fuel with no problems.
I am running 40-1.
Thanks
Jim
11-01-2015, 04:20 PM
#2
How are you checking the compression , with a gauge or just by feel ?a good engine needs more than 90 psi to achieve optimal combustion , tested with a compression gauge at wide open throttle accumulated during 6 to 8 compression cycles.
The exhaust needs to be un restricted .
The cylinder does need to be wet , not dry ,but not flooded.
Things that can cause loss of compression
Worn or stuck rings , check ring end gap .it should be more than .002 but less than .010 , ideally. .003 to .007 .
Nicasil cylinders don't wear out easily ,but they do loose plating sometimes and it would be easily noticed when inspecting the cylinder.
If the engine is run too hot the oil will flash off and create a dry cylinder effect ,the oil actually seals the ring to the cylinder ,without it the seal is lost and so is compression.
So , I would check the following
Wet the cylinder and see if compression returns to normal then check for proper oil mix and or engine overheating.
If it does but only temporarily ,check ring end gap and cylinder surfaces
If it doesn't check for , cracked or burned through piston , cylinder surface damage .
The usual causes are overheating the engine in almost all cases.
The exhaust needs to be un restricted .
The cylinder does need to be wet , not dry ,but not flooded.
Things that can cause loss of compression
Worn or stuck rings , check ring end gap .it should be more than .002 but less than .010 , ideally. .003 to .007 .
Nicasil cylinders don't wear out easily ,but they do loose plating sometimes and it would be easily noticed when inspecting the cylinder.
If the engine is run too hot the oil will flash off and create a dry cylinder effect ,the oil actually seals the ring to the cylinder ,without it the seal is lost and so is compression.
So , I would check the following
Wet the cylinder and see if compression returns to normal then check for proper oil mix and or engine overheating.
If it does but only temporarily ,check ring end gap and cylinder surfaces
If it doesn't check for , cracked or burned through piston , cylinder surface damage .
The usual causes are overheating the engine in almost all cases.
11-01-2015, 04:53 PM
#3
My Feedback: (19)
No fancy explanations here .... but if you found metal it came from somewhere and is most likely the cause of your problem. Do some inspections for damage bearings, damaged cylinder walls, etc. Most of the time when an engine looses compression "all of a sudden" it is because of a stuck ring so make sure your ring is loose in the piston groove. Regardless, a fairly quick inspection should reveal the cause of your problem and any engine guy would have to do the same to reveal the problem. Not something we can just tell you here on a forum with the info provided.
11-01-2015, 05:24 PM
#4
Senior Member
Check the piston retainer clips on the wrist pin. I just had an engine lose compression suddenly and one retainer clip had popped off, wiping out the piston, ring and liner. That's after around 800 flights but still I never expected this engine to do THAT...this engine never had a problem in 5 years of operation; it's first time was pretty disastrous.
Last edited by MTK; 11-01-2015 at 05:28 PM.
11-01-2015, 06:48 PM
#6
Thanks for the info guys.
Yes this afternoon I dropped off the cylinder head and the rings are free with no carbon anywhere.
Spark plug has that perfect tan color.
I am using regular Steel (sp) oil mixed 40-1.
I flushed everything out with a gas hose from electric pump and reassembled and squirted with soapy water looking for bubbles around fins and spark plug.
Nothing...... with lose of compression at TDC.
Beats me as this motor has never had a prop broken and always had high compression.
Always flown off a grass runway and minimum dirt exposure.
Is mounted on a Hanger Nine Pawnee with plenty of airflow.
Yes this afternoon I dropped off the cylinder head and the rings are free with no carbon anywhere.
Spark plug has that perfect tan color.
I am using regular Steel (sp) oil mixed 40-1.
I flushed everything out with a gas hose from electric pump and reassembled and squirted with soapy water looking for bubbles around fins and spark plug.
Nothing...... with lose of compression at TDC.
Beats me as this motor has never had a prop broken and always had high compression.
Always flown off a grass runway and minimum dirt exposure.
Is mounted on a Hanger Nine Pawnee with plenty of airflow.
11-02-2015, 05:07 AM
#8
I have Re read the op's post and I keep getting hung up on the "at TDC" and I feel like I am missing his point somehow. I have been a proffessional mechanic for over 30 years and have not heard of compression related to top dead center . It may be me or the op is describing something in terms I'm not understanding.
Compression ends at TDC and therefore is lost at TDC , it does not stay compressed long . I do know many people have had concerns about TDC over centering or that loose rocking feeling at TDC , which is normal.
My questions are , does it still start and run?
Do you feel any compression build up at all when flipping the prop?
Please build me a better picture of what is happening when you are noticing the loss of compression at TDC .
You have me very curious.
Compression ends at TDC and therefore is lost at TDC , it does not stay compressed long . I do know many people have had concerns about TDC over centering or that loose rocking feeling at TDC , which is normal.
My questions are , does it still start and run?
Do you feel any compression build up at all when flipping the prop?
Please build me a better picture of what is happening when you are noticing the loss of compression at TDC .
You have me very curious.
11-02-2015, 10:28 AM
#9
TDC....Yes I hear what you are asking.
As the piston enters TDC it loses most of it compression that is enough that flipping the prop there is no carry through...it's dead at that point of rotation. No, the engine will not start. On the last flight I heard a change of sound and on throttling back it quite on landing. Never did that before.
I think I lost some of the cylinder wall plating (chrome??) at TDC? Remember I said above that I found a couple flat pieces of thin metal.
The engine is being boxed up and sent to someone that knows this stuff better then I.
I am afraid of what W8ye is telling me above. Not that much time on this engine.
Jim
PS...what makes me unhappy is I have two chain saws and a leaf blower that gets treated like crap and at least ten years old and never a problem with them using the same oil gas combination. This hobby stuff is something else!
As the piston enters TDC it loses most of it compression that is enough that flipping the prop there is no carry through...it's dead at that point of rotation. No, the engine will not start. On the last flight I heard a change of sound and on throttling back it quite on landing. Never did that before.
I think I lost some of the cylinder wall plating (chrome??) at TDC? Remember I said above that I found a couple flat pieces of thin metal.
The engine is being boxed up and sent to someone that knows this stuff better then I.
I am afraid of what W8ye is telling me above. Not that much time on this engine.
Jim
PS...what makes me unhappy is I have two chain saws and a leaf blower that gets treated like crap and at least ten years old and never a problem with them using the same oil gas combination. This hobby stuff is something else!
11-02-2015, 11:01 AM
#10
mx I hear what you're saying about the rc stuff . I have 30 year old saws that have cut hundreds of cord of wood and always runs flawlessly , yet the plane engines running the same fuel keep me busy with repairs.
Let us know what your guy finds wrong with your engine , it may help others..
Let us know what your guy finds wrong with your engine , it may help others..
11-02-2015, 11:43 AM
#12
Before you splurge on that DA, try Scott Swirles at http://aeroscott.com/. Yes those Fox engines use some of the same parts as the Brisons. The cylinder and piston assemblies are made by Mahle. Fox used to service both. I do not know if they still do. Give Scott a try. Dan.
11-02-2015, 01:22 PM
#13
TDC....Yes I hear what you are asking.
As the piston enters TDC it loses most of it compression that is enough that flipping the prop there is no carry through...it's dead at that point of rotation. No, the engine will not start. On the last flight I heard a change of sound and on throttling back it quite on landing. Never did that before.
I think I lost some of the cylinder wall plating (chrome??) at TDC? Remember I said above that I found a couple flat pieces of thin metal.
The engine is being boxed up and sent to someone that knows this stuff better then I.
I am afraid of what W8ye is telling me above. Not that much time on this engine.
Jim
PS...what makes me unhappy is I have two chain saws and a leaf blower that gets treated like crap and at least ten years old and never a problem with them using the same oil gas combination. This hobby stuff is something else!
As the piston enters TDC it loses most of it compression that is enough that flipping the prop there is no carry through...it's dead at that point of rotation. No, the engine will not start. On the last flight I heard a change of sound and on throttling back it quite on landing. Never did that before.
I think I lost some of the cylinder wall plating (chrome??) at TDC? Remember I said above that I found a couple flat pieces of thin metal.
The engine is being boxed up and sent to someone that knows this stuff better then I.
I am afraid of what W8ye is telling me above. Not that much time on this engine.
Jim
PS...what makes me unhappy is I have two chain saws and a leaf blower that gets treated like crap and at least ten years old and never a problem with them using the same oil gas combination. This hobby stuff is something else!
11-02-2015, 02:33 PM
#14
My Feedback: (19)
That 4.2 cylinder and the related 5.8, 3.2 and 2.4 Sachs / Dohlmer / Mahle cylinders are some of the best in the industry when it comes to quality. If yours failed it is a fluke happening and not the norm. A very quick visual inspection and run your finder around the top of the cylinder bore will tell you in short order whether there is a problem in that area.
