OS50SX - Hissing sound around piston ring
07-27-2005, 12:50 PM
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OS50SX - Hissing sound around piston ring
Hi,
OS50SX had been running great for awhile (about 30 hours of flight), but had to change the rough running bearings last week. In the process of putting the piston back into sleeve, broke the piston ring. Therefore had to replace with a new piston ring. Engine fired up, run richer than previously set, to let the new piston ring seat with the piston sleeve. However, I noticed substantial loss of power compared with before bearing change.
Removed just the muffler, introduced some lubricant through exhaust outlet into cylinder, hand turned crank shaft to compress - notice hissing sound around piston. Next, with piston top/ring between TDC and exhaust outlet level, introduced some more lubricant to create a pool, hand turned the crank shaft to compress further, I saw bubbling around piston ring the compression stroke. Most bubbling seen around the area where piston ring gap laid.
Questions needing your advice - should I be bothered with continuing the "breaking-in" of the new piston ring, or should I ditch this piston ring, and go for a new ring, and perhaps a new sleeve.
Your recommendation will be appreciated. Thank you!
gc
OS50SX had been running great for awhile (about 30 hours of flight), but had to change the rough running bearings last week. In the process of putting the piston back into sleeve, broke the piston ring. Therefore had to replace with a new piston ring. Engine fired up, run richer than previously set, to let the new piston ring seat with the piston sleeve. However, I noticed substantial loss of power compared with before bearing change.
Removed just the muffler, introduced some lubricant through exhaust outlet into cylinder, hand turned crank shaft to compress - notice hissing sound around piston. Next, with piston top/ring between TDC and exhaust outlet level, introduced some more lubricant to create a pool, hand turned the crank shaft to compress further, I saw bubbling around piston ring the compression stroke. Most bubbling seen around the area where piston ring gap laid.
Questions needing your advice - should I be bothered with continuing the "breaking-in" of the new piston ring, or should I ditch this piston ring, and go for a new ring, and perhaps a new sleeve.
Your recommendation will be appreciated. Thank you!
gc
07-27-2005, 01:32 PM
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RE: OS50SX - Hissing sound around piston ring
With ringed model engines, it's normal to have some leakage past the ring when you turn the engine over slowly by hand. This would be true whether the engine is brand-new or has been used a while. Once the engine's properly broken-in, the amount of leakage will be less, but still present.
The reason you get the leakage is that the compression and combustion forces actually help to drive the ring against the cylinder wall and lower ring groove to provide the seal. You cannot generate enough pressure to do that when you turn it by hand, even with brisk flipping of the propeller. You can, though, get a good-enough seal to allow the engine to start.
When you replace only a piston ring, it must be run for a while to allow it to hone itself against the cylinder liner. Because the cylinder liner has had some wear from the previous piston ring, it may not be as round as it should be. It will take some time for the new ring to work itself to fit the liner. This is especially true if the liner wasn't honed a bit before the new ring was installed. The same is true for the ring fitting into the piston ring groove.
It may also be true that the piston and cylinder liner, that weren't replaced, have had enough wear that the new ring will never achieve a good fit. In that case, your engine's performance may be down permanently, and a new piston, ring, and liner will be the only solution to get the full power back.
The reason you get the leakage is that the compression and combustion forces actually help to drive the ring against the cylinder wall and lower ring groove to provide the seal. You cannot generate enough pressure to do that when you turn it by hand, even with brisk flipping of the propeller. You can, though, get a good-enough seal to allow the engine to start.
When you replace only a piston ring, it must be run for a while to allow it to hone itself against the cylinder liner. Because the cylinder liner has had some wear from the previous piston ring, it may not be as round as it should be. It will take some time for the new ring to work itself to fit the liner. This is especially true if the liner wasn't honed a bit before the new ring was installed. The same is true for the ring fitting into the piston ring groove.
It may also be true that the piston and cylinder liner, that weren't replaced, have had enough wear that the new ring will never achieve a good fit. In that case, your engine's performance may be down permanently, and a new piston, ring, and liner will be the only solution to get the full power back.
07-27-2005, 06:31 PM
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RE: OS50SX - Hissing sound around piston ring
Hi,
Thanks for your quick and informative response. I now feel more worthwhile to give myself more time to let the new ring to set in, and see how it goes after 10 or 20 more flights.
My sincere thanks.
gc
Thanks for your quick and informative response. I now feel more worthwhile to give myself more time to let the new ring to set in, and see how it goes after 10 or 20 more flights.
My sincere thanks.
gc
