Restoring compression seal on a Norvel .074
01-24-2010, 04:38 AM
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Restoring compression seal on a Norvel .074
Here's a run on this Norvel .074 http://www.youtube.com/watch?v=jW7Sq...&feature=email
With a great many hours of running, this Norvel .074 had lost nearly all compression seal. Resizing the cylinder to recover the fit was tried. But Norvel cooling fins are integral to the cylinder so they had to be removed. Exposing a clean cylinder is required to resize this engine for proper pinch at TDC. This is done with a single split collar with the ID turned out to match the OD of the cylinder. The split collar is tightened around the cylinder at 5 minute intervals. Going by feel, you can see that the wrench requires less and less force as you cinch down the diameter of the cylinder. Every cylinder will respond differently so no specific directions can be given. Just keep going round and round while tightening and loosening the wrench. The piston should be trial fitted as you go along. When you're satisfied, that you have the desired pinch at TDC, put the engine back together and give it a run.
You will notice in this clip that the new, detachable cylinder fins, have been left off. This was done because the run shown was done at -10C. The finned head was all the cooling that was needed at these ambient temperatures. We didn't want the engine to run too cool when new and potentially wearing out the new fit prematurely.
The prop is a Cox 7 X 3.5 and we see that we have recovered just over 1,000 RPM.
More work in this area will be done on various other baffed engines.
With a great many hours of running, this Norvel .074 had lost nearly all compression seal. Resizing the cylinder to recover the fit was tried. But Norvel cooling fins are integral to the cylinder so they had to be removed. Exposing a clean cylinder is required to resize this engine for proper pinch at TDC. This is done with a single split collar with the ID turned out to match the OD of the cylinder. The split collar is tightened around the cylinder at 5 minute intervals. Going by feel, you can see that the wrench requires less and less force as you cinch down the diameter of the cylinder. Every cylinder will respond differently so no specific directions can be given. Just keep going round and round while tightening and loosening the wrench. The piston should be trial fitted as you go along. When you're satisfied, that you have the desired pinch at TDC, put the engine back together and give it a run.
You will notice in this clip that the new, detachable cylinder fins, have been left off. This was done because the run shown was done at -10C. The finned head was all the cooling that was needed at these ambient temperatures. We didn't want the engine to run too cool when new and potentially wearing out the new fit prematurely.
The prop is a Cox 7 X 3.5 and we see that we have recovered just over 1,000 RPM.
More work in this area will be done on various other baffed engines.
01-24-2010, 07:30 AM
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RE: Restoring compression seal on a Norvel .074
Excellent, if some one had asked me if it can be done, I would have said that there is no way it would work...
Guess it is mainly works on aluminium cylinders?
This way the livespan it extended. So they can stop making 1/2a engines but there is no way they'll have us flying electrics!
Guess it is mainly works on aluminium cylinders?
This way the livespan it extended. So they can stop making 1/2a engines but there is no way they'll have us flying electrics!
01-24-2010, 01:25 PM
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RE: Restoring compression seal on a Norvel .074
Interesting fix...
Do you think the original loss of compression was due to wear only?
Or perhaps from pressure cycles or thermal cycling over time or the like?
Maybe you could have just turned some cooling fins on the resizing collar and had adjustable pinch
Not a glow engine, but Contental recently updated and lightened their 0-200 (think Cessna 150), and found that they didn't need fins on the cylinders for cooling. The did find that the fins were needed to keep the cylinders dimentionally stable though.
Do you think the original loss of compression was due to wear only?
Or perhaps from pressure cycles or thermal cycling over time or the like?
Maybe you could have just turned some cooling fins on the resizing collar and had adjustable pinch
Not a glow engine, but Contental recently updated and lightened their 0-200 (think Cessna 150), and found that they didn't need fins on the cylinders for cooling. The did find that the fins were needed to keep the cylinders dimentionally stable though.
01-24-2010, 03:57 PM
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RE: Restoring compression seal on a Norvel .074
Silvaire,
That's interesting about Continental. In fact, that seems to prove a conclusion I came to some years ago that air cooled engines, (at least our small stuff) don't suffer from apparent over cooling. I had read for years that 1/2As could not be made to throttle properly because, according to the math, they were grossly overcooled. Those folks obviously never burned themselves on a Cox .049. No, small engines can run pretty hot. And Norvels that have massive cooling fins compared to Cox, run just fine.
But this experiment was carried on to the next day and at the same ambient air temperatures. I installed the detachable fins and found, to my surprise, that I picked up a decent 800 RPM. So I took the fins off and promptly lost that extra power. NOW I think I know why Norvel set up their small engines so tight. The cylinder appears to grow more than the piston and you lose some of your fit as the temps go up at full throttle. This is evidenced by the piston of this engine while not all that tight at TDC when new, did have SOME pinch. But in time, power was lost, and inspection showed staining of the piston from blowby with just small polish marks, here and there.
So in this case, what's happening is that even with the tight fit achieved by re-sizing, and with no fins, and despite the low ambient temperatures, that cylinder was growing to the point of a slight loss of compression seal, at full throttle. I know this because when cold, the engine snaps over smartly. When hot, not so. It still runs and runs well but by cooling it with a LOT of fin area, the cylinder doesn't grow so much and a shade better compression seal is evident.
This effect doesn't seem so pronounced in the larger engines, .15 and up because of the three or four of each that I have owned, none have the arm busting fit of the .049s and 06s.
That's interesting about Continental. In fact, that seems to prove a conclusion I came to some years ago that air cooled engines, (at least our small stuff) don't suffer from apparent over cooling. I had read for years that 1/2As could not be made to throttle properly because, according to the math, they were grossly overcooled. Those folks obviously never burned themselves on a Cox .049. No, small engines can run pretty hot. And Norvels that have massive cooling fins compared to Cox, run just fine.
But this experiment was carried on to the next day and at the same ambient air temperatures. I installed the detachable fins and found, to my surprise, that I picked up a decent 800 RPM. So I took the fins off and promptly lost that extra power. NOW I think I know why Norvel set up their small engines so tight. The cylinder appears to grow more than the piston and you lose some of your fit as the temps go up at full throttle. This is evidenced by the piston of this engine while not all that tight at TDC when new, did have SOME pinch. But in time, power was lost, and inspection showed staining of the piston from blowby with just small polish marks, here and there.
So in this case, what's happening is that even with the tight fit achieved by re-sizing, and with no fins, and despite the low ambient temperatures, that cylinder was growing to the point of a slight loss of compression seal, at full throttle. I know this because when cold, the engine snaps over smartly. When hot, not so. It still runs and runs well but by cooling it with a LOT of fin area, the cylinder doesn't grow so much and a shade better compression seal is evident.
This effect doesn't seem so pronounced in the larger engines, .15 and up because of the three or four of each that I have owned, none have the arm busting fit of the .049s and 06s.
