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Old Apr 15, 2014, 12:48 AM
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O.S. 75AX - What Could Be Wrong?

I just installed a brand new O.S. 75AX into my airplane. I did the initial break-in of the engine per the O.S. manual, where I ran a full tank starting from a very rich (4-stroke sounding) needle setting for 1 minute. Then I transitioned between the very rich setting for 10 seconds and just to the beginning of where it stops missing and it sounds like a 2-stroke for 10 seconds. Going back and forth until the tank was empty.

Afterwards I took the propeller off and put the cowl on. Then I put the prop and spinner back on and tightened them down. While I was doing this I noticed that the propeller is not spinning in a smooth fluid motion when I rotate by hand. I can hear and feel a rough metal-to-metal action occurring inside the engine. It does not feel right.

Could I have damaged something while tightening the propeller? Is this possible? Or, could it be that the engine suffered damage during the running-in period? Does this happen with new engines?

I used Byron's fuel with 15% nitro.
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Old Apr 15, 2014, 03:10 AM
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DarZeelon's Avatar
Israel, Ramat HaSharon
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Joe,


The break-in procedure outlined in the OS manual is wrong for a tapered-bore engine (a.k ABN/ABL/ABC/...). This is what their US dealer's web site says about it.

This is what I wrote about this issue some ten years ago.

Nevertheless, even following that misleading procedure in the manual should not cause real damage to a loose-fit engine such as your OS - not getting the break-in done is what I'd expect... and reducing the engine's longevity somewhat - not breaking it outright!
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Old Apr 15, 2014, 06:02 AM
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Joe, I don't mean to be offensive but are you sure the spinner and cowling are not rubbing? That has happened to me before and gave me a bad shock 'till I found out the engine was not hurt at all. Just asking
Jim
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Old Apr 15, 2014, 06:12 AM
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Quote:
Originally Posted by mosquitowings View Post
Joe, I don't mean to be offensive but are you sure the spinner and cowling are not rubbing? That has happened to me before and gave me a bad shock 'till I found out the engine was not hurt at all. Just asking
Jim
+1 Yeah check and see if the cowl is rubbing, I have experience this and thought there was something wrong with the engine. The only other thing I can think of maybe the woodruff key has shifted.......? Good luck!
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Old Apr 15, 2014, 08:53 AM
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I noticed that there is a thin shim or washer that is placed behind the drive washer and the engine front bearing. That washer may have come off and lost on the floor or became jammed up or buggered up causing the drive washer to bind up against the front bearing of the engine. Without the little washer or spacer in between, the drive washer would contact the front of the engine then.
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Last edited by earlwb; Apr 15, 2014 at 08:54 AM. Reason: add more info
Old Apr 15, 2014, 09:36 AM
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This is my first new glow engine, so I have no idea what to expect as a newbie. I spoke with one of my fellow club members that is a long time glow/gas flyer and he said that it is perfectly normal for the engine to feel tight and have the metal-to-metal scraping/grinding feeling for a few tanks. I feel much better about the situation now.

I was comparing it to an old OS 46FX that was given to me by the same gentleman, and that engine shaft rotates smooth as silk.
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Old Apr 15, 2014, 06:58 PM
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+1 on what Earl said. look for a thin black washer.
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Old Apr 15, 2014, 11:20 PM
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The thin black washer is there.

So I am a little confused because I have a person in my club that is telling me the tightness and metal-to-metal scraping feeling is normal during the break-in, but it seems like that may not be the case since some of you do not seem familiar with that aspect of a new engine.

I guess I will just keep running it at a slightly rich setting from now on and see if everything smoothes out after a while. Hopefully that is the case.
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Old Apr 16, 2014, 08:26 AM
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It's unclear whether you're feeling the normal piston to liner interference of ringless designs at TDC or something else. It's normal for the piston to bind in the liner when turned over slowly near top dead center. Flipping the prop briskly it should glide right through TDC. If you feel grinding, rather than unwillingness to turn over, something could be wrong.

Greg
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Old Apr 16, 2014, 09:54 AM
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When you test ran the engine did it feel like there was that feeling you were talking about when turning the prop over? Then after to pulled the prop and put the cowl on and then put the prop back on did it have that same feeling or was it different when turning the prop?

Like gkamysz stated the engines using ABC or ABL designs can feel quite tight when cold and that could cause them to feel like the metal is grinding in some cases too.
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Old Apr 16, 2014, 02:48 PM
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New, out of the box, the motor felt very tight but smooth when turning the shaft by hand, and as it would get closer to the highest point of compression it would become difficult to turn, as expected.

After the first fuel tank "running-in" process and installing a new prop, it still feels the same way when turning the shaft by hand, except for one difference. Now I can feel/hear a metal-to-metal scraping/grinding sensation coming from inside the motor.

Everything rotates fine and the compression feels just as good as before the running-in process.

What I have been told by another pilot is that this is normal, because nitro engine components need to re-shape and essentially scrape away metal as they break-in. I was told that after a few tanks of fuel that the sensation of scraping/grinding will disappear and it will be a smooth feeling.

Does this sound correct?
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Old Apr 16, 2014, 02:57 PM
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No it doesn't in my opinion. If you ran the engine it ought to get smoother fairly quickly, no more rough feeling like metal is rubbing or grinding on the inside. I would suspect that something is wrong somewhere with the engine. Unfortunately I can try spinning the prop here through the internet. It maybe OK, but I don't know of course.
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Old Apr 16, 2014, 03:08 PM
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If you ran the engine until the tank was emptied, the engine might just be a bit dry inside now, compared to when it was fresh out from its box. So the pinch at TDC might then make some noise...

Remove the muffler and put a drop or two of after run oil on the piston, turn the engine over and check if that changes anything. Most likely it is just a bit too dry inside, but it is very hard to check that over the internet...

How much oil is there in Byron's fuel?
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Old Apr 16, 2014, 04:20 PM
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Remove your glow plug. With the piston at the bottom rock the crankshaft back and forth without approaching top dead center where the piston clearance will be tighter. The crankshaft should turn over pretty much silky smooth. If it doesn't, something is definitely wrong and further investigation is needed. Don't run the engine like that, you are going to ruin something and will probably void your warranty. Get it to someone that actually knows what they are doing or send it in for warranty repair, Shoo
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Old Apr 16, 2014, 06:55 PM
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After discussing this with more people that use glow power at my club field, they are all in agreement that it is normal for a glow engine to have that scraping/grinding feeling when it has not been completely broken-in. They also all were in agreement that I should change out the glow plug after the first 3-4 tanks of fuel because it will most likely get fouled-up by metal scrapings during the process.

I thank everyone for chiming in on the concern, and it is a bit disconcerting that I have such differing opinions from all long-time fuel flyers on this board and long time fuel flyers at my club field. I will just go with it being normal and fly it. It makes sense to me that a glow engine needs to be worn-in over time, which would mean that it rubs tightly metal-to-metal and that material will be removed during that process.

Thanks again!
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