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crashed airplane - engine question

Old 08-21-2008, 11:21 AM
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hsink
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Default crashed airplane - engine question

hi,
i crashed my trainer airplane and the mds .40 size engine seems to stick at the top dead center when you rotate the engine
by hand. it will turn over and i have had it all torn down and cleaned but not sure if this will be ok.
what are your thoughts ??

thanks

Old 08-21-2008, 11:26 AM
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Jetdesign
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Default RE: crashed airplane - engine question

It's normal; my engines do it too. Don't know what the reason is though. If I had to guess it has something to do with the position of the arm and the related pressure changes in the cylinder. Pretty much it means you have good compression; it's a good thing.
Old 08-21-2008, 11:33 AM
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Default RE: crashed airplane - engine question

Is it a new engine? On the ABC type engines the cylinder wall tapers ever so slightly, getting tighter at the top, so you get the most compression around TDC. On a new engine, it can be so tight that it seems to "stick" at the top. As the engine breaks in, the stickiness will go away.
Old 08-21-2008, 12:35 PM
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Default RE: crashed airplane - engine question

I had the same thing happen, but mine has a little knock or catch at the top of the stroke. If you turn it real slow when it get to the last 1/4" of travel you feel a slight catch then it releases. Someone said at my club it feels like the connection rod has some backlash (play in it). I called ST and they were no help, just said send it back for repairs. I'm not sure if this is something that will cause more damage if I run it??? I'm thinking the worst cause would be that the engine might get hot from the extra friction and not run right, or once the ABC lining heats up and expands, maybe it will go away?? Any ideas guys?

DG
Old 08-21-2008, 01:54 PM
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Default RE: crashed airplane - engine question

Normal for an ABC engine. The cylinder is narrower at the top to hold compression. It may even get tighter with some oil in the cylinder. As the engine heats up the fit between the piston and cylinder will change. It is normal for the ABC engines.

I read about one guy who was a car mechanic for 30 plus years or so and just knew something was wrong. He honed the cylinder until the brass was showing and could not understand why the engine would not run. Some times we think it is wrong to find it is right. You can check under the engine forum and read about ABC engines and why they are that way.



Dru.
Old 08-21-2008, 09:33 PM
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Default RE: crashed airplane - engine question

ORIGINAL: gregoryda
...mine has a little knock or catch at the top of the stroke. If you turn it real slow when it get to the last 1/4" of travel you feel a slight catch then it releases.
This is quite normal with some ABC engines. As others have said, the liner has a taper when cold and gives an interference fit to the piston when it's very close to top dead centre. Turn the engine over slowly and the piston sticks in the liner. This puts quite a bit of load on the conrod and crankshaft so when you turn the crankshaft that little bit extra all the loads suddenly release and you hear that clicking noise. Most of that click comes from when the rod clearances suddenly move to a different part of the rod, mainly at the wristpin in the piston. It's a very bad idea to turn over a brand new ABC engine slowly before it's had some fuel with proper oil through it. In fact it's best to never do this even after it's run in just to be on the safe side.
Old 08-22-2008, 11:01 AM
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Default RE: crashed airplane - engine question

thanks guys for all of your thoughts. have not tried to fire the engine up yet . but i think i will.
will let you know what happens.
thanks
Old 08-22-2008, 11:14 AM
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Default RE: crashed airplane - engine question

My first thought was to tell you to pull the head and inspect for foreign objects, but you've already torn it down and cleaned it. Sooo, in my VERY limited experience Id say the same thing already said. Its probably just fine to run so give her a flip and see if she fires
Old 08-22-2008, 12:01 PM
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Default RE: crashed airplane - engine question

after a crash...I don't rotate Nothing.

You know..dirt gets in the carb.

it gets jam between the crank shaft and housing.
You know...the front part of the crank shaft..with the opening..between the inboard/outboard bearings.

Flushing the head is not going to do doo..doo. You need to slide out the crank shaft to makesure there's not dirt anywhere and everywhere.
If you rotate the engine..it'll score the sleeving.

Cross your finger that the shaft is not slightly bent.

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