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HomeRC & PowerAircraftHelicopterEngines Plugs Mufflers Fuel › How to tell if an engine needs rebuilding
10-11-2011 01:38 AM  7 years agoPost 1
loewermx

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Alexandria, La

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So, I have an os 91HZ that i cannot for the life of me remember what it came out of. That being said, whats the best way to tell if it needs to be rebuilt? I dont have it in a machine and i dont have a test stand. The piston and sleeve dont look bad at all. Any suggestions?

Cajun Aircraft

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10-11-2011 01:39 AM  7 years agoPost 2
airdodger

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Johnston USA

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Put a bearing in it and try it out.

Chris

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10-11-2011 01:47 AM  7 years agoPost 3
JPhillips

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Waco, TX

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If it's been sitting a while, it might be a good idea to put a bearing in it just to keep it from imploding and otherwise ruining your engine. Check out the internals of the motor, if they're rusty, clean it up and change the bearing. If your ring is nice and black and not silver and shiny, it's probably okay but if you go so far as to change a bearing, I'd change the ring too, just for peace of mind.

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10-11-2011 01:49 AM  7 years agoPost 4
helitom

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Pine Grove, Calif, USA

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Assuming it doesn't feel like bad bearing when turned over by hand, a big clue for me is the engine not holding a needle setting. When the mixture changes during a flight it's a good sign the piston-ring-liner are not working properly. Of course, this is after the normal checks - plumbiing, glo plug, carb dirt, etc.

Strap it in a heli and fly it....

The older I get, the fewer things seem worth waiting in line for.

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10-11-2011 02:15 AM  7 years agoPost 5
BladeStrikes

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Shelby TWP,Mi

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Take the head off and make sure the sleeve isn't scratched up bad and check the piston ring color..The ring should have a blackish color to it but if it's shiny,it need's replaced.While the motor is out,change the bearing for the hell of it.There cheap and you'll know there new.Here's some video's I made a while back if you need help ..

Watch at YouTube

Watch at YouTube

Watch at YouTube

Just my opinion but if it was me,I would slap a bew ring and bearing in the motor..That way you know for a fact everything is perfect and the motor will run strong..It's already out and the ring/bearing's are pretty cheap.

BTW,
If the sleeve isn't scratched bad,you can reuse it many time's as they don't go bad..After a few hundred flight,they start loosing power because the ring get's weak and doesn't press/seat on the sleeve good anymore..

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10-11-2011 04:37 AM  7 years agoPost 6
CX1

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Canada

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How to tell if an engine needs rebuilding
if there is rust on the bearings
if the ring is shiny (can be seen from exhaust port)
if it's been sitting for along time neglected in a corner somewhere
if you take it apart ang look inside the sleeve and see scoring mark running the length of the cylinder

then chances are: you need a rebuild

" Team Protoast "

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10-11-2011 03:28 PM  7 years agoPost 7
loewermx

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Alexandria, La

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The ring is still black but what has got me wondering is I can turn it over by just the crankshaft and the glow plug in it. Just seems low compression. I will probably just rebuild it for good measure. I would rather do that than have to pull an engine again.

Cajun Aircraft

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10-11-2011 03:53 PM  7 years agoPost 8
BladeStrikes

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Shelby TWP,Mi

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loewermx
The ring is still black but what has got me wondering is I can turn it over by just the crankshaft and the glow plug in it. Just seems low compression. I will probably just rebuild it for good measure. I would rather do that than have to pull an engine again.
If it's low on compressing,im guessing it sat for some time and the ring stuck to the piston or the ring is dry.If the ring is dry and doesn't have oil/fuel on it,the motor will feel like it's low on compression like your saying.Just something good to know later down the road but like you said,it's best to toss a new ring and bearing's in it.When you take the sleeve out,run some emery cloth or VERY fine sand paper on the inside of the sleeve.That will take the glaze off the sleeve from the old ring so the new one can seat good..

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10-11-2011 04:11 PM  7 years agoPost 9
CF1

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Olympia, WA

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I have found that bronze wool in 0000 works great for this. You can get it from gun smithing supply places such as Brownell's. It is used to make mirror blued finished on guns. It will pull all the varnishing off, shine up but is soft enough that it can't do any harm. I've put my crank in a drill and spun it while running this along the shaft to remove the crud before I slide the new rear bearing on. Comes out clean and shiny.

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10-11-2011 04:17 PM  7 years agoPost 10
BladeStrikes

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Shelby TWP,Mi

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CF1
I have found that bronze wool in 0000 works great for this. You can get it from gun smithing supply places such as Brownell's. It is used to make mirror blued finished on guns. It will pull all the varnishing off, shine up but is soft enough that it can't do any harm. I've put my crank in a drill and spun it while running this along the shaft to remove the crud before I slide the new rear bearing on. Comes out clean and shiny.
You don't wanna do that with the sleeve because you want some small/fine scratches in it,that's what help's the ring seat to the sleeve's wall.If it's smooth,the ring will never seat and the motor won't tune right.It's good to use a very fine sand paper because it take's the glaze off and put's tiny scratches in the sleeve.It's like useing a hone...

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10-11-2011 04:30 PM  7 years agoPost 11
CF1

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Olympia, WA

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I have fat fingers so it's too difficult to do in the sleeve for me. A little 2000 grit lightly run through for me in there.

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10-11-2011 05:15 PM  7 years agoPost 12
loewermx

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Alexandria, La

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This engine has been sitting for probably about a year to year and a half. A friend of mine just reminded me where this came from. Its out of a T700 and had less than a gallon through it before a bad crash left the machine at a total loss. I pulled all equipment out of it and just put it in a box for later use.

I know, how do you not remember something like that?

Cajun Aircraft

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10-11-2011 06:42 PM  7 years agoPost 13
Jeff polisena

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westpalmbeachflorida usa

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Pull apart and inspect , as long as you don't see rust or hear crunching you will probably be fine . Yes the proper way would be to put new bearings , ring and sleeve but depends how you look at it . I make sure parts move freely , clean good and install to see how it runs . If motor gets destroyed after this than parts replaced are just added to the part that was bad .

I would be more worried about carb

I stole it ,flew it and gave it back ;)

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10-11-2011 10:45 PM  7 years agoPost 14
airdodger

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Johnston USA

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If the engine has only a gallon of running time, just change the bearing and put it back together, You will do more harm than good tinkering. Compression can't be checked by turning over by hand, so that test is irrelevant. If the ring is stuck just soak it in some fuel when it's apart, it will loosen up, and don't take the ring off the piston. If it runs correct which it should, you are ahead, if not you had to buy the parts anyway.

Chris

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10-11-2011 11:40 PM  7 years agoPost 15
JasonJ

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North Idaho

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If you are going to do any surface work to the sleeve, do it right. Go to an auto parts store and buy a brake cylinder hone. Chuck it in a drill, use hone oil or ATF like I do. Run it slow and up and down to create a crosshatch pattern. Works brilliantly, I have done it many times on several nitro engines wit awesome results. Don't go overboard, you are just breaking the glaze and getting rid of any scratches that may have formed.

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10-12-2011 12:25 AM  7 years agoPost 16
airdodger

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Johnston USA

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I honed many engines, by hand and with a CK, it is very demanding. First you need to know the material of the cylinder, then the correct stone to leave the finish needed for the ring and the use it will be put to. Using the wrong stone grit or fluid or improper crosshatch the engine will never perform up to it's potential. The correct way to hone would be with a Sunnen rod hone, that way you can keep the pressure without bell mouthing or tapering the cylinder, assuming you know all of the above. The problem with break hones is you can't keep enough pressure and you need to move the hone fast, if you dwell at the top for even a little you can lose some of the precious seal by opening the dimension just where it is most important. A good cross hatch not only seats the ring but holds the oil for lubrication. Bear in mind I did this for a living for a while and I would not hone a liner , I would just buy a new one. This does not mean it won't work, but your chances of getting everything correct are astronomical. Just my opinion...

Chris

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10-12-2011 01:21 AM  7 years agoPost 17
BladeStrikes

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Shelby TWP,Mi

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loewermx,
Trust me,use some very fine sand paper.People have been doing this for year's,never had issue's and the motor's run perfect.Ive rebuilt well over "100" 50/90 size motor's for friend's along with my own and never had problem's.Around 300 flight's I do the same thing and toss a new ring in..

This is a rebuilt motor with well over 200 flight's on it and every video I have is with a rebuilt motor..It's a waist of money buying a new sleeve..

Watch at YouTube

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10-12-2011 02:04 AM  7 years agoPost 18
madman22

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Alexandria Louisiana

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You bring ot to me andet the guru see the goods.

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10-12-2011 02:14 AM  7 years agoPost 19
airdodger

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Johnston USA

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Yeah factories spend tens of thousands of dollars to do things right, they should just use sandpaper a torch and call it a day, won't make any difference.. Who Knew! ROFL

Chris

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10-12-2011 02:39 AM  7 years agoPost 20
BladeStrikes

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Shelby TWP,Mi

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airdodger
Yeah factories spend tens of thousands of dollars to do things right, they should just use sandpaper a torch and call it a day, won't make any difference.. ROFL
You said you have no experience in doing this but feel fit to give your unexperienced expert advice ...Your getting good at trashing other member's thread's.Maybe you should watch SmackTalk and learn something from real expert's ...

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