Hey all, I think I bent my crank shaft today out at the field. I thought it was just a unbalanced propeller, but when I purchased a new propeller and threw that one on, I started it up and it was vibrating like mad. Is there any easy way to tell if that is what I really did? Am I going to have to take it apart to find out.

It is a OS46LA

Thanks all

did you balance the new prop? even new ones can sometimes come REALLY unbalanced.

If the prop is properly balanced, take the engine to a machine shop. They can put a dial indicator on the crank end and check the TIR (Total Indicated Runout). If it's more than about .0005, then the crank is bent. They can probably straighten it. For what it will cost you, you can probably buy a new crank or engine.

Dr.1

The cranks usually bend in the prop area,

Enjoy,

Jim

brocja01, has the engine been involved in a crash, or has the prop hit a solid object while running at high revs ? If the answer is NO to both these questions , then it is highly unlikely that the shaft is bent , and you only have an shockingly out of balance prop. Get it balanced before starting the motor again.

Gentlemen;

The third possibility, is that the pilot envolved didn't use a spacer to properly seat /centre the prop on the shaft.

Keep in mind that this is the 'Beginer's Forum' and missing such basics is common place.

Most engines come with a series of 'Spacers' of different thicknesses, these slide onto the Prop shaft to ensure a snug fit with the prop. Propeller manufacturers have different diameter centres - Engine manufacturers have different diameter shafts and compensate with "SPACERS"

IF you do not use these spacers the prop is not centred on the shaft and you will have the biggest out of balance propeller condition imagineable.[:@]

Before you spend any money, look for the free spacers - I hope that you didn't throw them out saying "What the He!! are these?"

bbbair hit on one possibility. That one is dead simple to test or prove. Look at the prop on the shaft. Look to see if the hole is larger than the shaft. If you can see any size difference, you need to fix it. If you can actually feel more than a tiny movement, you certainly need to fix it.

It's also dead simple to check for a bent crank without taking it to a machine shop. Simply put something in front of the airplane and turn the prop. Look at the tip of the shaft and you'll see quite easily whether it's not straight. This might be easier if you take out the plug. It might be easier if you sit the models tail up on something so the airplane is level and the prop has clearance on whatever is sitting in front of it. It might be easier if you move that something close to the prop shaft. It is always easier if you simply think about what you want to do and adapt.

to be contd.

Since this IS a beginner's forum.....

Pulling from my 40 years of experience, and using yesterday's experience as the freshest........

Two guys yesterday asked me to help them figure out what was wrong with their engines. One guy has been flying ARFs for a couple of months. This guy is SHARP. He is absolutely dependably a safe and really decent, reliable flyer. I'd let him fly anything I own and would go off and help somebody and not even bother to watch the guy. He isn't going to do anything with my plane that he thought he couldn't do etc etc etc His engine was loose. One bolt had come loose. It really was the first time he'd ever encountered that trouble and when I showed him the engine-to-mount looseness, he said, "guess I need to check the mount (to-firewall) too, hunh". This guy is sharp but inexperienced and didn't realise those suckers would loosen tight bolts.

The other guy has been flying ARFs for 3 years. He is really good. He's got talent and "the touch". He is also sharp at a tack. He had a brand new ARF he'd just finished sticking together. Engine hadn't even been run yet. It ran pretty good considering. It was hard to start. All day. Toward the end of the day, after fighting an engine that was breaking in, the airplane developed a bad vibration. The mount was loose. And three of the engine bolts took a quarter turn. He was "pretty sure" he'd tightened everything good enough last night. He probably had.

That was just the last flying day's experience. It looks like it's going to be an excellent flying day today. Think I'll swing down to the basement after I finish this bowl of cereal and check the charge on everything and check all the motors for looseness. Won't be going until this afternoon when it's up to 40degrees, but checking that stuff while it's quiet here is so much easier. And it needs to be done while I've easily got time. And it's easier to pull a muffler if I have to while I'm in the shop and it's warm and I don't have the frequency pin and somebody's waiting and......

What happened yesturday with my engine was I landed the plane and the wind pushed it off of the landing strip. We have about a 6 to 9 inch enbankment on our new field and since it was all the way at the end of the runway, I thought I could just run the plane up the embankment and I should be good to go. Well, I gave it the nuts and it went up the embankment, taking a bunch of gravel with it. I then proceded to take the plane off again, once it was in the air I new instantly that there was something wrong with it. I brought it down and shut it off and noticed the propeller. So I didn't think anything of it, went to the LHS and picked up a couple propellers. Put one on it last night to run out the fuel and the vibration was still there. No I haven't taken anything apart, I have just taken the engine off of the model to take a better look at it. If I can't see anything wrong with the crank, I'll check the prop. I don't have a prop balancer yet so I'll have to figure something out.

Any other suggestions would be great. Before I took the engine off the plane I didn't check to make sure all the straps holding it on were still tight, that could most definatley been an issue after hitting the gravel like I did. So before I take the engine apart, I'll check all the things mentioned and see if it is simply a out of balance prop, engine not tight on mount, or bent crank.

Thanks all!

After reading what happened I would put my money on something else has happened. It's harder to bend a crankshaft than most people think. I once crashed a plane with .20 OS on it straight into the pavement at full speed, I was sure the crank was bent but after taking it to a machine shop I found out that it wasn't. Do you have an engine test stand so you can run the motor when it's outside of the plane? If so, fire it up on that and see what happens.

Hope this helps

Ken

If the crank is bent, the prop tips will not track the same when you turn it over or run it. I'd bet on it having been loose in the airplane.

That's what I think -- check the engine mount bolts for tightness & check the mount itself for cracks. Also check to ensure that the firewall is not partially seperated from the airframe.

I'm with Ken on this. Bending a crank is usually the result of a full-bore nose dive into pavement. Kicking up a little gravel won't do it.

I took it to the LHS this morning after I got the engine remounted. The guy at the LHS said the same things you guys have been saying, I'm betting that the engine came loose and that what was causing it to rattle so much. He also stated that I need to balance my prop, anyone have any plans for a prop balancer I can make at home. I know they are only $20 but if I can make one for $5 I would rather do that.

AustinCraft (?) used to have one that was a U-shaped piece of aluminum. You attached double edge razor blades on either side of the U with short bolts. There was a short steel rod which inserted in the prop hole. Sit the rod down on the exposed edges of the razorblades and see which side of the prop would drop. I had one at one time. No idea where it is now.[]

I had one of those Austincraft jobs. There is another more recent economy tool box model that is just a 1/4" piece of drill rod that is pointed on the ends. You hold it between your fingers. Whatever you do, don't waste your money on the Tower Magnetic balancer. The magnetism is not enough to hold the arbor except on the small props. I have a couple of the High Point Balancers. They work real nice. They will do the big props and weed whacker flywheels too.

Enjoy,

Jim

Gentlemen , i want to take this opportunity to wish you all a Merry Christmas , and many happy hours of flying.
RcKen, as you can see my computer is fixed , the problem was the grafics card.
Thankyou for all your help throughout the year, Merry Christmas
the_madgenius ,downunder

the_madgenius,
Glad to see you fixed and back on RCU.

Hope you and your's have a very Merry Christmas

Or as Minnflyer says,

Happy Chrismahannakwansaka

Ken

Why thank you, and a Merry Christmas to you too

that can also be caused by a prop hub not being reamed/drilled out straight also, which will give the exact same indication as a bent crank.

how do you take out a crank shaft out of an os .15 cause I barely bought it off this one guy and he broke it and then he grinded it down so it would look fine???

What model OS 15 do you have. Some of the cranks you cannot get anymore?

You have to take off the head & backplate, remove the cylinder sleeve to get the piston/rod out. Then you can get the crank out.

Will probably require heating in the over to 275 for 20 min for the sleeve to be loose enough to push up with a dowel