Jetdesign

My Feedback: (8)

I noticed on my new engine that there is a track where the con rod rides around inside the crank case. There are two spots, on the sides, where the rod actually rubs against the crank case, very slightly. I'm assuming this is normal and OK, but I figured better safe than sorry. So is this OK? (trying to upload pics, not really working).

Also, is it only the tension of the prop nut that holds the shaft forward? With the engine off the plane and no prop, the shaft can be pushed in far enough that the con rod rubs against the back plate. Is this also normal/ok? Edit Ok for this part I guess once the prop is on for the first time the shaft gets seated in the bearings and doesn't slide around.

Thank you.

JohnW

My Feedback: (6)

Which motor? On the sides of the case,not the back plate? Is there a burr on the con rod maybe? If you can turn the motor over by hand I'd think it is probably OK, but I can't say I've ever really seen one do that. Granted, by recent experience with glow is pretty much YS only.

Jetdesign

My Feedback: (8)

120AX. Less than 30 flights on the engine.

downunder

Normally the rod shouldn't touch the crankcase but it's possible in yours that manufacturing tolerances have stacked up enough to allow a slight touch. If it were my engine I'd check the rod to find the high point where it's touching and carefully file/sand that area to give some clearance, maybe enough for about a .002" feeler guage to slip through. Chances are that if you leave it as it is then you won't have any problem other than maybe a blown plug if a bit of alloy rubs off and gets on the plug coil.

As for the crankshaft, with no prop fitted there's nothing to stop the crankshaft being pushed back inside the ball races other than whatever fit is used. There's a steel shim washer fitted between the front bearing and the prop driver and that shim sets up any axial play inside the bearings as well as putting prop nut tension only onto the front bearing's inner race. This movement doesn't happen in engines that use a split collet arrangement because even without a prop the collet still grips the crankshaft tight enough that hand pressure won't shift it.

da Rock

Sometimes the rubbing you see on the inside of the backplate happened when you jammed your electric starter against the spinner and hit the switch.

Don't worry about it.

Depending on the windage of the drive washer, some engines will give the drive shaft enough slack to get the crank pin back to the backplate. Some engines have drive washer dimensions to keep that crank pin off the backplate. Some engines have thrust washers behind the drive washer that're there for that purpose. If you have lost that trust washer, that is if your engine uses one, there could be a problem. But as you've seen, it isn't a big one.

Does the engine seem harder to turn over with your electric starter? Does it seem to bind when you slowly turn it over by hand? We don't usually push in too hard when turning by hand.

When running with a prop on the shaft, that shaft is not going to be pushing against the backplate.

If you're not seeing any problems, check back after awhile and see if the marking has increased.

Jetdesign

My Feedback: (8)

It's on the sides of the crank case, not on the back plate.

ChuckW

If there was serious metal to metal contact there, I imagine you would have heard it or seen a failure. Is it just a discoloration or is the metal of the crank case actually gouged or raised? It could just be a discoloration from oil slinging of the crank pin, heat or something like that. It's also possible that you had a small piece of debris in it at one time that is long gone.

Have you measured the clearance between the rod and crank case?

The magic question here is how does it run? If it were seriously rubbing, I doubt it would run well for long at the RPM's it sees.

Why did you pull it apart in the first place? Did there seem to be a problem with it? I've never had the back plate off of an engine unless the bearings were shot or it had some othr mechanical failure making it necessary.

I say that if it aint broke, don't fix it.

Jetdesign

My Feedback: (8)

I took the engine apart because the rear bearing was making noise (and in hearing from other big OS 2 stroke users the rear bearings go fairly quickly, so it's been replaced with a Boca bearing). When I went to reassemble the engine I was reinstalling the pison/pin assembly and noticed some wear marks in the track where the rod rides, and sure enough when reassembled the rod just touches in that spot. With the back plate and head off the engine, I can feel the rubbing when turning the shaft. With everything reinstalled the rubbing is less noticeable (due to pressure, etc).
The engine runs great, so I'm thinking that with proper lubrication it will just wear itself in, though now I'm thinking that this might cause extra vibration?

Just wanted to hear a few opinions before I fired the engine up again. To sand the rod, the best thing I have is fine grit sand paper. Sounds to me like it might be more trouble than it's worth?

alfredbmor

Personally I think that it is an extraordinary situation and I would fix it for once by sanding with a very thin black sanding paper. Chances are that this rubbish could generate a moderate vibration that can cause that the back bearing wear sooner. I have the same engine and I will check the same situation, mine has less than 10 flights but I have not seen anything wrong, either way is a good time to check it up.

Mr Cox

Is this the same engine as in the other OH my AX!!! thread [link=http://www.rcuniverse.com/forum/m_7889585/tm.htm]here[/link]?

Might help to keep everything in one thread to get the "full picture" of things

Jetdesign

My Feedback: (8)

Sounds good to me, I'll sand it down.

da Rock

lol..........

I was looking at a similar thread some hours before jumping into this one. They had an excellent picture of the backplate, btw. When I jumped, I thought I was in that one. Should have noticed the picture was "gone."

Not the first time I've sounded off on the wrong topic.....

Gray Beard

My Feedback: (-1)

Did you mark the rod before you tore the engine down??? Sometimes if the rod and piston don't go back in the same way it came out it could rub on the case A little. I have seen A lot of rough machining inside A case that looked like there was A problem but there really wasn't but you said you can both see and feel it hitting, that leads me to the rod and piston going back in the wrong way. 30 flights should not bother A bearing at all. However, stock OS bearings do feel loose and rough right from the get go, it even says that in there rebuild book I have. Off topic A little. does the new Boca Bearing you got look exactly like the one you pulled from the engine?. I used Boca on my last rebuild and they were different and I had to pull the sheild off the front and the rear had no seals, Boca said these were correct but OS said no way?? I didn't worry about the rear bearing but the way the front bearing was sealed made the engine run real hot very fast until I opened up the engine again and removed the seal. Just wondering if your bearing looked just like the stock one you pulled?

flyX

It's normal. I've seen slight marks or groves on some of my engine from the bottom of the rod rubbing against the casing ....OS and other brands.
That's why you break it in slowly. That's one of the reason you won't get maximum power until after a gallon or two.
There's drag coming from somewhere.

I threw a rod only once on a nitro engine..it was on a car engine. I lean it too fast. Only a couple tanks after break in..but the weather was cooler , so
I leaned it too much.

Bascailly the bottom of the rod stretched. The hole where the crank slides into got enlarged.

Jetdesign

My Feedback: (8)

I did mark the rod before disassembling the engine. Since there were marks on the crank case noticeable when the engine was apart, this would indicate the rubbing was occurring before I took the engine apart.

I've got a shielded stainless bearing from Boca. Different from what now comes in the OS engines, but the same as they use on some other, more expensive engines (this recommendation came from some very experienced pattern pilots, one of which is an OS rep, to use the shielded bearing). The engine came with a shielded on the front and an open in the back. Apparently there is a short life span of OS rear bearings.

I called Hobby Services and they said to send it in, that there must have been a machining error and that the rod should not touch the crank case. I think this is what I'll do - it's under warranty and I'd rather not remove material from the con rod, and plus I just got a NEW VENUS 40 to play with while the VII is down for service

ChuckW

I have never heard of or seen such a problem. Nothing mechanical lasts forever but a good bearing should hold up for a long time so long as it is lubricated properly and not allowed to corrode. I suppose a bearing that is not up to the application could cause problems as well but, like I said, I have never heard of this problem.

Could it be that the noise you heard was the rod rubbing the crank case?

How is the crank pin to rod fit? Is it sloppy? Does the crank pin look bent?

Could a bad bearing have allowed the crank to wobble and caused the contact?

What is the lubricant content and type in your fuel?

Did you over speed the engine at any time?

This sounds like the thing to do. They are absolutely right: no engine should have the rod contact any part of the crank case. I would be sure to send them your original bearing too.

I wouldn't sand or grind anything if you can avoid it. Hypothetically though, I would add clearance to the crank case if I had to rather than removing material from the rod. The rod needs to be as strong as possible.

I ordered a set from them too for a .46-AX. The rear bearing they sent was an exact match but their front bearing had a shield on both sides where the OS only had a shield on the front. I removed the shield from one side of the Boca bearing because I had concerns about oiling. Truth is, I could have re-used the original bearings but I had it apart so I figured I would replace them. The Boca bearings are great but I have found that I can get the from a local bearing supplier for the same or less money and no shipping costs.

Gray Beard

My Feedback: (-1)

An engine part hitting another part of the engine is not why you break in A new engine, there should never be anything like A rod hitting the case!! Chuck, my bearings from Boca were for A 1.20 OS FS and Boca sent me an opened rear bearing and an inclosed front bearing. Then I discovered I could get my new OS bearings from my LHS. If I would have known they had them at the time I would have just gotten the OS bearings, the first set only lasted about 10 years of very hard use. If I ever build another engine for racing I would still like to try there high end ceramics but I don't think that will ever happen here.

ChuckW

Yep, I've been inside countless engines. All the way from RC stuff to lawn & garden to big blocks to twin cam, high performance engines. A rod hitting the block would be very, very bad. To be really technical there should be no metal to metal contact of any kind while running. Everything should ride on a lubricant film.

Jetdesign

My Feedback: (8)

Well thanks guys - I have to admit I felt silly sending back a new engine with such a slight amount of contact, but I agree, the rod simply should not be rubbing on the crank case.

The rod appears to be in great shape; not bent, nice snug fit with the shaft. The noise is definitely from the old bearing, which has quite a big of play in it. The noise stops with the new bearing, though contact remains. Using 10% nitro fuel with 20% lubricant, 18% synthetic/2% castor. Top RPM achieved was 9000, though only for a flight or two then switched props and got 8000.

As far as bearings go, Boca was going to send me the high speed steel bearing, open at both ends, which is what's in the engine originally. Some top pattern contenders told me they all go bad after a hundred flights or so, and that most guys running OS 120-160 engines in pattern go with the stainless Boca bearings, and get 300+ flights out of them. My front (stock) bearing is covered - don't know the difference between sealed and shielded - it has red plastic or rubber covers over the balls. The original rear bearing was completely open, the new one has blue rubber or plastic covering, again as recommended by talented and experienced pattern pilots and an OS rep.

If I get a new engine or new bearing, I'll keep the bearing in it until it goes bad - we'll see what happens. After 30 flights the bearing is discolored, making noise, and wobbly, and I use after run and take care of my engines.

ChuckW

A sealed bearing is packed with lubricant (usually grease of some sort) and has the plastic or hard rubber insert between the inner and outer race to seal it. The seal serves two purposes: keeping debris out and the lubricant in. Some reference material that I have read says that sealed bearings aren't capable os as many RPM's as open bearings. I can see th theory behind this due to the friction of the seal, viscosity of the grease and tendency for the seal to become hot and fail.

Shielded bearings have a metal piece rather than the plastic seal that is intended to keep contamination out. I've found that these are generally pre-lubed also but the lack of a good seal means that light lubricants can flow into or out of them. Most RC engines have a metal shield on the outside of the front bearing to minimize air leakage and to help keep debris out of the bearing.

Personally, I think the rear bearing on an RC engine should be an open type. That way it gets the benefit of the lubricant in the fuel. Typically you only see open bearings in things like engines and gearboxes where they receive good lubrication and are protected from debris and moisture. Yes, a sealed bearing might work but I would think that its life would actually be shorter. Maybe I'm wrong, who knows.

As long as you use good fuel with plenty of quality oil and don't allow the bearings to corrode, they should last a long, long time. Of course that also means no excessive RPM, no excessively lean mixture and no overheating.

This site has some good information: http://www.reliance.com/prodserv/motgen/b5021.htm

Jetdesign

My Feedback: (8)

Thanks Chuck.

ChuckW

You know... for some reason I keep thinking about this problem. I'd venture to say that your rod rubbing on the crank case is a fairly recent occurrence. I really doubt it could have been doing it from day one. Here is why:

1- It had to make noise or give some other sort of indication.
2- Metal pulled off would have migrated to other areas and caused damage. Maybe this took out the bearing?
3- In order to make a mark, you have direct metal to metal contact. This means no room for an oil film and would have generated intense heat. This in turn would have cause the rod and crank case to eventually seize to one another, especially after 30 flights.

Regardless, Hobbico has some pretty good customer service so they'll hopefully take care of it.

Do you have pictures of the old bearing? Do you see pitting or flattening on the balls or races? any evidence of it locking up and the crank spinning in the inner race? When bearings are dry they will feel a little sloppy and noisy. They need an oil film to work properly. They can also be stained in a glow engine. I've seen people condemn perfectly good bearing because of this. Since your new sealed bearing is packed in grease, it won't fee sloppy or noisy.

Jetdesign

My Feedback: (8)

Now that you mention it:
I had really really dark grey stuff in my exhaust. My instructor was saying that it's just the engine breaking in, but some times it was really bad. The LHS said I was running too lean, but I know better then that. At the pattern meet I was told that running lean wouldn't do it - dark grey means aluminum; the ring and sleeve are NOT aluminum. I did find that the muffler baffle had been vibrating, and there was some slight wear on the edge of the baffle. Removing the baffle took care of MOST of the grey, but not all of it. The wear on the case is very slight, and extremely difficult to see, if any really, on the rod. I've been running the engine pretty rich since day one, so maybe it's not as bad as it could have been.
Maybe the extra heat and vibration from the rubbing caused the bearing to start to degrade.

flyX

obviousely , you don't want the rod to rub against the casing....Just own more engines and you won't tripp out so hard.
It use to bug the heck out of me too.
Not 50% of my enignes has bad bearings out of the box...lol

I've had never thrown a rod on any of my aircraft models yet, half of them has wear marks from the rod rubbing
against the casing.

Hello ?....did some one see or felt aluminum grits in there unburnt fuel ?.

Well in a automobile or a lawn mower...it would totally sucked if the oil slashing thingies hits the oil pan.lmao

jib

If you choose to sand the bottom of the conrod, take as little off as possible and make sure the surface finish is free of marks, ie, no scratches from the sanding. Scratches are a point source for cracks to start.

Also, I'm wondering if it is possible that the aluminum case will expand enough when it is operating (warm) so that the interference goes away. Aluminum, especially with a larger diameter, like the case, will expand a surprising amount when heated.

Jack