ETpilot

I purchased this engine new in 2014 when I got back into RC. It had been performing good until my last crash. On that crash I broke the carburetor mounting flange and so I had to get a new crankcase. Since that time the engine has performed poorly. I could never get it tuned up. It always seemed to be running rich and it would kick back and loosen the spinner nut. So I took it to the club and had a member look at it. He adjusted the fuel system and got it running somewhat better. But on a last flight it quit in the air and he had to make an off runway landing, no damage.

I removed the engine and put it on my engine stand. While running I noticed that fuel was seeping at the cylinder head. A check revealed a defect in the gasket. So I decided to order a new cylinder head assembly. Yesterday I got to install it and engine ran, no leaks, but with the same symptoms, rich and kicking the spinner nut loose. Throttle up the engine it would stumble and die. Also removing the glow igniter it would die. So I set the mixture control screw back to its original as new position.

I finally got a good run. Idle was good, full throttle was good. No signs of stumbling. Good power from idle to full throttle. I ran it going from idle to full a few times and no problem. So I decide to stop the engine and let it cool and do a restart. On the restart I got a powerful kick back. The spinner nut, prop nut, washer and prop went flying. I recovered all but the washer. I had another washer so I installed all the parts and figured I would close the mixture control screw about 25-30 degrees.

Once I had the parts installed I tried rotating the engine and it came to a solid stop in both directions. Not good. I opened the engine up and found the connecting rod had broken at the crank pin. Now I'm not sure how to proceed. Keep engine for parts or buy a connecting rod and try a repair. I'm a fix it guy so I'm leaning towards the repair. I'm thinking all of the kickbacks just weakened the connecting rod. Anyone have a similar problem. Thoughts,ideas, suggestions,etc. Prop used 12-8 fuel 10% nitro. Thanks.

aspeed

I think maybe the crank is bent from the crash, but that would not explain the kicking back. It could be harder to turn it over at certain points. For the price of OS parts, I would think hard about fixing it. Maybe use this one or another one for parts. My opinion, it is free. Then again, maybe the rod took the impact and broke, saving the crank from breaking. A rod is not too expensive. (a crank is) Another head spacer wouldn't hurt, at least until it is re-broken in.

ETpilot

Thanks for the reply. I just looked over all the parts. They all look good, no indication of a problem. I put a dial indicator on the threaded part of the crank. It did not indicate any problem. The crank rotates easily in the case. The flywheel part of the crank seems to rotate evenly in the case. Like you say, i need to give this more thought before repairing.

I just did a run-in on another new 55AX. That run-in went well. Not sure why this one is uncooperative, lol. Thanks.

aspeed

Check the head space. Maybe the new case is different, The .55s are usually pretty good. I have two.

blw

My Feedback: (3)

A dial indicator, even a lesser priced one is always good to have around. Sounds like you and your buddy did everything like you should. That's the thing with OS pricing- is it worth it or is it better to toss the engine in the parts box? However, the $22 price tag at Tower surprised me. I would be spending the money considering this is a $170 engine new.

ETpilot

Well, I have gone over this engine with a fine tooth comb. I cannot find any flaw. All parts look like new. No indication that any part is hitting somewhere. I measured with a caliper and have good clearance from piston to top of cylinder sleeve to cylinder head. The new cylinder head measures the same as the old. Not much else I can do. So I'm going ahead and order the connecting rod. This engine will run or it will self destruct. blw, thanks for the heads up on price. I had written the wrong price and thought it was more expensive. The part is not in stock but I will get an email from Tower when the part is available. I'll update this thread when I get to tinker with this engine again. Thanks all.

blw

My Feedback: (3)

There's all the hobby shops online too. Also, ordering from England is always a good experience. Prices tend to be slightly higher but postage is slightly lower. The only downside is you may have to wait 2 weeks for a package. I've ordered quite a few times from England. For my radio hobby a company was late shipping out an antenna due to manufacturing delays. They kept me updated, so no problems there. That company mailed the TV antenna sized antenna to me overnight from England. It arrived the next morning and they never charged me for the extra shipped costs. You can expect this level of service with model plane parts too from what I've experienced.

the pope

I think once youve had a significant crash then all bets are off . Cant blame the engine . Its like when some people reuse their receivers after they have been in big crashes . Would you buy one from the shop after the salesman just threw it as hard as he could against a brick wall ? I think not but I digress .I would get another engine or use the one u have just run in .

Propworn

My Feedback: (3)

When you install the rod look at the bottom end that fits on the crank pin. If one side is chamfered or relieved that side must face the counter weight. The chamfer provides clearance needed if there is any radius where the pin joins the counter weight. If its assembled backwards it can cause the rod to rub on the back plate.

Dennis

earlwb

Well it sounds like the compression is too high. I would make a head gasket shim or two and try that. You can find thin brass and aluminum sheet from the hobby K&N metal displays at the hobby shops. A piece of aluminum from the bottom of an aluminum pie pan works well but it might be a bit too thin so you would need more than one.

One thing about the OS 55 engines is they use a rather narrow or thin shelf/ledge for the cylinder to crankcase fit. With running and repeated disassembly and assembly of a engine, the cylinder could wind up going in slightly deeper into the crankcase, thus causing the compression to increase.

Depending on where the parts come from, you might have gotten parts for the European engines instead of the USA engines. I would not think that the crankcase would cause this but I never know anymore. They could easily have shaved a little off the top, so to speak. Usually they sell lower compression engines in the USA due to the availability of higher nitro content glow fuels. In Europe they don't use nitro in the glow fuel much and it is harder to get and more expensive. So they sell the engines with higher compression in Europe for using no nitro glow fuel. But I am not sure if OS does that or not with the 55 AX.

Oh yeah, you aren't using RC car fuel are you? RC car fuel typically has much less oil in it and also it has a lot more nitromethane in it. It is ill suited for RC model airplane engines. The low oil content can cause the rods to fail as does high nitro causing the engine to act like the compression is too high.

airraptor

My Feedback: (66)

When you have a a crash and the engine was running you have a good chance of a broken or cracked rod. On our race engines any time you have a crash the rod must be replaced or it will fail with in a few engine runs. Also the compression is just fine as I have run them with out the head shim on low and high nitro and high oil content no issues. With your kicking back you have to much fuel going into the engine. either your tank is to high flooding the engine or needles set to rich. The extra fuel in the engine on start and transition to full throttle increased the compression causing it to fire before normal. Doing this with a starter is very bad as it can break the rod too.

How far open is your carb to get it to idle? take some pictures once installed again.

Also parts may be high for some but you are paying for quality and well designed parts.
Earlwb you can see in the picture lack of oil wasnt the problem as it was force that broke the rod. Also high nitro with the wrong plug can do what you say but not the nitro alone. More oil content can will raise the compression not nitro so by running less oil lowers the compression. I have run these engines on car fuel with no issues at all, just have tune them right since there is less oil you cant tune by smoke.

ETpilot

Thanks for the replies and info. I am learning more about these engines. I am familiar with the chamfer on the connecting rod. I'm running 10% nitro with 18% oil content. This is within specs for this engine. I do have a shim I can use for the engine. Crashing a running engine brings up a good point. This engine has had 2 good running hits and one minor one. So it could be the rod was weakened and finally broke. The kickbacks I attribute to a rich mixture and was planning on making an adjustment. The idle run was done with about a 1/16th opening which appears to be about right. I have an idle adjust screw on my test stand so I always returned to the same idle.

Messing and giving thought to this engine I've changed my plan a bit. I'm going to order a new engine for my balsa model. I'm going to see if this engine will run with a new rod and fuel adjust. If it runs fine I'll relegate it to my SPAD airplane. The SPAD is more expendable than my balsa models. Plus they are easier to repair or just plain build another. Lol. I'll update this when I get that new rod.

aspeed

I am sure it will be fine with a new rod. SPADs are pretty cool. Almost as good as the reset button on a flight simulator.

blw

My Feedback: (3)

Did the rod itself break or did the crankpin break? The crankpin is usually at the weak link.

ETpilot

I like SPAD airplanes. They didn't exist back in the 70-80's. I found them on my return to RC. I'm getting ready to build an SPA 3DT. Hope it is a fun fly.

blw, the donut on the crank pin end of the connecting rod is what broke. Broke in 3 pieces. The bushing survived. The crank pin on crankshaft is ok.

ETpilot

Well good news. I checked the Tower website and the connecting rod is in stock. So I placed a small order. They had a special on free shipping plus I applied an e Cash certificate and a discount coupon. I did ok getting this connecting rod. It should be here sometime next week. Yea.

aspeed

How's the weather where you are? We hear stories here like 50 inches. You should be good with a new rod.

ETpilot

I'm 3 to 3 1/2 hours NNE of Houston. Several forecasts showed the system moving my way. So I prepared for it. But the system always stayed South of the county line. All I've gotten has been light rain and no wind. Going South it would go down hill fast until Houston which is flooded. This system appears to be moving East so it should not affect me at all unless it takes a more Northerly path. I've been thru a few of these hurricanes. I don't need any more experience.

I can't wait to to get that rod and put the engine back on the test stand. Will see how it does.

airraptor

My Feedback: (66)

crank pins on OS engines are fine and rod will break before the pin. magnum and ASP engine have pressed in pins and break or shear easily.

ETpilot

If you want to see what is going on in Houston, do a search. Houston Ch13 live feed It is live feed reporting. Absolutely horrible. This event will be on going for a long time.

blw

My Feedback: (3)

I think we'll get the very last of it when it is about spent.

ETpilot

Harvey made a nice curve just outside my area. Since it made landfall all I got was scattered light showers I dumped the rain gauge a couple of days ago at 3.25". Yesterday was my worse day. Constant rain day n night plus some wind. Rain gauge at 3". So a total 6.25" for the event. Should be sunny tomorrow. I haven't seen the sun since last week.

Good news today. I received the connecting rod. Fast delivery by Tower. I will get this engine ready to see how it will perform. The Good, the Bad or the Ugly to follow.

ETpilot

I got the engine repaired and mounted on the test stand. I need to put a balanced prop on it, hook up the fuel lines, fill it with fuel and see what she does. I closed the mixture control screw about 25 degrees and will start there. I probably can't get to run this engine until Saturday.

rcjetflyer0718-RCU

My Feedback: (1)

I just replaced a connecting rod on the 2nd OS 55 that a bearing came apart. It's expensive to replace the piston/liner on two 55s. You might want to replace the bearings of your 55 with some premium bearings from rcbearings.com or bocabearings.com

1QwkSport2.5r

OS engines are throwaways if you need to replace hard parts - like the piston and liner. Buy an ASP/Magnum and put your OS carb on it. Same or better power, better technology, and parts are respectably priced.