OS 75AX- Do I need to change the piston liner (or anything else)

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OS 75AX- Do I need to change the piston liner (or anyth... - 4/24/2012 8:28 PM

ameyam

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This post will have a large number of pics and a lot of description on my previous problems with the engine for obvious reasons.

The engine in question is a OS 75AX which I bought three years back. I was in my second year of aeromodelling then and not much adept with tuning glow engines. I used to run plain castor, no nitro then. This was my second engine and 2 1/2 airplane. It was UCD60 clone on which we installed the 75ax inverted vertical. We didnt actually run-in this engine in properly, the club trainer ran the engine rich on the ground for a few minutes and then took off with its still very rich. The engine flew ok for a couple of months and then began deadsticking. It was a frustrating period- no matter who tuned it, what we did, how much we cleaned the carb, kept or removed the baffle, how rich it was tuned, it would run for a few minutes, then deadstick. I never figured out whether it was a tank height (that model did have a tank height issue, it got sorted out later when I put a FS91 on it and lowered the tank) or an engine problem or some other problem for that matter. At the end I was starting to lose it and we decided to move on to other engines and models.

Then last year, after I lost confidence with a 55ax in similar fashion, I ported the unused well-oiled 75ax to the Reactor 46. It continued to deadstick. Now I was running Klotz with 5-10% nitro in the peak on the Indian summer (40Deg C outside) and the engine would overheat- you could feel the engine radiate heat when you sat in front of it. As it got hot it lost compression. It would also lose power and deadstick in a hover. Subsequently, I put the FS91 on that airplane and it resolved most of my issues with almost the same setup.

So this year I was considering replacing the piston-liner on the 75ax now so that I could get it running again at low cost for some of my 'cheaper' models. Someone suggested to clean the engine and then run once to check before I changed. Having got the screws heads spoilt by over tightening during the deadstick times, I had them opened some months ago and washed the engine with alcohol then, so its a lot cleaner in the pics. I just took apart the engine to check today. I have never completely taken apart an engine before, the parts came apart...well... 'by themselves'. I have also cleaned the engine with a rag, so take that into consideration. These images were taken on my phone, so will look much darker than is actually. There is one image that is much brighter than the others, thats what it actually looks like

Its really surprising- I expected the piston and liner to be really cuffed but as you can see, they quite clean (the 55ax I had similar problems with is much worse). If you look very closely, you can see minor lines on the piston side but those are so small, they look like machining marks, I dont think you can see them in the pics

The inside of the crank case is also very clean

The crank has some wear marks but I havent rubbed it with a soft cloth as well as the piston and liner. I didnt expect to be able to remove the piston and liner assembly, they came apart purely by chance, so then I gently tapped out the crank. I ran the rag over it and then took the pic

The bearings are definately rusted but they do turn over without any problems or roughness. The rag did dark reddish brown though when I tried to clean them. If someone can tell me how to get them out of the crankcase, I will try to get them out as well. Light coaxing with a mallot didnt help.

The carb has one of the retaining screws on the sides stripped. I found metal shavings from that screw in the carb body but not in the engine. Will clean out or replace before I restart the engine. Its happened recently, not earlier when I had engine problems and I havent run the engine since

So my question is, based on these pics, do you guys think I need to replace the piston-liner? Where else could be the problem?

Ameyam

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/24/2012 8:29 PM

ameyam

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/24/2012 9:11 PM

earlwb

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The engine in the pics looks OK. The bearings may have some castor oil varnish on therm than lets them appear to be rusty when they are not. It isn't to say that they could have some rust on them. But likely not, if they are feeling pretty smooth. If you were running no nitro or FAI fuel with castor oil it tends to cause the ferrous parts to get a castor oil varnish layer on them from the engine heat.

Well in the summer for us here in Texas when it is over 40 degrees C (104 degrees F) the engines tend to develop a heat sag and lose power. It is even worse if the humidity is high too. So we have to run the engines extra rich to compensate, some engines wind up four cycling a lot because they are so rich in this case too. You can usually fly around Ok but with the noticeable power loss you may not be able to do some maneuvers such as hovering, etc. It depends on how much surplus power you have with the engine though. So you may simply be in that bad time of the year when the engines do not like to run good.

Otherwise if the temperatures are lower, try adjusting the engine with the fuel tank about 1/4 full. Then fill up the fuel tank and try flying the plane like that then. It is possible you may still be a little too lean.

The other thing to get you, is the fuel line clunk inside the fuel tank may be getting stuck in the front of the fuel tank on the inside. It then winds up exposed to air when the engine is in various nose high attitudes. I have had the clunk react slow to flight attitudes, so maybe a heavier clunk is needed to help gravity do its thing to keep the clunk inside the fuel and not get exposed to air.

Finally if the engine has a remote needle valve on it and it is mounted to the rear of the engine. It may be getting enough heat from the engine to cause the glow fuel to vaporize inside the fuel line from the needle valve to the engine and generates air bubbles, which causes the engine to lean out and stall on you too. This happens more when it is hot outside than it does in the cooler months. The fuel line in between the needle valve and the carb has lower pressure inside of it, as the carb is sucking fuel though it, thus it is easy for it to let the fuel vaporize inside of the fuel line causing the engine to go lean.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/24/2012 9:16 PM

pe reivers

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There seems nothing wrong with that engine. Excellent condition. rusted bearing? put in a couple of new ones and start treating your engine better.
The dead sticking may be something in the muffler. Clogged pressure nipple or kinked pressure line maybe? Tank plumbing OK? Inside clunk strainer clean?
Also look into the fuel tract inside the carb. Perfectly clean? Smooth bores?

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 2:28 AM

ameyam

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Yes, I checked (or I had checked all of those)

Couple of years back- we used to tune with the engine pointed nose up and still rich in that condition. We opened and cleaned the carb several times. We even used a pump in all the holes to dislodge anything that could be stuck. However, I dont recollect whether I had compression problems then

Last year, when I had those compression, this was the second engine I had the problems on. Could be fuel related but my FS91 flew the same fuel fine. It had nearly 10% nitro in though. I had replummed the tank before I had these problems. The stuff was fairly new.

Thing is, on the last deadstick with the 75ax, I thoroughly shattered the Reactor's already weak gear block and did major repairs on it subsequently before I put in the FS91. I also replaced the stock generic tank with a Sullivan flexitank and the standard clunk with a OS bubbleless clunk. Dubro tubing was used instead of the local make I was using before. All of this was in a bit of desperation- it was the last chance for the setup and for glow (I had already started assembling a gas engine airplane then out of frustration with gl;ow engines). I dont use a filter in the tank (there is a superfilter and inline filter in the fueller) and others were using the same local tubing as me, so these shouldnt be issues. I have, however, stopped using anything but the bubbleless clunks / sintered brass Dubro clunks subsequently and had almost no problems.

Those loss of compression problems I had last year were not normal- from a standing start and running the engine at WOT for around 30 seconds or less while I tuned the model, it would suddenly cut. There was no rhyme or reason for it- going up-down in throttle, at and stick location whatever. Then, you could feel the radiative heat when you tried to turn it over and it would simply turn over with a finger without any effort, as if there was no plug installed. Then compression would return as the engine cooled. Even the most experienced club members gave up. I had the same problem with the 55ax and then I felt something similar with the FS91 as well. The 55ax was massively cuffed piston liner, so I didnt push my luck but the 91 kept running and it improved when I dropped the nitro% (by adding some synthetic fuel, the 91 doesnt tune perceptibly without some nitro) to 4-5%

Ameyam

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 5:42 AM

Turk1

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No problem at all with compression.Forget it.No problem with piston/liner.I think Earl hit the nail,remote needle system.Or stripped carb retainer screw(leaks air).Nothing else wrong with engine.TRy to use different glow plug(brand and heat range).What brand and # was the glow plug on it?

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 6:42 AM

ameyam

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OS #8

Ameya

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 6:58 AM

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most of the AX engines DO NOT LIKE TO BE RUN HOT 3d/ hovering really puts them in a high heat situation. low air flow over the engine around engine. best ones are the profiles where they get the most air to them. Cowled in ones need a good baffler set up to work well. keep the rpm up and rich.

if you do change the bearings the front one hardly ever need changed at all. its always the rear ones specially in the AX line up. order a Boca SS bearing. they will last twoce as long as the OS ones. change the ring also if you like never hurts since its apart now. replace any screws if loose on the head.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 8:29 AM

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quote:

ORIGINAL: airraptor

most of the AX engines DO NOT LIKE TO BE RUN HOT 3d/ hovering really puts them in a high heat situation. low air flow over the engine around engine. best ones are the profiles where they get the most air to them. Cowled in ones need a good baffler set up to work well. keep the rpm up and rich.

if you do change the bearings the front one hardly ever need changed at all. its always the rear ones specially in the AX line up. order a Boca SS bearing. they will last twoce as long as the OS ones. chlange the ring also if you like never hurts since its apart now. replace any screws if loose on the head.

While the engine is apart, rethread the carb retention screw hole, if that is what is stripped. Leaving that screw hole open to the atmosphere can cause the problems that you are describing. Try running a prop that presents less load to the engine if the problem persists after reassembly.

Ed Cregger

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 9:29 AM

ameyam

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Had overheating without the cowl, sorry I didnt mention that.

Well, I reassembled the engine after I put the WD40 into the bearings (couldnt get them out but they seem to be ok, this engine really hasnt been run that much). I dont have access to better screws for the head and backplate right now so I will final tighten later.

I need to replace that carb. Its got metal shavings inside from the stripped screw. Better than to be sorry afterwards. Its $70 for the carb! Thats 1/3 the price of the original engine. But I will try to get it because if the engine becomes operational, it will open up a lot of avenues with my smaller models (Now, the FS91SII is the smallest engine I have. So much so that I was trying to sell of some of my smaller NIBs). Lets see how it works out

Ameyam

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 11:51 AM

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I had another thought and that is the front bearing may be allowing air to get into the engine through a front anti-oiling system that could be there.
OS and some other brands too, had tried different methods of keeping the oil from oozing out of the front of the engine when it is running.
The front bearing seals don't have to go completely bad for this to be a problem, it only has to be enough of a air leak to cause the engine to lean out in the air some. Sometimes the air leak happens as the engine heats up good and then the front bearing heats up and the weak seals start letting air in.

Now I don't know what method OS uses on the AX engines or if they changed their method a few times during production either. But it might be worth checking out. I have used J-B Weld epoxy to carefully fill part of the anti-oiling groove on a couple of engines in the past and I have plugged the hole in a couple of engines in the past too. I do not mind a little oil oozing out of the front of a engine as most of my engines are quite messy when in use.

But in your AX engine pics above, the front bearing appears to allow for a large air gap all the way around the crankshaft. So if the crankshaft isn't a close fit inside the crankcase, then the front bearing, if the seals are going bad, would allow it to suck air in past the bearing causing the engine to lean out on you. This could happen as the engine heats up and the front bearing gets hot.

Some engine manufacturers use a shallow, spiral, anti-oiling groove cut into the crankshaft. As the crankshaft turns the spiral allows it to more or less, pull excess oil back from the front of the engine. But the groove only extends part way forward so that there isn't a chance of a air leak developing. But machining in a shallow spiral groove into the crankshaft is a expensive machining process and some companies may have opted for a cheaper method.

I don'tthink anyone uses these methods anymore, but I don't know, they could still be using them or people have older engines with these methods in them.

This method they used on some engines has a groove cut into the front of the engine so that when the engine is on its intake stroke, the oil is sucked out of the front bearing too. But if the seals on the bearing have gotten weak then it starts to suck air as well causing the engine to lean out.
These pics are courtesy of Konrad who made them a while back.

Another harder to see method is a small hole drilled through from the front to the intake hole under the carb in the front of the engine. This has the same effect but you can't see it very well. But if the front bearing seals have gotten weak or gone bad it can start to suck air too.

I thought I had a better picture of the apiral anti-oiling groove that you see on some engines. But the only pic I can locate at the moment is one for a Fox Quickee .40 engine. They cut the groove a little more deeply on this one.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 3:14 PM

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That sand blasted area on the top of your piston is caused by detination. Use a cold plug such as a Merlin blue seal which should also help with over heating. Another fix is to put a shim under the head to lower the compression. Use 5-10% nitro in your fuel which will allow you to keep your power in richer conditions. If you have (need) a tach set the needle 500 RPM lower than max then pinch the fuel line quickly and the RPM's should go to max and return. Atleast use the cold plug. Your engine appears to be in very good condition.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 3:21 PM

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Bolt her back up, throw in a OS Type "F" 4-stroke plug, tune it and then see how many deadsticks you have...

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 3:38 PM

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I concur with earlwb, the varnish is on the cage and outer race, exactly where you would expect varnish to be. Rust is usually not that uniform, unless rusted so bad everything is rusted. Then the bearings would hardly move.

I think your problem might be from switching from 0% to higher nitro fuel. The higher nitro fuels need to be set richer than the low nitro fuels. Althought they will run cooler when set to the proper amount, they will actually run hotter when run too lean. Setting it up with a 1/4 full tank is an excellent way of making sure it is rich enough to run a full tank.

About coming apart easily. Screws do loosen up with time. They need to be tight but not too tight. Nomal or slightly higher than normal pressure on the long side of an allen wrench is enough. Suggest that you use Tite Lock when assembling the engine.

BTW did you or someone remove a shim between the head and cylinder? That would be done to help run more efficiently and powerfully on no nitro or FAI fuel. If so the shim needs to be added for high nitro fuel as this could cause the very problems you are having, if not run very very rich.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 4:21 PM

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quote:

Those loss of compression problems I had last year were not normal- from a standing start and running the engine at WOT for around 30 seconds or less while I tuned the model, it would suddenly cut. There was no rhyme or reason for it- going up-down in throttle, at and stick location whatever. Then, you could feel the radiative heat when you tried to turn it over and it would simply turn over with a finger without any effort, as if there was no plug installed. Then compression would return as the engine cooled. Even the most experienced club members gave up. I had the same problem with the 55ax and then I felt something similar with the FS91 as well. The 55ax was massively cuffed piston liner, so I didnt push my luck but the 91 kept running and it improved when I dropped the nitro% (by adding some synthetic fuel, the 91 doesnt tune perceptibly without some nitro) to 4-5%

This has me thinking. Somehow the engine runs way too hot. This can have several reasons already mentioned here.
Lean mixture (vapour lock, clogged spraybar, restrictive fuel lines etc)
Lack of cooling.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 4:57 PM

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Ameyam, I have a 3 year old OS 55AX that ran perfectly until one day last year the engine suddenly stalled in the air. I new nothing had changed with that engine and I was running it often enough not to get gunk buildup. I discovered that the high speed needle has a small o-ring and mine tore up. I replaced a new o-ring (OS sells them) and I never had a problem again. Check the o-ring on the hsn if you haven't already.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 5:18 PM

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To clean out the needle assy, I carefully run a drill that fits the needle bore down to the needle seat. This lifts out all debris that has accumulated around the needle tip. An airgun is not of much use here.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 7:17 PM

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I used WD40-it came with a small plastic capillary that I could plug direct into the needle holes and force the WD40 through them. There was no clog, I subsequently took the carb apart, washed it with plain water under tap pressure, dried, reoiled it and reassembled. The new carb is $75 and even after spending that, there is no sure way to know the engine will work. One of the carb installation screws is still ok and the other can be held and a seal obtained with silicone sealent, I will bench test the engine that way. Awaiting replacement head and backplate screws.

Ameyam

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/25/2012 9:20 PM

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Another thought is that Ameyam is in Mumbai India. I assume he has a more or less tropical climate there with warmer temperatures and high humidity levels since it is near the ocean. So the engine could be experiencing some heat sag. Here in Texas when the temperatures are high in the summer, I lose maybe as much as 20% to 40% of the engine's power and speed in the summer heat. It depends some engines aren't as affected by the high temperatures as other engines are. But I have had to fly them sometimes with a 4 cycling rich setting as you just can't lean them out at all on a really hot day.

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/26/2012 4:39 AM

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I don't know if was asked but WHEN is the engine quiting:

Idle?
Full throttle?
Hovering AKA-helicoptor mode
????

1st thing I would do is mount the engine on it's side.

A great, reliable engine can turn into a very frustrating piece of machinery, when mounted inverted...
Just a thought.....

I recently mounted my Saito 1.25 on it's side because it wouldn't do well when Idling, and it was a PITA. to start.

Moved to to a side mounting position, and it has been ultra reliable since....

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/26/2012 1:27 PM

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Yes, the original airplane had the engine inverted. On my Reactor it was at 45deg from inverted. The only airplane I can test the engine out on now has also the same orientation but I cant use that since I modified that (and the holes in the firewall) for a DLE20 (which is slightly wider). So now I will need to make new holes for this engine and mount at 45deg from vertical.

By the way, in the first stint with plain castor, no nitro, it would cutoff nearly anywhere and everywhere for no apparent reason, sometimes it would cut by the time I got the model with the engine running 5 feet to the flight line. In the second stint it would usually cut going into a hover. The Reactor 46 didnt have the ground clearance to run anything more than 14" but on the Phoenix Extra (my test stand) I have already run a DLE20 with 16x8 so I have a much wider test range

Lets see...

Ameyam

< Message edited by ameyam -- 4/26/2012 5:10 PM >

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RE: OS 75AX- Do I need to change the piston liner (or a... - 4/27/2012 12:06 AM

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quote:

ORIGINAL: ameyam

I used WD40-it came with a small plastic capillary that I could plug direct into the needle holes and force the WD40 through them. There was no clog, I subsequently took the carb apart, washed it with plain water under tap pressure, dried, reoiled it and reassembled. The new carb is $75 and even after spending that, there is no sure way to know the engine will work. One of the carb installation screws is still ok and the other can be held and a seal obtained with silicone sealent, I will bench test the engine that way. Awaiting replacement head and backplate screws.

Ameyam

I once serviced an engine with exactly the same problems. The user also cleaned the spraybar like you did.. When I used my dril bit method, a lot of debris was removed. problem solved. I must have a picture of that stuff around somewhere. If I find it I will post it.

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(in reply to ameyam)

Post #: 22

RE: OS 75AX- Do I need to change the piston liner (or a... - 4/27/2012 8:39 AM

TimBle

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Joined: 4/10/2010
Last Login: 5/22/2013
From: Cape Town, SOUTH AFRICA
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Have you tried a new can of fuel?

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(in reply to pe reivers)

Post #: 23

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