DC12VOLT

I've had my $25 swap meet SuperStar 40 and Thunder Tiger GP-42 built for some time now, now that I think about it it's been about 9 months! But my engine never got broken in. Both the plane and engine were old-new "stock." Yesterday I was running the engine as per instructions and I noticed a gradual drop in RPMs at WOT. It eventually slowed and ran out of fuel, so I filled it back up. It wouldn't start. I checked the glo plug. Checked the fuel lines, and I noticed my fuel tank worked its way backward, cutting off fuel. I made a shim so that it would stay in place, and started it. I alternated throttle settings as per instructions, and still there was that drop in RPMs. It eventually quit, and I tried to start it again. Nothing. I poured a little fuel into the carb. I cranked the engine, and I then saw the "windshield" was coated with glow fuel! I couldn't see where the glow fuel came from! After some close examining, I found it must have leaked out of the head-block seal. The head bolts were a bit loose, so the engine must have been losing compression gradually as the bolts worked their way out until the point where it failed. So I put some blue loctite on, tightened the bolts, charged my radio, did my homework, and went to bed.

My question is: Why did the bolts come loose? Why didn't TT have lock washers or something on those bolts? Was the engine just running a little hotter than I guess it's "supposed" to? I was running 15% nitro (I'm sorry little engine, my hobby store didn't have 5 or 10%!) so maybe that gave it some punch?

Also: EVERYTHING has to be checked after you run the engine on the plane for the first time! I had the fuselage restrained with 2 brake drums and 2 cinder blocks mind you, but that little thing vibrated every screw out a little bit! Loctite everything you won't be messing with and lock-washer everything else! You don't want that plane coming apart on you mid-air or worse: mid-landing! My nose gear was really lose after that!

Campgems

There are a lot of places for locktite, but your engine isn't one of them. If you tighten the screw down, it will stay tight. Why your head came loose, I'm not sure, but it must have been that it wasnt tight to begin with. Ditto for the other stuff that worked loose. Engine bolts never have lock washers. Ido use them on the bolts holding the engine to the mount though. Thread the mount and a lock washer under the head of a socket head cap screw and it never comes loose unless I put a wrench on it.

If you are getting that kind of viberation, you best check your prop balance. A single cylinder engine is going to viberate some, especailly as some rpm settings, but it shouldn't shake your plane apart. Make sure the prop and spinner are balanced. If they are, then it is as good as you are going to get it. The plane is going to viberate more on the ground than flying.

For your engine, get a tach. Adjust the needle for maximum rpm then go rich at least 500 rpm on a new engine. Don't lean it more for at least a gallon of fuel through it. Set up the engine so it is running really rich for the first few tanks. After about two to three tanks, fly it for the final brake in.

Things that bolt to a plywood base, espically the softer ply used in the ARFs, should be tighten well and here is a place for lock washers on the heads of the bolts. It may also be a good place for your blue lock tite, but be warned that you can spin a blind nut when you use the locktite and it will make your life miserble trying to get it out.

Generally speaking, tighten things down. File a flat on landing gear where a set screw on a collar or a stearing horn will contact it. That gives the best hold. Not much of a flat, but one big enough so the whole tip of the screw is contacting metal vs the tiny marks you will get on a round shaft.

Tighten things before you start the engine the first time. Check them after a break in run. If the engine is in balance, then things that were tight to begin with will stay tight. Don't over heat your engine with a lean run ever. Finally, a little bitty tube of blue locktite should be enough for a couple dozen planes.

Don

proptop

My Feedback: (8)

I had the same thing happen w/ my TT .42 GP...head bolts not quite tight enough when it rolled out the factory doors...
You might also want to check the backplate screws...mine weren't all that tight either. A leaky backplate can cause erratic running too.

RCKen

My Feedback: (9)

The above advice is good. And as they have said, sometimes bolts are loose coming out of the factory. However, there is something else at play here as well. You problem could simply be part of  the normal break-in process. The act of "breaking-in" an engine is done for a simple reason. The engine is a collection of metal parts that have been roughly assembled at the factory. When you first start running an engine all of these parts are now starting to run together and they will start to "wear in", or start meshing, with each other. As this happens there are fine metal shavings that are produced as these parts start wearing together. This is one reason why we run these engines rich during the break-in process, the extra fuel suspends the metal shavings that are produced and carry them away out of the engine. Because of this wear-in process parts that were tight can come loose, such as your head bolts.
'
Part of any break-in on an engine should include checking all of the bolts on an engine to ensure that they are tight. I usually do this once after the first tank of fuel, and then again after about 10 tanks of fuel. It's not uncommon to find loose bolts after that first tank of fuel, and I even find them again at the 10 tank mark as well. This is perfectly normal and nothing to worry about. In your case just tighten down the bolts and move on. And it's too late now, but as Campgems pointed out, Locktite should never be used on an engine, especially head bolts. You may find those bolts difficult to impossible to remove later on after the engine has been ran.

Ken

da Rock

Also consider that many break-in processes include altering engine rpm with THE NEEDLE valve. The idea is to heat the engine up and cool it down, all with the needle valve, not the throttle. Adjusting the mixture instead of just moving the throttle insures the engine cools properly from the inside out, not just runs at different speeds.

Can't remember ever seeing a mfg suggest, ".......alternated throttle settings as per instructions," in their break-in recommendations. It's common to see them talk about using the needle, however.

combatpigg

My Feedback: (3)

The quality of the engine mount and strength of what the mount is bolted to has a bearing on how smooth the engine will run. I'd give this a look, especially after having a shaky run that loosened everything up. It's not unheard of to have one of those hot glued ARF firewalls rip loose.

opjose

I've NEVER had a two stroke glow engine of ANY sort from ANY manufacturers, where the head bolts DID NOT loosen after the first few runs.

The engines are usually assembled with the engine cold, so most of the bolts typically loosen after the engine is run.

I run the engine at least TWICE with the cowl off, and while the engine is still warm after it's first runs I tighten down all of the bolts.

It is surprising how loose they can get, especially when seals/gaskets are employed, as the seals compress over time.

Mufflers and muffler fittings also need to be checked.

Once re-tightened the head bolts rarely come loose again.

Muffler fittings are another matter because of the extreme temperature ranges, and in this case a bit of lock-tite helps ( up to a point... ).

Campgems

We used to do that with our car engines on a rebuild also, Put the oil clips on the rockers so there wasn't oil flying everywhere and run the new rebuild for few minutes and the torque the head bolts again. Some guys would go under and torque the bearing caps and crank bearings again also.

Don

TedMo

Hey Rock! You may be the first one I have noticed that mentions varying the engine speed by the needle setting not the throttle, during break in. This has been my system for many years and is the proper way. Also glad to see someone spell break-in correctly. Not brake in.

jester_s1

There aren't many places on RC airplanes that locktite is needed. If the screw is going into a hole that is tight, such as into wood, there is plenty to hold it there unless something is wrong. The only place I use it is on the engine mount, both the mount to the firewall and the engine to the mount. I use very little, and mostly to seal the threads from oil getting in. The problem with locktiting a screw in is that it can wear loose without turning, but when you check it it still feels tight. Imagine that scenario on head bolts! You'll go nuts trying to find the demon in your engine.

BTW, the biggest reason for head bolts on these engines going loose is because of the large amount of expansion that aluminum has with heat. The hotter the head gets, the tighter the fastener gets. If it's too tight cold, then you get deformation in the aluminum when hot which results in a loose screw down the road. If it's perfect hot, then it's too loose cold and can vibrate looser. Torque specs are much more crucial for aluminum engines than iron because of that property. A proper head bolt stretches with that expansion, holding well enough to seal but not so well to make the aluminum deform. A lot of that break-in loosening has to do with finding the right middle ground torque wise, and letting the bolts take that small amount of permanent stretch that they will when new so they settle in and return to the same size cycle after cycle.

DC12VOLT

Okay, so I came bark today and ran it, and luckily the loctite didn't take. I had to tighten the bolts once more, but hopefully I won't hawe to in the future. I figured out that it was my prop causing excess vibration. It had rubbed on something just slightly before and was a little bit imbalanced. I changed out props today, and now it runs like a champ. I'm running it rich like you guys told me to, and hopefully it will run well for years to come. I have to make a trip to the hobby store, though, becasue I found that my glow plug is not sealed very well. I can feel air leaking out of it when I hand-spin the prop, and I tightened it up and it still leaked. Then I tried a dead plug. That doesn't leak. I'll get a bunch of plugs today or tomorrow and hopefully will maiden in the next month if weather permits.

Campgems

Take the copper gasket off the glow plug that doesn't "leak" and put it on the one that does.

Don

Campgems

brett65

My Feedback: (18)

Are 4 strokes more likely to grenade like that than 2 strokes? It seems like I've seen more fragged 4 strokes than 2 strokes.

combatpigg

My Feedback: (3)

The damage in that photo is from a crash. Look how relatively weak the design is compared with a much more compact 2 stroke.
ARP, a leader in the bolt business has a slogan, "tightness is what keeps bolts tight".
The acquired feel for how tight this is is easier understood if you can visualize tightening the bolt to the point of stretch, but not to the point beyond the bolt's elastic limit to return to the original length when loosened.

JOHNS3D

My Feedback: (5)

never trust factory bolts tights specs...............i have learned all that from helis....you need to check every bolt, most of the time they come ok, but is necessary to be sure and add some locktite if is necessary.

Campgems

I hate the new editor. Some how, this pic I posted got moved to this thread from a PM that I was attaching it to. One bump of the wrong key and things go to hell.

For those interested, I trashed the O.S 52 for stroke by getting disorentated and upside down and pulling full up and full throttle. First contact wht the ground was the rocker cover. the photo shows the rest. It has nothing to do with this thread. Sorrry for the confusion.

Don

N429EM

Judging from the condition of the spinner backplate, I figured that was crash damage.

da Rock

Having flown C/L combat, ratrace and Goodyear......... I've seen a lot of 2strokes go.

Having flown mostly weekend R/C for about as long, I can't remember any 4stroke in ruin that wasn't just a meltdown from the owner setting the sucker by ear. Certainly never saw a crank go and don't expect to.

But then, I've only seen a few R/C 2cycles ruined. One guy did most of them in. He likes to go fast. (He's the only guy I've seen trash an OS, btw.)

Campgems

The crank in my photo above was about the only thing I salvaged from that engine. Other than dirty and a dinged threaded end, it came out fine. I can't beleive how tough they are.

Don

jester_s1

Getting that little bit of stretch is the key to torquing bolts right. If you're worried about them vibrating out, put a dab of paint on the side of the bolt head where it meets the engine. You'll be able to tell at a glance if it is moving.

CGRetired

My Feedback: (1)

Jester: Your post was the first time I saw a reference to "torque" in any of the above replies. I would think that there is more than just tightening the head bolts, I would think that applying the right amount of torque to the head bolts is necessary.

Is that so?

Hmmm.. the right amount of torqueage... sounds like a line from "My Cousin Vinnie"

CGr

mclina

I used a Crafstman, laboratory grade, signature series, torque wrench. The kind used by Cal-Tech high energy physicists and Nasa engineers.

And a split second before I applied the wrench, it was calibrated by federal and state employees from the Department of Weights and Measures.

And I routinely twist to maximum allowable torqueage.

One of my favorite movies.

jester_s1

Well I did mention it in my first post too, but to answer yes, the right torque is the key to good head sealing. If you apply too little, the head won't seal cold and the bolts will eventually work loose and then it won't seal at all. If you apply too much, you overstretch the bolts and ruin them or you ruin the head or block. Our hobby isn't nearly as precise and proper as a car engine shop would be, but the goal is to get the screws tight enough to seal but still have some stretch left for when the engine heats up. Using the bolts supplied with the engine is also important, because they were chosen for their stretching characteristics.
When you are tightening screws, you can feel the stretch limits if the threads are clean. As you go in, you feel the bolt seat, and then a steady rise in torque needed to turn it more. If you keep going, you will then come to a point where the torque needed to go in more rises substantially. That's the bolt's limit of stretch. Not long thereafter, the bolt will break. All this assumes that the threads don't fail first, but in our engines they will. For our purposes, tightening down until the screw seats and then just a little more (1/3 to 1/2 turn) will give enough stretch to keep the head sealed cold and keep the screw from vibrating, but will still allow plenty of stretch when hot to keep the bolt from fatiguing. You need to retorque after a few thermal cycles because the stretching when hot puts a permanent set in the bolt. Guitar players see this in the initial string stretch they have to do when replacing a set. Once that permanent set has been made, the bolt will stabilize dimentionally and will keep its torque setting. The better quality bolt you use the faster this happens and the longer it lasts. Eventually, the metal will fatigue and start taking a permanent set again, but our engines will be long gone before that happens as long as we are using the stock bolts.

raydar

Hooke's law