Hi,
I'm trying to figure out how I broke my first header yesterday. Its a Hatori 661 cooling header coupled to a hatori 660 pipe (I think its similar to the 791 header and 699 pipe?). The engine is a 110, on a similar mount to the MK one (engine bolts to aluminium beams, which are held by hard rubbery blocks, which are connected to the airframe). The pipe is mounted with the hatori beam mount that comes with the pipe at the back, and a dave brown (?) pipe mount at the front. The header has the MK mount with the plastic inner part sliced (as was suggested to me to let the header vibrate). The header broke where the flexi part starts. It had about 6 runs with the engine running. It never flew as I had problems getting the mount set up right. It was suggested to me that it could have been the pipe mount not allowing the pipe to vibrate back and forth with the header. I wasn't to sure about that because its the header that just slides into the pipe, allowing it to vibrate back and forth in the pipe inlet.
I did notice that the engine was vibrating at full throttle at a high frequency, ie the rocker cover was a blur, but I thought that that was normal for YS engines?
The other thing that I thought could have something to do with it was the cold, it has been under 0 degrees (celcius) for the last few days, and the shed where the model is stored must get very cold at night. Would that have anything to do with it? I did give the engine time to warm up before tuning it but the header might have cooled down again after I stoped the engine for 5 mins to get the belly pan on.
Any ideas? At so much $$$ a pop I don't want to break another one.

Thanks!

Stuff doesn't break from just sitting around, so there is a real good argument that vibration was the cause.

Your engine is on a shock mount from the sound of it. Shock mounts work by letting the engine vibrate without as much connection to the "solid" rest of the airplane. So no matter how much more or how little more the engine was vibrating, it was moving more than it would had it been rigidly nailed down to the airframe. So there was "more" vibartion induced movement from the engine in your setup, or at least more than less.

If you ran all the runs on the ground you actually added "some more" shock to what the broken parts probably would have seen had the airplane been in the air. So there was "more" again. It's really worthwhile to not break an engine in on the airplane, especially if you do the breakin with the airplane sitting on the ground.

Whenever there are flexible parts in a system, they are there for a reason. And a lot of the materials that're flexible get lots less flexible when cold. So there was "more" again. It sounds like the soft parts in the present system weren't really appropriate for the operating conditions they saw. So there is another "more".

I wish there was a picture to look at, but whatever..... If you can, you might re-engineer one of the joints (like where it broke would be a good joint to use) to use some of the very flexible, blue tuned-pipe connector hose that's available in our hobby. It looks like huge fuel tubing. It's even somewhat see-through like fuel tubing.

I'd bet that what happened was just an accumulation of small additions of "more".

If you can use the hose, it'd be worthwhile to see if you can make the joint longer. Just a few more millimeters of length can do wonders to increase the flexibility.

Thanks for the help. I'll get a new header and see how it goes.

Hey, if it's broken where a silicon tube can be pushed over it, slip a length of tube over it and see if that holds.

And if it's where it can be brazed, brazing is way stronger than most people think. The brazing won't stand the vibration any better than the metal did (that is unless the metal was flawed to begin with) but.... if you can then slip a length of silicon tube over the weak area where it broke originally....... BTW, aluminum pipe brazes just as easily, you just need to have aluminum brazing rod. You probably already have a propane torch, right.

You can also find brass tubing in sizes that "fit" over the break. I've bent big brass tubing to match the bend and then parted it in half. Took one piece and brazed it over the hole. The result was a covered rip where the patch reinforced a bad weak design. I could have used both halves but didn't need to. I bet the guy is still flying that engine. BTW, I got him to introduce a section of silicon tube just downline, at the next connection, and the new layout looked bulletproof.