Stripped Glow Plug

Hi Balogh
I do tend to tighten pretty good the head. Maybe I am needing to add the oil I never have done that before. But after a run plenty of Castor and Klotz oil is there so thought would be ok?

Thanks for the temperature 140 c that should be about 284 F.

Hope you're getting some more flights at Balaton?

Take Care my friend.
Darren

Thanks, Darren,

I add the after run oil to moisten the cylinder, not the threads just wanted to say that I am also often removing the head while others may add the after run oil through the exhaust ports and remove the head only if the glow filament has been blown...

Luckily the owner of the small friendly airport near my summerhouse at lake Balaton is a modeler himself playing with RC gliders, and has all the empathy for glow engine aero-modelers grounded by the crazy drone law of Hungary...so I fly an outlaw in a very friendly field, inherently dedicated, situated and geared to flying...

Well another high compression glow plug has completely stripped on me. One of the best running ones I have at the moment.I tried some copper foil to wrap the stripped threads but didn't work.  I tried lock tighting the glow plug in left 24 hours but after one great run. Next start the glow actually popped right out of the cylinder.I caught it in my hands.
I guess there is no solution other than to throw it away although running perfectly? On the motor bikes used helicoil but this is the head not the cylinder stripped. Any other amazing solution?
Thanks
Ice

Hi Darren, your glow head thread stripping puzzles me... I have never had such experience with COX glow heads, new or aftermarket both..are you sure you correctly screwed it in without forcing the head on the cylinder threads, thus stripping the softer aluminum thread?

Once stripped, I would not experiment with Loctyte or similar glues whose application temperature may be limited as well...the peak pressure inside a COX cylinder when the charge burns at TDC is near 15...20 bar or similar, and you do not want the glow head to become a projectile, while you may be just  leaning over the engine during start-up.

Hi Andras
Good suggestion about the head popping off as a projectile. I'll not do that.
No I always am extrememly careful and never cross thread a plug. However this plug always seemed to loosen? I thought just because it was working loose from expansion/contraction. However each time I tightened it after a run it would "screw" in another partial turn. I suspect the plug had started to wear and every time I tightened it again it stripped. The head is older about 5 years old and flown maybe 100+ flights but runs I mean ran so sweet.

Thanks
Ice

If the engine is running with too much compression, the head usually comes loose. Another problem is the head gaskets, they only technically work one time. Once you've tightened it, the copper squashes and it offers rebound. When it comes loose and you re tighten it, it further compress offering even less. You shouldn't be removing the head unless necessary. Oiling it through the top obviously makes it easy, but it's ruining your gaskets.  New gaskets usually take care of a head that's been loosening.The head will come loose one or two times but after that, it won't loosen. There's no need to crank down on the head, only snug it.  You can also try placing a piece of 600 sand paper on glass using some oil lap the mating surface of the head a little. This will insure full contact. I've taken old gaskets and heated them over a gas flame until glowing orange and let them air cool which puts a little elasticity back into them.

If the threads are gone there just isn't much to be done. You have to remember you're working with aluminum and maybe 3 full threads. I could look into how much torque it could actually handle, but clearly you were yielding the thread, and/or wearing it. It's possible the head was on the low end of the tolerance to begin with.

By lubricating the thread, you reduce the friction so a given torque increases clamping pressure. If you already put a lot of torque on the head, this means it will be even tighter and load the threads more. So, don't over torque it.

The gaskets work harden in use and lose there ability to do the intended job after a few cycles. Annealing the copper gaskets can be tricky, they are extremely thin and will melt. Proper annealing calls for quench in water, but being so thin, it's probably not necessary. The soft gasket crushes to conform to the surfaces the first time you use it. After it work hardens, it's less effective.

Ken's suggestion to check the sealing face for defects is important. Burrs or small dings from handling in production can make it much harder to get a seal. This is one of the things I try to be very careful about in my parts.

I think that you can use stripped glowplug. Tie the glowplug fin to cylinder fin using soft iron wire as criss cross. Two tying need at 180 degrees. I didn't try it. I'm going to send a photo.

It used to be standard instructions on the Cox 049 engines not to overtighten -but just to 'snug the head up gently' [quote] -I have been messing about with Coxes for 55+ years-and NEVER stripped a head-(burnt out a few over the years  though!)-so you are doing something very wrong with your approach. Gaskets are cheap and available in bulk from Bernie and crew-good idea to buy a pack of 10. You can soften copper gaskets that have work hardened by heating them to red heat, and allowing to cool naturally-ie do not quench them. You can do this 2-3 times, not indefinitely.

It is perfectly normal for Cox heads to loosen up with continual use-you are putting them through multiple heating cooling cycles-in an environment where they are closely clamped to two dissimilar metals with different expansion coefficients-of course stresses are setup up in that process that will need to self alleviate-and that's on top of any vibration induced effects. I would under normal condition expect a greater occurrence of loosening in a situation where multiple gaskets are used to get the correct head clearance....eg TD 049/051s used to be supplied ex factory with 3 gaskets installed. There was a specific separately printed note to this effect included in the packaging. These multiple gaskets will naturally compress with time and running. Even worse is the situation with aftermarket heads such as the Galbreath unit-excellent unit though it is-but its high compression sometimes requires the use of 5 gaskets to get the deck height right.

ChrisM
'ffkiwi'

thanks Chris  @ffkiwi and Greg  @gkamysz that is very helpufl info to me. for years I had no problem with aftermarket or cox glowheads coming loose. Then I started having problems and it was driving me mental. thinking back now, I believe the problem lines up with when I started oiling the cylinder and head threads before doing them up. I always used to just snug the heads down dry and had no problems, but............  Anyway, Im going to go back to new gaskets and no oil on the threads just like I used to do before and hopefully the plugs will stay tight like they used to.  small changes can have quite a difference in the way things work, and I for the life of me couldnt work out or remember what I was/am doing different.  Now I remember and your explanations make sense that relates to what I am doing and the result I am getting.   thanks heaps.

I have to honestly say that I have more issues with stuck heads-especially on second hand engines...than heads loosening up. castor of course-after the passage of a few months-and a lot of heating cooling cycles makes a very passable glue...and you then run into the problem of how to get the damn thing off...heating will soften the castor...but heating will ALSO expand the head threads-making it grip even tighter-and many of the Cox cylinders do not have the spanner flats....the ideal of course-is to heat the cylinder -and cool the head...which is rather problematic.

One thing I am aware of-but have never tried-is you can buy 'cooling sprays'.....generally intended for rapidly cooling electronic components in localised areas on a circuit board. One of the products MIGHT prove useful in the situation where a tight head needs removal-and the engine has been heated to soften any congealed castor, as it can be accurately directed via the plug in straw-so might offer some assistance in rapidly cooling the head whilst still keeping the steel cylinder fairly hot...

The exact opposite of the thrust of this thread, of course-but pertinent to the general issue of heads themselves....

ChrisM

Not sure why this particular head loosened so much. I used all variables. Multiple copper gaskets. Tightened loosely. However stripped plug. The Kamtech ones I have are really good and never experienced a single issue with them. The turbo plug actually seems to have a bit more kick even. Thanks Andras for the suggestion.
Ice

I actually had a Gilbert 7 head pop right off as I was flicking it over to start. Fired once and flew right off. Thread gone. It's hard enought keeping the 7 heads from burning out but to have a brand new functional head which can't screw into an engine is most disappointing.