I just ordered a55AX and got some OS F plugs with it because lots ofknowledgable folks recommend it forthe AXseries.Just curious, why is the F plug the plug of choice instead of a hotter plug made for 2 strokes, like the OS #6?

Use the F or a #7 plug in the 55AX. Bottom line is there is more platinum in the element to keep it lit.
That's also the reason they are more expensive.

What about the OS #8 that comes in the box ? is the F a better choice than the #8 for this engine ?

Yes. It will help from mid-range flameouts.

Very interesting. I have a Magnum XLS .52 in a OMP Pro Yak, it has an OS #6 (formerly A3) and sometimes it has mid-range flameouts, some day I'll try the OS F.

Thanks

OS F plug is the only way!!!!!! Lots of folks including myself had issues with their 55AX's until installing the F plug....do it before you loose a plane from the "mysterious flameouts".

Have you tried some other 4 stroke glow plug ? perhaps the Fox Miracle ?

Glow plugs are not the place to try and save cash. One flame out at the wrong time and you re-build or re-place an airframe.

AGREED!!!! Along with learning how to actually tune the engine well.

I put a Funtana down in the pond this week due to engine cutting out(OS 1.20ax). Seemed like a flame out, not 100% sure. I had the F plug ready to put in engine, but hadnt yet as I had just broken the engine in. I dont know what I was waiting for. Thankfully just a little damage to the cowl from the impact - but was able to more or less fly it down. Electronics, engine and frame are good to go. When I get back to the field next week, the F plug will be the first thing to get done.

Ive also been flying the heck out of a proyak with a 55ax and F plug. Its been flawless. It would cut out on me on the first 5 or so flights, but Im guessing I could attribute it to the engine not being well broken in yet, i m guessing(might also have been tuning but im pretty good at it). I must have at least 75 flights on it now and has performed perfectly.

That happened to me yesterday. The trusty 55 AX was not trusty, in fact it's been a piece of junk. Way too many deadsticks with. But I put it in a plane that I just got and have never flown and you guessed it, it dead sticked and I could land the plane and the wind caught it at the wrong time and totalled the plane. [>:]

I got some F plugs.  Gonna try them on my 75AX tomorrow.  Its always seemed like it needs a hotter plug. 

for the 55AX, should I break it in on a smaller prop or is the 13-4 apc sport ok?

I have two 55ax running with a3 plugs. the engines are mounted on their sides and I have never had a flameout with them. one plane is a mojo and the other is a omp yak, I have always thought of my os 55 ax as one of my most reliable engine. they start easily by bumping them clockwise and until this post now I have never heard of them having any problems.

<span style="font-size: small">Iwas having some deadstick issues on my UCD46 with an OS55AX and a pitts style muffler. My pitts style muffler had one of its pipes break off so Icovered the hole but would still have some deadstick issues.So Itried an OS #7 plug and it seemed to run better with that plug, then it quit again.
Iput the stock muffler on with the #7 plug and it ran better. Had four consecutive flights with no deadsticks. The 55 is a little short on power for my UCD 46, as Iam not used to flying much above 1/2 throttle. Ihave however swapped it out for a OS91 FX just to see how it flies. Power now is unlimited vertical with a 16x4 APC. This 91 is a bit too much but it's all Ihave.

Les</span>

Pitts mufflers don't provide enough back-pressure for smaller 2-strokes. The 7 plug is hotter plug and should help flame-outs.

I put an F plug in my 75 AX and it really helped the transition.  Needles were not nearly as critical.  I was running it with a nice smoke trail, all seemed good but the plug burned out in a few flights.  The element was bright white but the base of the plug was oily.  DId I do something wrong?  I've been running OS motors successfully for 30 years, pretty much trouble free.  I feel pretty confident about my ability to adjust needles, but burning out this plug seems like operator error....
I want to use this plug in my new 55AX but I would like to be sure I'm treating it right.
thanks for any thoughts.</p>

Might be too high on the Nitro. What are you running in it?
I run 25% in my 55AX's but with an added head shim.

10%, no shim. 

What exhaust and is fuel foaming possible?

stock muffler.  Yeah, actually foaming might be an issue.  Its in a fatty.  Tank is in foam but it could have slipped, causing vibration to get to the tank

try the a3 with a standard exhaust, I have had no problems running 15 % fuel

So is 3D that hard on this engine, or is 3D just make engine tuning difficult? I've got one on a 4*40, snaps and flat spins don't phase it. 10-15% with A3 plug. Rarely touch the needles. 12x5 APC.

3D requires responsive midrange. It's not the attitude or the fact that it's being thrown around.
Most sport or pattern is near full throttle most of the time where the engine is just tuned for max power.

Download this video of McDDD flying a Primo with a 55AX. Listen to the throttling.
[link=http://www.smilexpress.net/photos/RC/ProBroVideo/2009_mcddd_no_music.wmv]http://www.smilexpress.net/photos/RC/ProBroVideo/2009_mcddd_no_music.wmv[/link]

wmb I use a3 plugs with my 55 ax doing 3d using 13x4 props and trust my engine implicitly. the only problem occured in the first five flights breaking it in and fine tuning the low end. it is the only engine that I hand start because it is easier to grab the spinner and give a quick back flip and start it then to pick up the electric starter

i couldn't get my 95ax to idle until i went to an OSF plug. I tried everything from a fox to A3 and #8. I don't know why OS provides anything other than an OSF plug for their AX engines.

F plugs in every single engine out there that uses a plug and many problems will disappear.