I consider myself a beginner, or novice today as I used to fly years ago, but have been away from the hobby for several years.
I fly 40 size planes with 2stroke nitro engines.
I have had more issues with my glow plugs than ever before. So, I need to order some new glow plugs.
I used to buy standard long glow plugs with an idle bar.
Today I see #3, #4, #6...etc. etc.. I do not know what these designations stand for.
Can someone please explain how to choose the correct glow plugs for my aircraft engines, and what is the difference between #3, #4, etc.?
Thank You

The big differences are length ( reach ) and heat range. To actually make recommendations I would need to know what engines you are running and the nitro content of your fuel.

The glow plugs that I used to order years ago were "standard long" with an idle bar.
I don't remember there being all the different variations (numbers, etc.) back in the day. I thought my stuff was pretty much standard stuff.
I have a couple of engines, a Tower Hobbies .46 and an O.S. 60 (I think). I run 10% nitro in both, and always used the same glow plugs for both.
Any advice / help is greatly appreciated. Thank You

I believe the tower hobbies .46 uses a long plug with an idle bar. The OS .60 uses an OS no. 8 plug.

On most 2 stroke glow engines running 10% to 15% nitro the OS #8 or Enya #3 are my go to plugs. Idle bar plugs were good when fuel contained more castor oil or for use in loop scavenge engines.

Boy have you opened up a can of worms. Every self-proclaimed expert is now going to lead you through the minefield of public opinion and why you should listen to their advice. They will have all kinds of reasons their advice trumps everyone else's. There aren't that many actually glo plug manufacturers left. Most engine manufacturers that sell their brand are just repackaged from the regular plug manufacturers. I have tried lots of plugs and was flying a Hanger 9 HN3005 in all my two strokes. When they stopped carrying it I called and was told it was a McCoy plug, in fact a MC14 which they listed as a heli plug on the hot side. I bought a few they looked identical to the Hanger 9 plug and ran just fine in my 2 strokes. I purchased in bulk and I doubt I will run out before I leave this hobby. As to four strokes I used only the OS F plug. When Central Hobbies started selling their own branded plug it turns out it was a repackaged OS F but was near a 1.50 cheaper per plug. I also bought these in bulk and have enough to last. So I have through trial and error whittled down my supply to a plug for 2 strokes and a plug for 4 strokes. I still have all kinds of differing plugs and they all work but not as consistent as the two I have chosen. These plugs I give away on occasion when someone at the field might need one. I have a fair supply of those as well, enough to last.

Idle bar plugs became harder and harder to find switched to regular plugs never looked back even with my old engines that recommended an idle bar plug.

Most here will make suggestions. Try them and see what ones fit your needs. Besides performing a plug in regular or everyday use should perform well and last a long time. Most of mine last well beyond a single season as long as I don't get a lean run.

Thank you so much, that was the answer that I was looking for. I've seen glow plugs advertised today in a multitude of numbers #3, #6, #8...etc...etc...I think I will try what is working for you. I too bought in bulk years ago and thought I'd never run out, but this year (first year I've flown in about 15) I've had most of my stockpile go bad in my first few outings. I think they must have gone bad in storage, if that's possible. I did notice some tarnish on the outer body of them. Maybe that had an effect on the element as well. So, Thanks Again Propworn !!