Glow Plug length - general guidelines?

Does anybody know of some general guidelines for choosing between short, medium, or long glow plugs?

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I wouldn't go by general guidelines. I would go with the manufacturer's recommendations. Some engines can be physically damaged by using the wrong size glow plug, such as the HP.21 and .49 VT engines, or running an R/C long in a Fox .15BB.
However, if you pull the cylinder head of a two-stroke, screw in the glow plug and nothing sticks down into the cylinder below the contour of the head, you should be safe. This includes the aforementioned Fox .15BB.
There is no reasonable way to perform this same test on the HP VT series four-strokes that are mentioned previously.
Using too short a glow plug is not a problem, as you may have surmised. It is when the glow plug can mechanically interfere with the movement of an internal engine part that problems arise.
This answer is not meant to offend you by assuming that you aren't very smart. Not my intention at all. But it is meant to help others with little experience in these matters.
Ed Cregger
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No offense taken, Ed. I just started flying last year and I'm simply trying to accelerate my learning a bit. My engine manuals are a little too specific - I have a Magnum XLS .52A manual that only says the Thunderbolt #3 glow plug is the ideal plug for the engine. I don't know exactly what a Thunderbolt #3 glow plug is, but I think it's a hot, medium length plug.
You stated that "using too short a glow plug is not a problem," but if that's the case then why make different sizes of plugs? Shouldn't they all just be short?
The manual for my O.S. Max .46 FXi and my O.S. Max .40 FX says that O.S. plugs No. 8, A5, and A3 are all fine. These plugs range from hot (#8) to medium-hot (A3) to medium-cold (A5). O.S. doesn't describe the length of any of these plugs, however. Are they all medium length? When I go to the Tower website instead of the official O.S. website, the A3 is described as hottest while the #8 is medium hot. Which is actually correct? Tower also describes the #8 as a "Standard Long" plug, but then describes it as medium length.
I also have a GMS .47 engine with a Fox Medium Long glow plug installed in it. When I first started breaking in the engine, I had an O.S. A3 installed and I couldn't keep it running. Switching to a longer plug made a big difference. My problem is I don't have the instructions for the GMS motor and I can't find them online. I don't even know what plug is recommended for the engine.
I was guessing that short plugs would be best for .25 cu in or smaller engines, medium up to .75 2-strokes, and long glow plugs for .90 or larger 2-stroke engines. This was a random guess based on antecdotal comments I've noticed.
While I'm learning a lot about R/C flying in general, glow plugs are still mostly just a mystery to me. I hear guys that I fly with claim one thing, I'll read something different online. I try different things myself, and I get totally unexpected results. I'm just hoping for a little more detailed information.
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The Thunderbolt #3 is alledged to be a close copy of the Enya #3, but much less expensive. You will hear the Enya #3 plug being mentioned in hallowed tones. It has a cult following (me too!) and is a great glow plug where a hot two-stroke plug is needed, or when needed as a standard four-stroke plug. I prefer the OS Type-F for most four-stroke applications, but, having used the Enya #3 for four-strokes, I can see where it might be preferred in certain instances. Anything with the Enya brand name is expensive. The Thunderbolt #3 is an attempt to copy the performance of the Enya plug, but at a lower price. I haven't had the chance to try the Thunderbolt line of plugs as yet, so the jury is out for me. Others might want to jump in and comment on their experiences with the Thunderbolt plugs.

I was saying that it wouldn't physically damage the engine via mechanical interference, not that the engine would run as well as with the proper glow plug. You are an astute student, Ed. <G>

In truth, it depends upon the amount of nitro used in the fuel, the prop load placed upon the engine and the amount of cooling available to the engine. The OS #8 is a safe plug and will work with the greatest amount of variations in the aforementioned things that determine which plug to use.
Our glow engines, in spite of the fact that they do not use a spark plug, actually do have and utilize ignition timing. Things that affect this ignition timing are all of the things I mentioned that determine which plug to use. This is why there are different glow plugs of the same length. There are hotter or colder plugs, which you have previously ascribed to.

As long as the plug does not physically interfere with the operation of the engine, it is fine to experiment. Any recommendation made by the manufacturer should also include how much nitro and a prop sized are being used.

Your opinion sounds logical. A good starting point for most two-stroke engines is the generic plugs sold by several outlets, such as Tower Hobbies. Use the long version in engines over .15 size, unless otherwise instructed. There is no reason to spend a lot of money on a two-stroke glow plug if you are running normal sport fuel, a stock or near stock exhaust system (non resonant) and props within the manufacturer's recommended range.
Medium length plugs are a head scratcher for me. Why OS did this is beyond my experience. However, the only OS plugs that I bother paying the high price for are the Type-F four-stroke plugs. While I do love the Type-F plugs, I also experiment with other brands. Especially less expensive brands. <G>

Other than mechanical interference, there is no reason not to experiment in order to discover which plug works best for you in a particular application. To me, that is part of the fun of this hobby. We're always looking for that magic combination that gives us a better idle, transition or a few hundred more rpm. If we can save a few bucks in the process, all the better. <G>
Ed Cregger
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Much good information at Alan's Hobby, Model & RC Web Links http://homepages.ihug.co.nz/~atong / under : "Carburettors, Tuning & Performance Testing, Glow Plugs." in particular: Glow Plugs- Brian Hampton http://www.holdfastmac.com.au/howglowplug.html Glow Plugs - Selection Guide - cross reference of most plugs. http://209.15.36.61/rdrc/gloplugs.html Glow Plugs - The technology of http://web.archive.org/web/20040723045301/http://www.rcnitro.com/rn/articles/glowplugs.asp
others - Airplane flight,model airplanes,engines,aerodynamics - Pé Reivers Cen Racing - good tutorial on tuning, care & maintenance *** C.H.Ignition Engine Technical Notes-VERY complete! Extend Engine Life - M.A.N article For the new modeller - How To... Fuels - Brian Hampton How to Rebuild Your Nitro Engine - RC Nitro Magazine,*** Installation of Model Engine Bearings K&B owners guides - Carbuettor tuning etc Modifying Your Engine - Expansion Chambers etc NitroReview_reviews of almost every car_truck engine One-way valve - Gasoline Check Valve One-way valve - Glow Fuel Performance Testing of Engines - Martin.Hepperle Perry Pumps & Carburettors Plasma Liners (Ceramic piston & liners etc) see also Norvel engines. Prop and rpm performance figures - RCFAQ Radial Engine FAQ (RC Warbirds) Regulated fuel supply for large engines - Pe Reivers Running In Engines - Brian Hampton Setting the Idle Mixture _ Brian Hampton SmartPlugs - Applications Squish Clearance - RC Boat.com Tuning and setting Car & Truck engines*** Super Tigre set up - by Pe Reivers How To Tune Your Engine+#01 ***

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Thanks Alan, good info here on the relationship between fuel content and plug temperature.

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Alan's site should be on every RCer's favourites - it's a brilliant resource!

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To the best of my knowledge, the 'short' plugs are basically used in the smaller engines (,19 ~ ,09). 'Long' plugs can be used in just about any engine as long as the bottom of the plug doesn't hit the piston. BTW, the long and short refer to the reach, or threaded depth of the plugs; I've never heard of a 'medium' reach.
On the other hand, there are heat ranges for plugs which are cold, medium, and hot.
And when all else fails, use the manufacturers' recommendations :-)
If you're new to R/C you might find some helpful information within my Web Site; the article on four-strokes also generally applied to most all model engines.
Cheers -- \_________Lyman Slack________/ \_______Flying Gators R/C___/ \_____AMA 6430 LM____ / \___Gainesville FL_____/ Visit my Web Site at www.LymanSlack.com

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Some of the OS plugs are alleged to be of medium reach. They are a tad shorter than normal long plugs. Advertising them as such is a fairly recent phenomenon.
Ed Cregger
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Ed Cregger wrote:

The OS A3 is 1mm shorter than the typical long plug. Why, I dunno. That's always puzzled me.
Geoff
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