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.
Much good information at Alan's Hobby, Model & RC Web Links
"Carburettors, Tuning & Performance Testing, Glow Plugs."
Glow Plugs- Brian Hampton
Plugs - Selection Guide - cross reference of most plugs.
http://188.8.131.52/rdrc/gloplugs.htmlGlow Plugs - The technology of
Airplane flight,model airplanes,engines,aerodynamics - Pé Reivers
Cen Racing - good tutorial on tuning, care & maintenance ***
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 ***
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
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
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
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.
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
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
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.
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 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
Cheers -- \_________Lyman Slack________/
\_______Flying Gators R/C___/
\_____AMA 6430 LM____ /
Visit my Web Site at