Here's some info I posted a while back about plugs, what % of fuel you use is what plug you use.

Information on glow plugs.

The general rule regarding glow plugs and nitro fuel is:

The higher the nitro percentage, the colder the plug should be.

Conversely:

The lower the nitro percentage, the hotter the plug should be.



MC-4C: 4-cycle engines

MC-55: The MC-55 is considered a hot plug and is designed to run on fuel with a low nitro content. Applications for this plug would be planes, tether cars, etc.

MC-59: Like the MC-55, the MC-59 is considered a hot plug. The difference between the 55 and 59 is that the MC-59 is designed to run on fuel with a higher nitro content (low to medium nitro content) than the MC-55. Applications: Boats, planes, heilcopters, and cars.

MC-8: The MC-8 is a medium to cold plug, deisgned to run on fuels with a medium to high nitro content. Applications: Boats, cars, and planes.

MC-9: The McCoy MC-9 is considered a cold plug and is designed to run on fuels with a medium to high nitro content. Applications: Cars, boats, ducted fan and Giant Scale planes.

MC-14: The MC-14 is a hot plug, designed for helicopters and 4-stroke engines.

From reading just these bits of info on McCoy's plugs, you can easily see that general rule...if you're running fuel with a high nitro percentage, you should be using a glow plug that's considered to be a "cold" plug. Conversely, if you're running a low nitro content in your fuel, you should be using a "hot" plug.

The following are lists of OS and Novarossi plugs. These plugs follow the same nitro/heat range rules as demonstrated with the McCoy's plugs.

OS PLUGS

OSMG6300- #0 Economy standard plug similar to the O.S. #8.
OSMG6304- #1 Hot plug for low nitro (0-5%).
OSMG6308- #3 Medium plug for medium nitro (5-25%).
OSMG6312- #5 Cold plug for high nitro (25% +).
OSMG6316- #7 2 stroke engines requiring an idle bar.
OSMG2691- #8 "The" standard. For use in all 2 strokes, any nitro.
OSMG6320- #9 For general use in 2 stroke engines.
OSMG2692- #F For O.S and other four strokes.


Novarossi Plugs

10001 R1 Extra hot from .8 TO 2CC
10002 R2 Hot from2 TO 3.5CC
10003 R3 Medium from 3.5 TO 6CC
10004 R4 Cold from 6 TO 10CC
10005 R5 Extra cold for nitro fuel and RC
10006 R6 Cold for nitro from 10 - 13CC
10007 R7 Cold for nitro from 13 - 15CC
10008 R8 Super Cold for nitro from 15-30CC
10010 RC Hot for RC from 2.5 TO 6CC
10020 RC Cold for RC from 6 TO 15CC
10011 G1 Hot for R15 Speed
10022 G2 Medium for R15 Speed
10033 G3 Cold for nitro fuel from 18%-30% for R15 Speed.
10044 G4 Extra cold for nitro fuel from 30%-15% for R15 Speed
10055 G5 Extra cold for nitro fuel from 50%-70% for R15 Speed.