Chip,
The original question was on reach not thread length but reach is thread
length and both of us made that mistake. The words are used incorrectly
interchangeably for thread and electrode protrusion.
A colder plug transfers its heat to the head faster than a hotter plug.
With better alloys and much thinner electrodes it is now possible to
make a projected nose plugs in colder ranges where you can't see the
difference visually, but the traditional way to make a colder plug is to
recess the electrode. I haven't looked for a thin electrode plug as a
replacement for T-series and don't know if they exist. Generally US
plugs are low numbers colder and higher numbers hotter and the reverse
for Japanese.
The extended tip has to be cooled by the mass of the head. At the
temperature of the tip incoming fuel wouldn't cool it but be ignited.
Douglas,
I use Champion N3C as two ranges colder. That is what BMC recommended
for MGBs with a milled head getting them to 9 to 1 compression and I'm a
little higher than that in my TD. If you have only done a mild plane
you can likely use standard plugs unless it detonates.
With apologies to the list,
Ron Parkinson
You're confusing "reach" and "thread length". They aren't the same
thing.
The original question was about the correct thread length for an XPAG
engine with a Wolsely head on it.
"Reach" refers to how far into the combustion chamber the electrodes
extend. These days an extended reach plug is preferred for most
applications. Extended reach does not equate to "hotter". Like
standard
reach plugs, extended reach plugs are made in a large range of heat
ranges. An extended reach plug usually works well under a wider heat
range than a standard reach plug because the plug tip is more exposed to
the cooling effect of the incoming mixture on the intake stroke.
Thread length, as the name implies, is the length of the sparl plug's
threads. The cylinder heads originally used on the TB, TC and early TD
require plugs with a 1/2 inch thread length. Due to a casting change
the
late TD and TF head requires plugs with a 3/4 inch thread length.
If you use 1/2 inch plugs in a head designed for 3/4 inch plugs, the
electrodes are recessed within the longer plug hole, so they aren't
fully
exposed to the mixture.
If you use 3/4 inch plugs in a head designed for 1/2 inch, 1/4 inch of
thread on the plug extends beyond the plug hole. Once it carbons up,
the
plug will be difficult to remove. Under some circumstances (especially
if
you use an extended reach plug) the piston can colide with the plug.
--
Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271
Cub Hill, Maryland
fold@bcpl.net
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