On Tue, 13 Jul 2004 10:58 -0400, Parkinson,Ron wrote:
> For my $.02, I have a modified late TD, and use the shorter reach plug
> (colder) because of the higher compression. The theory as I remember it
> is that the longer reach (hotter) plug can act as a glow wire like in a
> diesel and cause a pre-detonation or knock.
>
> Remember if you are using a sport coil you can run a wider gap than spec
> as the addition volts will still arc and that will help.
>
> I don't have many miles since it returned to the road but it is running
> great! That is my experience.
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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