Home
Page
welcome to our website
About Us
information about our Club
Contacts
our Officers and Committee
Events Diary
our events for the
months ahead
Event Reports
from the last four months
Member's Cars
a selection of
our active "Sevens"
Joys of being a 7'er
and a few stories
Do you fancy an Austin Seven?
advice on buying a "Seven"
Photo Gallery
of our past activities and
"Sevens" in close up
From the Past
Items from 1920/30s
magazines
Registration Marks
in the UK from 1903
Help at the roadside
fuel and electrical problems
Austin Seven Journal
advice for Austin Agents
& Service Depots
Austin Seven
Handbooks
information for the
new owner
Technical Articles
our Members help you
with various tasks
Austin Big Seven
Sixlite & Forlite
Magazine Articles
relevant to Sevens
Road Tests
for the most popular "Sevens"
Parts and Services
sources of spares etc
Quick Tips
a few handy ideas
Other Articles
of interest, not technical
Austin Seven Books
some of the books available
A7CA and FBHVC
CA7C is an active supporter
Links to other Austin
Seven Sites
Website Policy
on advertisements and links
Last updated:
23 September, 2023
Sparking Plugs
Firstly I will not profess to be an expert on this subject but that
it enlightens the reader into the dark side of the workings of the spark
plug in our Austin Seven engine.
The
removal of the sparking plug [easy
but be careful]
1. To remove the plug from the engine, one must first
disconnect the insulated lead called the ‘HT lead’ [HT = high tension]
from the plug, this is by either a simple pull off cap, or there is a
small brass nut which screws onto the top thread of the plug and the HT
lead is held on by a brass terminal.
2. The plug now can be removed from the cylinder head
with the aid of a suitable spanner. This spanner not only must be a
proper fit on the hexagon part of the plug but ideally it should be a
box or socket type which bears on all six sides of the plug. The
ordinary open ended spanners, particularly adjustable variety, should
not be used, because the body of the plug is comparatively thin and is
liable to be distorted.
3. Even when using the correct box or socket one must
be very careful not to tilt the socket over to one side. Damage to the
insulator can easily happen if care is not taken. Also watch out for
your hands and knuckles as the seven engine bay is not that large!
Types of sparking plugs
Plugs with detachable centre electrodes, made up of several
components were widely used before the war. The cleaning of these
plugs is a lot easer as all the parts can be dismantled. The
majority of new replacement plugs are not detachable and the cleaning of
these is more difficult, This is best done with a scribe or small
‘brass’ wire brush and some paraffin to remove the oil/carbon deposits.
Spark plug lengths
Another important point about a plug is its ‘reach’ i.e. the length of
the part which goes into the engine. On the right is a plug with too
much reach; the earth and central electrode are exposed to the
combustion gases: pre-ignition is a likely result. The threads are
exposed and may cause damage to the cylinder head thread when the plug
is removed due to heat damage or carbon build up. Also, the plug
may touch on to the top of the valve and the consequences of this just
do not bear thinking about [so not a good idea to have them too long!].
On the left there is too little reach: this may make starting difficult
and slow running / tick-over erratic.
The correct reach is shown in the middle.
Polarity
With our ignition system it is best to ensure that the coil is wired up
correctly. The correct polarity is critical for the best performance
from the spark plug.
If the coil has been incorrectly set up with the SW [ignition SWitch
side] wire and the CB wire [Contact Breaker /or points] crossed over,
then you could lose anything up to 40% of the spark performance as the
spark has now to jump from what would normally be the earth electrode to
the central electrode.
So cold engines may be more difficult to start in the mornings,
perhaps this is worth a check?
The sketch shows the erosion effect of a reversed polarity on a sparking
plug.
It is a good idea to mark the wires on the coil so that it is difficult
for these to be crossed.
Spark plug heat range
There are ‘hot’ or ‘cold’ plugs. A 'hot' plug transfers combustion
heat slowly and is used to avoid fouling in engines with relatively low
combustion chamber temperatures.
A ‘cold’ plug on the other hand, transfers heat rapidly from the firing
end and is used to avoid overheating, where the temperatures are high
in, say, a high compression ‘race’ engines.
Spark plug indications for correct heat range
Spark plug condition:
Normal - Correct plug Insulator is white or very light tan to rust
brown.
Too cold [use hotter plug] Insulator is a dark grey or black sooty
deposits.
Too hot [use colder plug] Insulator very white.
Internal construction of a spark plug
Ththis cross section shows that the big heavy looking hexagon
on top of the cylinder head is actually pretty thin and delicate -
hence the need to use the correct plug spanner.
The detachable type, shown in the advert, had a second internal thread
that allowed the whole ceramic portion to be removed for cleaning.
Variants with multiple electrodes also appeared in the thirties.
Correct type for
Austin Seven as recommended by Austin Motor Co. which I believe was
correct around 1939.
Champion No.7 [18mm size plug]
1935-39 Champion L10 [14mm size plug]
Austin Handbook recommended the correct setting of ‘gap’ to be 0.018”
[-/+ 0.02”]
This article, written by Mike Davies, originally appeared in CA7C
Seven Focus in Dec 2008 pp22-25.
See also:
Website:
http://gsparkplug.com/technical-information
has excellent information on:
How Spark
Plug Works;
Spark Plug Construction;
Spark Plug Fault Diagnosis;
Spark Plug Gap Settings;
Heat Range.
Sparking plugs—curing heat range problems