The T Series
The poor old TA has always had a bad press. Form the first, it was vilified
by the O.H.C. owners as being effete and not a proper M.G., with hydraulic
brakes to boot. Later on, after the XPAG-engined TB and TC arrived, it was
then criticised as having a useless engine and thus not a proper T-Type! The
truth is that it really was the first M.G. that started the roll for these
cars to become volume produced sports cars that enchanted and swept the
world in export sales after the Second World War.
With road quality rapidly improving during the late 1930’s, the previous
M.G.s were seen as mechanically complex, somewhat delicate over higher
annual mileages and progressively less safe with their cable brakes. The TA
can now be seen as one of the seminal evens in M.G. sports car history, and
a saviour, in its way, of this run of cars that allowed the MGB to develop
into one of the best selling sports cars of all time. Without it, the M.G.
Car Company Limited would not have survived the war.
The following technical description of this car comes from the May 25th 1938
edition of the Motor Trader. There is quite a lot of detailed information,
so this month we will have the engine and electrical systems, and we will
round it off next month with the transmission and chassis.
APW
TA IN DETAIL
Maintenance is simple for both minor adjustments and major overhaul. The
owner, who is usually an enthusiastic motorist, can give the car the
necessary attention without gymnastics and major repairs call for no
elaborate technique or special tools.
GENERAL
Engine and gearbox in unit, mounted on rubber. Open Hardy Spicer shaft to
semi-floating rear axle. Spiral bevel drive. Four-speed box with
synchromesh on top and third. Lockheed brakes; Bishop cam steering.
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ENGINE
Four cylinders cast in one with deep skirt extending well below centre line
of shaft. Cast sump with one-plane gasket. Flywheel bell housing flange -
bolted to box.
Cylinder head with valves and rocker gear detachable. Cast aluminium valve
cover embodying oil filler.
To remove engine, having removed radiator, bonnet, disconnected batteries
and stripped ramp plate, floorboards, dynamo and starter connections and
exhaust pipe, uncouple clutch rod from lever.
Remove gear box remote control and uncouple propeller shaft at gear box,
marking the flanges to secure the same assembly. Disconnect exhaust pipes at
manifold, petrol pipes and all controls.
Place jack under box and take weight sufficiently to allow removal of four
rear main engine bolts securing gear box to rubber mounting. Remove nuts
from front mounting bolts, with rubber washers and retaining clips. These
cups are threaded and must be unscrewed. Unscrew bolts from runner mountings
and lift engine clear of car. If starter motor fouls steering column turn
the dust cover slightly.
CRANKSHAFT
Three main bearings, 52mm diameter by 38mm long, white metal lined, shells
dowelled in caps. End float 0.004in. Radial clearance controlled by fitting
the correct replacement bearing. If shaft is reground it should be done at
the factory, if not, to specified undersize to ensure that new bearings fit.
Bearings can be supplied for crankshafts reground 0.010in and 0.020in
undersize only.
End float controlled by centre main which is flagged at both ends. To remove
crankshaft, the flywheel must be removed.
This spigots against flange on end of shaft and is secured by bolts. Remove
flywheel housing, two bolts having their heads inside the crankcase. Remove
connecting rod caps and draw pistons from underneath, turning cranksharft as
necessary to allow each one to pass. Three bearing caps can now be withdrawn
and shaft removed after the front and rear bridge pieces are removed.
Flywheel starter gear integral, 8.636:1 ratio.
IMPORTANT FIGURES
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No of Cylinders
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4
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Bore & Stroke
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63.5 x 102 mm
2.5 x 4.01 ins
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Capacity
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1292 cc
78.84 cu ins
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RAC Rated HP
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10
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Compression Ratio
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6.5
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Firing Order
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1 3 4 2
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Tappet Clearances
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inlet 0.010"
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HOT
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exhaust 0.015"
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Breaker Gap
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0.010" - 0.012"
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Plugs (14mm)
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Champion L10
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Gap
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0.018"
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Up to
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Starting at
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Gear
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Engine No
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Engine No
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Ratio
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MPJG 683
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MPJG 684
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1st
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18.1
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16.84
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2nd
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10.73
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9.95
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3rd
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6.92
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6.44
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Top
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4.375
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4.875
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Capacities
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Sump
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11 pints
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Gear Box
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2 pints
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Rear Axle
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2 pints
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Water System
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1 7/8 gallons
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Petrol Tanks
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15 gallons
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Tyres
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4.50" x 19"
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Tyre Pressure
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F 24 R 26
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Wheelbase
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7’ 10"
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Track
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3’ 9"
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Turning Circle
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38’ 0"
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Weight
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15 3/4 cwt
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Battery Earth
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Positive
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CAMSHAFT
Three plain bushed bearings, centre journal embodying a skew gear to drive
oil pump. Distributor skew gear drive is separate, integral with shaft.
To remove camshaft with engine in place, remove both tappet blocks. If
engine is out and inverted this is unnecessary. Remove distributor and oil
pump. Uncouple timing chain and draw sprocket. Draw camshaft partly forward,
having removed dowel screw whose head is just above oil gauge tap and
connection. At this stage wire the two halves of the bearing together to
prevent their falling apart. Draw bearing free, untie and remove bearings.
To refit, install front and rear bushes and slide shaft part of the way in
from the front. Then assemble central bearing and wire up. Enter bearing in
its housing and remove wire. Push home and, with a tang or blunt scriber,
feel for the locking screw hole. Insert locking screw and wire up.
CONNECTING RODS
Big end bearings cast direct in rods. Replacement rods provide correct
clearance. Undersize rods available for ground shafts. To remove a rod with
piston from below, turn crankshaft as required. There is ample clearance.
Big ends will not pass through bores. Journal sizes 45mm diameter by 28mm
wide. End float, 0.002in - 0.004in. Gudgeon pin locked in small end of rod
with clamp bolt and tab washer. Floating fit in piston when warm. To fit or
remove clamp bolt, two brass or mild steel end pads are required which are
just less than diameter of pin holes and of such length that they stand
proud of the pistons. These can be made on a lathe or can be supplied by the
factory.
Clamp in vice in line with pin, so that no stress of tightening this bolt
comes on piston or rod. Pin is slotted so bolt must be completely withdrawn
for removal of pin. Clamp bolt must be to off side of engine.
PISTONS
Two types have been used. Up to engine number MPJG696 inclusive a plain
aluminium piston was used, having three rings at the top and one on the
skirt. Top two were plain compression, third a slotted scraper, all with
slant cut gaps. Bottom ring was stepped scraper, pegged in groove.
Clearance of piston in cylinder at 90° to gudgon pin 0.10mm or 0.004in.
From engine number MPJG697 pistons are of fully compensating type with three
rings above pin. Top two are plain compression; bottom slotted oil control.
Piston clearance 0.033mm - 0.061mm. or 0.0031in - 0.0024in. Ring gap for
either type of piston 0.004in - 0.006in. Rings easy fit in grooves.
CAMSHAFT DRIVE
Timing sprockets secured to shafts by single key. Timing chain is duplex
roller, two links being white. On each sprocket one tooth is marked "T".
Set chain so that "T" on each sprocket is in a white link with shorter side
of chain (13 black links) uppermost. This give inlet opening 11° early.
(Diagram shows the compete timing, which follows if the first point is
correct).
To find TDC, set the two bosses of screw holes in belt pulley on a vertical
axis, in a straight line with the arrow above on the timing case.
An alternative check on timing is on the flywheel periphery. Inlet opens 27.
8mm early and closes 149.3mm after BDC. Exhaust opens 141.7mm before BDC and
closes 60.7mm late.
Crankshaft sprocket secured by combined nut and starting handle dog which
grips pulley and distance piece, pulling sprocket against shoulder of front
journal. Camshaft sprocket held by centre bolt and tab washer.
TAPPETS
In blocks of four, dowelled below and held to cylinder block by two bolts
each. A circlip prevents tappets falling through block. Tappets not
adjustable. Hollow heads to take push-rods, flat base.
ROCKER GEAR
Rocker shaft carried in top of cylinder head in four plummer blocks, each
secured by two bolts. Rockers are of four different shapes, two to each hand
at different angles, and must be assembled correctly. Coil springs take up
end play in rockers and promote silence.
Valve clearance adjusted by screw and lock-nut in ends of rockers. Valve
clearance 0.010in inlet, 0.015in exhaust, when hot.
VALVES
Not interchangeable. Both have 8mm stems and are 122.3mm long, but exhaust
valve is 30mm and inlet valve 33mm diameter at the head. Face angle 45°.
Triple valve springs, split cone cotters.
LUBRICATION
By gear pump, skew-driven off centre journal of camshaft. Dipstick below
distributor. First three pints do not show on dipstick. Capacity of sump
13/8 gallons. Oil is drawn from sump by pump through a floating intake which
ensure cleanest oil being drawn in. Oil pressure is 60lb/sq. in., controlled
by spring-loaded non-adjustable by-pass valve. Oil is passed through
Tecalemit oil filter, which is arranged to by-pass oil if filter is left
unchanged. Renew element after first 1,000- miles and thereafter 10,000
miles.
Oil leads to internal gallery inside block and thence through drilled
passages to camshaft and crankshaft bearings. Big ends pressure-fed from
mains. Hole drilled in offside shoulder of big end sprays cylinder walls.
Front bearing of camshaft spills oil to chain wheel thrust face. Three
radial holes from chain wheel boss feed oil to chain. Clutch lubricated by
hole in centre of crankshaft.
Vertical pipe takes oil to hollow rocker shaft via rear rocker shaft support
block. Surplus oil from rocker chest flows down push-rod passages to sump.
Oil reaching clutch pit is picked up by starter ring and thrown into a
gallery on near side of flywheel housing, draining back to sump.
COOLING
By fan and impeller, belt-driven with dynamo. Carbon gland packing. If
impeller requires attention at the gland it is best to replace complete unit
with a service unit though the job can be done in the garage if essential.
At top tank of radiator is thermostat, inserted in water outlet pipe. This
is set to open at 80°C and is not adjustable.
CARBURETTOR
Two S.U. horizontal with hand control of jet; 1in choke; standard needles -
A.C. with M1 for rich, D for lean running. No service required except to see that piston
rises and falls. Check by removing air cleaner intake and inserting finger
at intake, raising piston and letting drop. If it does not fall freely it
may require a little oil. If this does not cure it the jet may be decentred.
A.C. oil wet air cleaner. Clean every 5,000 miles.
FUEL FEED
By S.U. pump, electric.
IGNITION
Coil and distributor, 12 volt. Distributor skew-driven off camshaft.
Automatic control with a range of 32° on flywheel from TDC at full
retard. Complete distributor retained in drive housing by set screw in
timing plate. Timimng plate has miocrometer adjustment. One division on the
scale is 4° on crankshaft. With clean engine and proper fuel setting
should be at zero when checking timing at TDC. Breaker gap 0.012in. Smear
light oil on cam to prevent wear of rocker heel.
GENERAL FEATURES
Electrical equipment includes 12-volt two-unit battery with earthed positive
pole; negative wired direct to pull-wire-operated switch on yoke of
starter, which is a four-pole machine with four brushes.
Dynamo is a two-pole machine with third-brush control, wired to control box
containing cut-out, field resistance, fuses and junction terminals of all
circuits. No chassis boxes are fitted.
Six panel lamps are included for instrument lighting and a map-reading lamp
for separate switch control.
CHARGING CIRCUITS
D and F2 dynamo terminals are wired direct to control box, where they are
connected in "Low" charge position by all the field resistance and the field
fuse. In "High" position terminal F1 - wired to panel switch only - is
bridged to terminal D, so that resistance is cut out and rate for given
speed increases. This also obtains "Side" position.
IN "Head" position all resistance is bridged out - also the fuse - so full
output is obtained. If field fuse is blown, test in "Head" position and
watch for cause of fuse failure such as flickering ammeter reading when
wires are manipulated, sparking at brushes, or bad connections. Do not race
engine as open circuit voltage may overload lamp bulbs.
Check condition of cut-out contacts by connecting voltmeter between D
terminal and top clip of right-hand fuse (A1). No appreciable volt drop
indicated good contact with dynamo charging.
LIGHTING CIRCUITS
Numbering fuses from left to right - one and two carry side and tail and
near side head respectively.
Off side head is supplied through contacts of dipper unit. Hornpush-dipper
switch on dash panel completes dipper solenoid circuit to earth, when unit
operates, dip contacts opening to insert hold-on resistance in series and
lamp contacts opening to cut out off side lamp, which is wired direct.
Blown dipper fuse indicates stiff action of plunger, pivots or contact
opening peg. Clean surface of contacts with magneto file.
Apply light machine oil to wearing surfaces. See that reflector is not
distorted when rim is fitted.
If side and tail lamp come on dim when brake is applied, see to earthling of
rear lamp body.
When testing stop lamp and brake switch action, remove No 5 fuse and connect
12-volt test lamp across clips. This will light dimly when circuit is closed
through switch.
AUXILIARY CIRCUITS
Fuses Nos 3 and 4 are fed via ignition switch and form junction terminals
for coil, petrol pump, and feeder from panel; switch "IGN" terminal.
All the panel lighting assembly is supplied by No 3 fuse, while No 4
protects screenwiper and trafficators where these are fitted.
A "30mph" warning light is supplied from common terminal of panel light
switch and its circuit is completed through contacts in speedometer.
WARNING LAMP
When fitting replacement use only correct type No 252MES. If bulb and
connections are intact and yet bulb does not light when switch is closed,
renew bulb-holder assembly which contains resistance.
Constant failure of bulbs indicates faulty cut-out adjustment.
Cut-out should close at 12.5 - 14 volts and should open when reserve current
reaches 2.5 - 3 amps.
If bulb does not black-out but dimly glows, clean cut-out contacts and look
for high-resistance connections. Volt meter test will reveal this defect.
IGNITION
When resetting timing note that tapped extractor holes in crankshaft belt
pulley mark TDC positions, where contact breaker should be just opening.
One degree variation allowed each way. On micrometer adjustment 1
division=4° crankshaft rotation. Breaker gap 0.012in.
Firing order 1-3-4-2. Auto-advance mechanism to be lubricated with thin oil.
STARTER
Check conductivity of each half of field windings and see to switch
contacts when overhauling.
Do not undercut micas. If field insulation is broken down, do not
remove pole screws unless pole screw driver equipment is available. Fit
exchange carcass.
ADDITIONAL FITTINGS
Extra circuits should not be wired to battery terminals but connected
between starter switch terminal and chassis, where current reading on
ammeter is not required.
For inclusion of ammeter connect to terminals A1 and E in control box: for
constant availability through fuse connect to A2 and E: and for ignition-key
control through fuse connect to A4 (No 4 fuse) and E
The original booklet also contains a copy of the wiring diagram, which
unfortunately would not reproduce here.
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WIRING SPECIFICATION FOR M.G. TA LUCAS 12 VOLT SYSTEM
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Dynamo
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C45NV
DA21
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Starter
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M418A
A84
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Cut out &
Fues Unit
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CJR3
L35
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Batteries
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2 x SLTW11E
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Distributor
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DK4A
A35
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Coil
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Q12
L
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Headlamps
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Lucas No 54
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Side, Tail
& Stop
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Lucas No 207
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Fog Lamps
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Lucas No 2
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Ignition
Warning
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No 252A
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Panel Lamp
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No 1224M
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30 Mph warning lamp, map reading lamp and dash lamps all use
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Lucas No 207
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Headlamps
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25 amps
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Sidelamps
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25 amps
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Dynamo field
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4.5 amps
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Accessories
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25 amps
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Dipping Refector
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6 amps
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..continued
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