ARA 326 tank
The factory standard plate pairs this tank with GRC 110 cap. MGOC describes the tank as plastic. A used ARA 326 example shows the molded number on a broad rib and two integral mounting ears.
1975 MIDGET 1500 / COOLING CONFIGURATION
Find the system
actually fitted.
British Leyland issued an overheating bulletin for 1975 Midgets in August 1975. It changed several connected parts. Use this guide to identify what is on your car before choosing a pressure cap, ordering hoses, or deciding what an overheating symptom means.
A lead on the outlet casting
Tentative photo reading: possible CHA343. The raised characters beside the filler plug in IMG_6895 appear compatible with this number. The angle and surface texture prevent treating that reading as a confirmed transcription. MGOC lists CHA343 as a 1500 thermostat housing; this is a catalog comparison, not proof of the fitted casting or interchangeability. Locate the marking in the owner photo ↗
The photograph positively shows a filler plug on this upper outlet. A seller’s plain elbow without a filler opening differs in that visible feature. We have not established a fault requiring outlet replacement, nor matched its joint, hose neck, branch arrangement, plug thread or seal. Do not choose a replacement from one tentative number or the shared Triumph engine family.
The CHA595 housing specified by bulletin 75-A-6 remains a separate historical reference. A possible CHA343 mark does not establish whether this car received all, some or none of that coordinated modification. Tank/cap, upper-hose tee, switch, small hoses and other kit parts still need their own identification.
- With the engine cold, photograph the entire raised marking square-on and with light from another direction. Copy every character; keep any unreadable character explicitly uncertain.
- Record both outlet fixing positions, the filler opening and every visible branch with surrounding landmarks. Keep upper-outlet and lower pump-housing features separate.
- Trace the complete upper hose, tank/cap and any added switch or tee independently. Keep those findings separate from the casting-number reading.
This observation changes the evidence record only. The CAD casting, dimensions, hose routes, threads, fastener assignment and removal path remain provisional.
When the bulletin applied
Its stated trigger was an instance of severe overheating on a 1975 MG Midget. It also instructs the mechanic to establish that ordinary faults, including head gasket trouble, loose head bolts, incorrect ignition timing and an improperly adjusted fan belt, were not the cause before making the modifications. The photos do not show that this car has overheated or that the bulletin was carried out.
Compare the bulletin with your car
The bulletin describes a package of changes. A single replacement part would not establish that the complete conversion is present. Record each finding separately.
URP 1148 tank
The factory modification plate replaces the original with a metal tank, URP 1145 double-seal cap, separate tank brackets and a BHA 5239 spill bottle. It also specifies the upper-hose switch tee and modified thermostat housing. Check the parts as a group.
In your photos: the pale shell is compatible with the standard plastic-tank description, but no readable molded number, cap underside, complete upper hose or spill-bottle view establishes the fitted arrangement.
Identify them together
Model year cannot distinguish the standard and modified systems. IMG_6849 shows a cap marked 14–17 lbs on a pale, seamed tank, but the cap seal, tank neck and molded part number are hidden. The standard plastic ARA 326 is a plausible comparison, not an identification. The bulletin says its URP 1145 double-seal cap is not interchangeable with the original cap.
On the car: when completely cold, photograph every side of the tank, especially the broad rib below the filler neck and both mounting ears, and transcribe any molded or stamped number. Photograph the cap's underside, the tank's inner and outer sealing surfaces, and the neck depth. Match tank and cap as a pair against the correct parts plate before buying either; have the cap pressure-tested if its condition is in doubt. For a closer virtual fit, record the tank shell’s three outside dimensions and its left/right, top/bottom, and front/back offsets from the finned radiator core in the CAD tank inspector. Never remove a hot cap.
Look for the added switch
The conversion places thermo-vac switch C-37430 in a 158308 T-sleeve adaptor in the top radiator hose, with vacuum and water hoses routed to the modified system. The current photos do not show the full hose clearly enough to call it present or absent.
On the car: trace the entire upper hose from radiator to thermostat housing; photograph any inserted sleeve, switch and small hose connections.
Check the right-hand shock mount
The bulletin adds no-loss bottle BHA-5239 on bracket CHA-585 at the right-hand upper shock absorber support plate. No current photo positively identifies this bottle.
On the car: photograph that mount area and trace any small hose from the tank or cap. Keep its actual routing in the record.
Do not infer them from the tank
The bulletin also specifies thermostat housing CHA-595, externally vented carburettor deceleration valve AAU-2346, replacement temperature gauge AAU-2641, repositioned horns and front number plate, plus a final pressure test. None of those identities has been confirmed from the published photos.
On the car: photograph part markings and the front bumper and horn area; record what you can verify and leave the rest unknown.
Check the switch vent and gauge
British Leyland's May 1976 bulletin 75-A-6A says some installed USA-300 kits retained a yellow shipping plug over the thermo-vac switch's atmospheric vent P-1. It says the blocked vent can leave timing excessively retarded under some conditions and raise engine temperature. The same bulletin stresses fitting the kit's replacement temperature gauge, because the original gauge would give incorrect readings with the modified system. These are conditions to check if the kit's switch is actually present; the current photos do not identify it or the gauge.
On the car: first identify and photograph the switch and its molded port labels, hose connections and vent. Check the gauge's part number or documented replacement history. Do not treat a gauge reading or a loose yellow item alone as proof of a complete conversion.
What to do with the result
1
No overheating history established
Complete the cold cooling inspection, including the fan-specific plan and record the parts actually fitted. Do not order a cap from the model year alone. The historical bulletin is a reference for identifying the system, not proof this car needs a conversion.
2
Overheating or coolant loss observed
Record when it happened, what the gauge showed, coolant loss and any visible leaks. Let the engine cool before inspection. Check the basic cooling system and belt first, then investigate the ordinary faults named in the bulletin. Do not keep driving a car that is overheating or losing coolant.
3
Repair or change proposed
Match the fitted tank neck and cap, hose arrangement and other installed parts to a procedure for that configuration. The bulletin describes a coordinated period modification and a pressure test; its old kit number is not a substitute for an available, verified parts list today.
Evidence and limits
British Leyland Technical Service Bulletin 75-A-6, August 1975 ↗ is the primary source for the trigger, parts, diagnostic warning and installation outline. The factory 1977 parts catalog, PDF pp. 86–87 ↗ distinguishes the standard ARA 326/GRC 110 tank and cap from the bulletin's URP 1148/URP 1145 assembly, and lists the other modification components. The May 1976 follow-up, 75-A-6A ↗ documents the P-1 shipping-plug and replacement-gauge checks. The Moss 1500 cooling plate, PDF p. 37 ↗ notes the original modification parts are no longer available. IMG_6849 is the primary evidence for this car's printed cap marking. A printed range cannot confirm its test result or fit. The part identities and presence of a complete modification on this car remain to be checked in person.
Save a cooling configuration project ↗ · Open tank photo locator ↗