Cooling the AC Light Six
This is the page to read before you top up. The AC Light Six is a mixed metal engine, and the wrong antifreeze in it is not a maintenance error but a slow structural one. It also has a water pump with a design fault that no amount of careful reconditioning will cure.
Which engine this covers
This page covers AC's own Light Six, chassis prefix AE. If your car has the Bristol 100D2, prefix BE, see cooling the Bristol six instead. The chassis guide explains the prefixes in full.
What the engine is made of, and why that decides everything
An alloy cylinder block with steel wet liners, a cast iron cylinder head, steel head studs and originally a copper head gasket. That combination, four metals in one water jacket, is the worst case for galvanic corrosion rather than the benign all aluminium case people often assume. Note the head in particular: it is cast iron, not aluminium, on every source the archive has found.
The consequences are documented rather than theoretical. Head studs corrode and snap, and can become fused into the block. Deterioration of the water jacket lets coolant climb the head studs and appear on top of the head. Water past the liners turns the oil to a grey sludge. And the copper gasket in contact with the alloy block corrodes the block, which is why the specialist who rebuilds these engines now supplies a copper and asbestos substitute gasket rather than the original.
AC6, who rebuild this engine, put the consequence plainly: it is a waste of time rebuilding an AC engine unless the cylinder block is in really good condition, meaning no corrosion present. Coolant is therefore not a consumable on this engine so much as a corrosion inhibitor that happens to carry heat.
What coolant to use
The Federation of British Historic Vehicle Clubs is unambiguous, and its guidance is reproduced by the MG Car Club and by the Bristol Owners and Drivers Association. Use an IAT antifreeze, usually blue but sometimes green in historic vehicles. IAT products inhibit corrosion with silicates, and silicates are wanted here precisely because they protect engines with mixed metals.
Do not use OAT, the long life coolants usually coloured red, orange or pink. The Federation's wording is that OAT antifreezes have been accused of destroying seals and gaskets and causing a great deal of damage in older engines. HOAT is likewise not recommended for historic vehicles. This is not merely enthusiast caution: Penrite's own product page for their OAT blue coolant states that it is suitable for classic and modern vehicles but not for vintage cars, because the OAT chemistry has been found to be damaging in older vehicles. When a manufacturer tells you not to use their own product in your car, that is worth more than any forum opinion.
Never mix coolant types without flushing the system thoroughly first, and renew within the antifreeze maker's stated interval, because the inhibitors deplete whether or not the antifreeze is still protecting against freezing.
Three further points specific to this engine, all from the AC two litre community, whose engine is the same unit. Keep an inhibited coolant in all year round rather than draining for summer, because the alloy castings are prone to corrosion and the inhibitor is the point. Use deionised or reverse osmosis water rather than tap water. And do not drain the system and leave the engine standing empty, which is the worst of all worlds.
The water pump, and its design fault
The pump on post war engines is mounted on the side of the block and driven by belt from the crankshaft pulley, and it pumps coolant into the engine. On vintage engines it was driven from the front of the camshaft instead.
The fault is in the pulley. It is not bell shaped as on most cars, so the fan belt load is offset from the ball race bearing, which places a side load on the plain bearing. That is a design problem rather than a wear problem, which is why AC6 say that reconditioning a pump to original specification does not last. Their redesigned pump doubles the front bearings inside an extended pulley, replaces the bronze bush with a synthetic one and fits a modern one piece seal in place of the old carbon thrust seal. Carbon water seals failing, pulleys, bearings and shafts wearing and alloy casings corroding are the failure modes they describe.
One practical piece of owner advice worth carrying: a short fan belt that will drive the fan but not the water pump, so that a failed pump bearing does not end the journey.
Temperature, and reading the gauge honestly
The system is not pressurised, and the normal running temperature is around 75 degrees, with a bellows type thermostat rated at 78 degrees. Some coolant loss through the radiator overflow is normal on an unpressurised system and is not in itself a fault.
The temperature gauge bulb sits in an adaptor to the right of the thermostat, and that adaptor tends to clog with scale. A scaled adaptor reads low, which is the wrong direction for a warning device. If a gauge reading looks improbably good on an engine of this age, suspect the adaptor before believing the needle.
Coolant tends to stagnate at the rear of the water jacket, where debris is deposited. That is the mechanism behind circulation loss and, in time, head gasket trouble, and it is an argument for flushing rather than merely topping up.
Running in traffic
The mechanical fan turns with engine speed, so it runs slowly exactly when it is needed most. An electric fan is the usual answer, and is particularly worth having for heat soak after fast driving rather than for the traffic itself.
Capacity, and an honest caveat about it
Seventeen pints, which is 9.6 litres. That figure comes from the AC two litre saloon rather than from an Ace or Aceca source, and the sports cars have a different radiator and different plumbing, so treat it as an indication and confirm against your own car or the club. The archive would rather tell you where a number came from than let you assume it was measured on an Aceca.
What the archive could not establish
Whether there is a thermostat bypass circuit. No source confirms or denies one, so nothing is asserted. The radiator core type and capacity, and whether the cores are soldered brass, which would sharpen the case against OAT further. The coolant capacity and thermostat rating for the Ace and Aceca specifically, as distinct from the saloon. And a pressure cap rating, since the saloon system is unpressurised and this was not separately verified for the sports cars. The authoritative answers live with the AC Owners Club, whose members' area and Weller engine forum board the archive cannot read.
A warning about specification websites
This applies to every page in this section. Several general specification databases serve Ford 2.6 or Bristol figures under the heading AC Ace, without saying so. Cooling capacities, valve clearances and plug types found on such sites are frequently for a different engine entirely. Treat any figure as unusable unless the source names the AC engine specifically. The archive has thrown away several plausible looking numbers for exactly this reason.
Related reading
- AC Light Six oil & filterGrade, oil pressure, the bypass filter and the zinc question.
- Servicing hubWhere each task fits in a full service.
- The AC Light SixBackground to AC's own two litre six.
- Parts and specialistsWho supplies parts for this engine, AC6 among them.
This guide is not a substitute for the official AC Workshop Manual. If anything here is wrong or can be improved, please get in touch.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.