Three clearly different heater layouts were used across Midget production, each with unique components that are not interchangeable between systems. All three work on the same principle: air is drawn over a hot-water-fed matrix, with temperature controlled by varying the water flow through a heater tap. The heater layout fitted must be identified before ordering any components, quoting the chassis number ensures the correct specification is supplied.
Early Heater System
The early heater system was fitted from 1967 to approximately mid-1970 on 1275cc cars. It uses a square-profile heater case with a separate external blower motor drawing air from the wing via a wire-reinforced flexible hose. The heater tap is mounted on the cylinder head. Within this early period a further distinction exists: 1967-build cars had the air control valve in the front panel, while subsequent cars relocated it to the blower assembly, resulting in a different control cable length (48 inches versus 28 inches).
The cable specification must be confirmed before ordering.
Later 1275cc Heater System
From approximately mid-1970, a revised heater was fitted with a longer casing incorporating an integral fan, eliminating the separate blower motor. The air intake hose changed from wire-reinforced to a moulded PVC type. The control cable is 28 inches. The heater tap remained on the cylinder head with a return pipe to the radiator hose.
This system was used through to the end of 1275cc production.
1500 Heater Layout
The 1500 heater is a mirror-image version of the later 1275cc heater, adapted for the revised engine bay layout. The matrix is the same unit, but the heater case, air intake, and water connections are 1500-specific. The 1500 heater tap is fundamentally different, rather than mounting directly on the cylinder head, it uses a water control tap and elbow assembly on a footwell bracket, with water supply from the water-heated inlet manifold via an adaptor on the thermostat housing. The return pipe runs to the water pump housing.
The 1500 control cable is 23 inches.
Heater Matrix
The heater matrix is available for all three systems, and an uprated matrix with improved heat output is recommended during any heater overhaul. When removed, the matrix should be back-flushed by connecting a hose to the outlet pipe and flushing in reverse to dislodge the sediment that accumulates over years of use. The foam seal wrapping the matrix within the heater case must be renewed whenever the box is opened, a degraded seal allows air to bypass the matrix rather than passing through it, drastically reducing heater output even with a new matrix fitted.
1275cc Heater Tap
The 1275cc heater tap is a manual valve mounted on the cylinder head, operated by the dashboard control cable. A tap washer provides the internal seal, and replacement washers are available individually for taps where the body is otherwise serviceable. The tap should be treated with anti-seize compound on the cylinder head studs during refitting, these studs are invariably seized after years in situ and will shear if forced.
1500 Heater Valve Kit
The 1500 heater tap arrangement is more complex than the 1275cc and is prone to seizure of the valve mechanism. A heater valve kit addresses these issues with all the components needed to restore full function, tap, elbow, seals, and associated fixings. The 1500 tap assembly should be dismantled and cleaned periodically to prevent the valve seizing in the closed or partially open position.
Control Cables and Hoses
Control cable lengths differ between all three heater layouts: 48 inches for the earliest system, 28 inches for the later 1275cc, and 23 inches for the 1500. The cable outer should have an insulating rubber sleeve where it passes through the bulkhead near the battery, this prevents the cable chattering on the heater case and protects it from battery acid and heat. Heater hoses differ between all three systems, and all hose connections use worm-drive clips. Hoses should be inspected regularly for softening and swelling, as a burst heater hose will empty the cooling system rapidly.