The MGB GT V8 instrument binnacle uses 80mm diameter gauges in a V8-specific layout. The main instruments are the speedometer (with integral odometer and trip meter) and the matching tachometer (engine rpm gauge), with smaller gauges for fuel level, coolant temperature and oil pressure. The binnacle is V8-specific and is mounted on the V8 fascia panel HZA4916E. Production change at V8 1825 (June 1974) revised the gauge calibration; cars before this chassis number used the earlier calibration.
Speedometer and Cable
The V8 speedometer reads to 140mph and was offered in mph specification only, no kph version was produced. The speedometer is driven mechanically by a flexible cable from the gearbox output area, with the cable rotating an internal magnet that drives the speedometer needle via a magnetic coupling. The internal odometer and trip meter are driven by gears from the same shaft, recording total mileage and trip mileage. Speedometer faults usually result from cable failure (the cable inner snapping, or wearing at the end fittings) rather than gauge head failure.
The Production Change at V8 1149 (November 1973) added an oil pressure gauge that had not been fitted from the start of production.
Tachometer
The tachometer is electrically driven, sensing the ignition coil pulses and converting these into rpm reading. The factory specification matches the V8's firing pattern, eight pulses per two engine revolutions in the four-stroke V8. Tachometer accuracy depends on consistent ignition pulse generation; misfires or weak coil output can cause tachometer needle flutter or low reading.
Fuel and Temperature Gauges
The fuel gauge reads the variable resistance from the fuel sender unit in the tank, indicating fuel level. The temperature gauge reads from a sender unit in the engine cooling system. Both gauges use thermal-strip operation common to the period, providing damped readings that respond slowly to changes, a deliberate design feature that avoids alarming needle movement under normal driving conditions.
Oil Pressure Gauge
The oil pressure gauge is mechanically operated by a small-bore pipe from the engine oil gallery to the gauge head, with engine oil pressure directly moving the gauge needle. The gauge was added at V8 1149 (November 1973); earlier cars had only an oil pressure warning lamp. Mechanical oil pressure gauges are reliable but the small-bore feed pipe can develop leaks at fittings, dripping oil into the cabin.
MGOC Spares
MGOC Spares supplies V8 instrument components, gauges, speedometer cables, sender units and binnacle service components for the MGB GT V8.