Thermostat for early DBS 6 or AM Vantage 6 cylinder with bypass hose from water pump to thermostat body, this is correct type of thermostat for this engine when using the bypass hose that connects the thermostat housing to the end of the water pump, this is not what is sold via other suppliers inc Heritage Dealers which is incorrect. 

This is used with our 069-006-0103 thermostat housing bypass plate, not the original plate on your engine which no longer fits the thermostat !.

Available in 74°c, 82°c & 88°c

Rough Description of how the bypass system works

A thermostat is placed between the engine and the radiator to make sure that the coolant stays above a certain preset temperature.  As the engine warms up the coolant will be below the thermostat temperature or if the coolant temperature falls below this temperature, the thermostat blocks the coolant flow to the radiator, forcing the fluid instead through a bypass directly back to the engine (normally sucked through by the water pump), this has the effect of quickly raising the temperature to or back to the thermostat temperature.  The coolant will continue to circulate like this until it reaches the design temperature, at which point, the thermostat will open a valve and allow the coolant back through the radiator.

So in effect it makes the cooling system more efficient.