The Chieftain's L60 engine and cooling system were designed into an integrated engine-pack which could be changed "in the field" using the crane of an FV434 Armoured Repair Vehicle, which had been designed for this purpose, and a complete engine change took around one-and-a-half, to 2 hours.

The requirement for an easily changeable engine pack was the result of a British Army analysis of previous tank battles that concluded that a likely future tank battle would last no longer than two hours and so the most demanding requirement expected for any tank engine during wartime would be for it to be run at full power for this total amount of time only, and so it would then be advantageous for it to be removed from the vehicle after the battle and exchanged for a fresh engine within a minimum of time.

This practice would also allow the removed engines to be worked on in properly equipped REME workshops rather than 'in the field', the engines being exchanged between vehicles and workshops as-and-when required.
This philosophy was also applied to the contemporary FV430 series of vehicles.
The Chieftain's L60 engine and cooling system were designed into an integrated engine-pack which could be changed "in the field" using the crane of an FV434 Armoured Repair Vehicle, which had been designed for this purpose, and a complete engine change took around one-and-a-half, to 2 hours.
The requirement for an easily changeable engine pack was the result of a British Army analysis of previous tank battles that concluded that a likely future tank battle would last no longer than two hours and so the most demanding requirement expected for any tank engine during wartime would be for it to be run at full power for this total amount of time only, and so it would then be advantageous for it to be removed from the vehicle after the battle and exchanged for a fresh engine within a minimum of time.
This practice would also allow the removed engines to be worked on in properly equipped REME workshops rather than 'in the field', the engines being exchanged between vehicles and workshops as-and-when required. This philosophy was also applied to the contemporary FV430 series of vehicles.
Leyland L60