Raising the JET maximum toroidal field from 3.45 to 4.0 T results in increased forces and stresses in the key machine components during normal operations and disruptions. These forces and stresses have been predicted for 4.0 T operation and compared with allowable values derived from tests on used and spare JET coils. In this assessment significant lessons related to the design, manufacture and operation of key Tokamak components have also been learnt. (C) 2001 The European Commission. Published by Elsevier Science Ltd. All rights reserved.