Brian,
Yes, I think that you are absolutely correct. The internal diameter is fine, but the outer diameter is something like 2mm too large.
However, There are multiwave compression rings thas can be obtained from : –
http://www.assocspring.co.uk/p/41/multiwave_cor
They produce a spring that will fit without messing about. It has a 28.6 o/d, 21.6 i/d and either a 7.6 to 3.7 compression or 12.6 to 6.3 compression ; the latter (or the former, if there is to be little force on the leg) may be best.
They cost (wait for it!) £12.16 each plus p/p ; expensive, but should save much messing about.
Chris.
