Mine measure just under 17mm. I used a 17mm ring spanner as (unintentionally) it seemed to have the effect of keeping the force low down, reducing twisting. I used moderate taps with a club hammer as it gives a steady accurate firm sharp one-off tap. Not perfect but for me it minimised marking on the tightest one. Sure enough as others find, the seals were flat and hard, and someone had over tightened the caps which were leaking slightly. I wonder if that special grip paste might help, or something like a shim to make a 17mm spanner fit more tightly, although my outcome was ok. The slightly marked one can be moved out of sight!