POI - my 559X20(interior) actually measure 24cm exterior width.
Sooooooo - you should be able to measure the exterior width with like a large crescent wrench, or calipers, or plier handles that don't move very smoothly, and subtract 4cm, which should give you your interior measurement - or damn close, anyhow.
Last edited by Wanderer; 10-02-08 at 06:48 PM.