Defective Rebound Damping Adjustment?

[markzxr400l3]

Hi, I am now in the happy position of trying to get my bike 'just right' so I started on the suspension settings and I've immediately hit a snag. I put the front forks back to standard (L1) with 6 rings showing on the preload and 6 clicks out on the rebound damping. Did the right hand side one fine but the left rebound screw didnt stop when screwed in!!! The measurement in the manual is clicks out from fully screwed in so I have just set it to the same height for now but I presume there must be summat wrong for the screw to carry on into the fork top? The swine of it is that I've just sold another pair of forks that have been kicking around the garage for ages or I could have just changed the tops (I assume).

Any advice? Can I just start counting clicks from level that other fork hits the stop or do I have to rebuild the forks?