Cassete spacer for CS 7800 and WH 7900 Should I use spacer on CS-7800 who is being installed on WH-7900 ?
Or just install the cassete without 0.5mm spacer ?

Cheers and thank you..
Shimano Dura Ace wheels do not require the extra spacer behind the cassette that every other wheel requires. So, no the 7900 wheel should not require a spacer with the 7800 cassette.