Piff 

Went looking for an answer but came upshort. The question is short and simple, will a 7800 cassette, meant for a freehub with tall splines, work with a regular hyperglide cassette? Specific usecase is '9 of 10 on 7'.

My inclination is: 'Of course', but wanted to double check here first.

Thanks.

georges1

the CS 7800 will fit on dura ace 7400,ultegra 6400 and 105 SC 8 speed hubs as well as on a dura ace 7700, ultegra 6500 and 105 5500 9 speeds hubs and even on 11 speed hubs such as Shimano 105 5800,Ultegra 8000 and Dura Ace 9000 with a spacer.

Piff 

Great! I was mostly worried about the freehub splines not being tall enough. Build is a go!

KCT1986

Yes, in most cases no issues. Just need to check that the outermost sprocket engages the end of the spline.

There a few 7 speed freehub bodies that have the shortened splines for use with an 11 tooth outer sprocket, but these are uncommon.

roadcrankr

I believe you need to use the spacer (that came with a new cassette) for it to work on 8 or 9 speed freehubs.
Conversely, Shimano took a brief departure with their 7800 freehub, making only 7800 cassettes suitable.

georges1

Here are some interesting articles about 10 speed cassettes compatibility on 8 speed hubs and 9 speed hubs Cassette Compatibility Chart and A guide to freehub body and cassette compatibility

plonz 

This extract from the second link states "And the fourth stipulation is in regards to 6 and 7-speed HG-style cassettes and freehub bodies. Both 6 and 7-speed HG cassettes are narrower in width and therefore can be installed on any of the above HG freehub bodies with the addition of a 4.5 mm spacer behind the cassette. However, newer 8, 9, 10, 11 and 12-speed cassettes cannot be fitted to old and narrower 7-speed freehub bodies." This indicates a CS-7800 10 speed cassette does not fit on a 7400 freehub. I think OP intends to remove an inner cog make it a 9 speed so maybe it will work on a 7 speed freehub. Guessing this will come down to trial and error.

KCT1986

Yes, it seems that the OP plans to use a 7 speed freehub body and run 9 of 10. With the CS-7800 the inner sprockets are on a carrier, so one of the 'dropped' sprockets will have to be one of the loose ones in the middle or outer end.

Removing 1 sprocket and spacer (if not integrated) should bring the 'stack' width down to 32-33mm, (taking into account the recessed mounting of the inner carrier). This width should fit on a 7 speed body. The thing to check is that the chosen outermost sprocket is able to engage the end of the spline. Generally, not a problem unless the freehub body is the 'C' version, which has a shortened spline for an 11 tooth outer.