wroomwroomoops

Bike has 8sp (revo)shifter, but only 7 cog cassette - something the previous owner did. I have had it with the shifting issues on this MTB, so I decided to actually get myself an 8 cog cassette for it (of course, I know I will have to tune the RD). Well, the LBS tells me the cheapest 8sp cassette they can order, would cost me 25 EUR - no way, I think to myself, and went home with the intent of fitting a 9sp cassette that I have in a drawer.

The problem is, I can't fit the last cog on the freehub, the freehub is too short. If I, however, remove that last (smallest) cog, the lockring that threads onto the freehub and keeps the cassette in place, doesn't keep the cassette firmy in place in that case - there's about 1/2 mm play there.


What to do?

Retro Grouch 

If you had size 9 feet would you expect them to fit into size 7 shoes? You're trying to force fit parts that simply aren't designed to fit together. Because of the differences in the spacing between the cogs, that 8-speed shifter is going to work better on a 7-speed cassette than it will with a 9-speed cassette that has had one cog removed.

wroomwroomoops

Actually, Sheldon Brown says this can be done:

Any 7-speed Shimano Hyperglide Freehub will actually work with 8 sprockets, without any modification! What you need to do is to use 8 of the sprockets from a 9-speed cassette, with the 9-speed spacers.

To make this work, you'll also need to use a 9-speed chain and shifters. Your old 7-speed derailer should work OK if it isn't too badly worn. The limit stops on the derailer will cause the useless 9th position on the shifters to be locked out, so this will work as a perfectly normal 8-speed rig.

https://www.sheldonbrown.com/k7.html#897

So what I am doing is not impossible, it's just that there is this 1/2 mm play that I didn't expect.

wroomwroomoops

Good point on the shifter - damn!

Back to the drawing board I go.