I need to replace my worn Sunrace 8 speed cassette but I can't find any anywhere. I am going to change it with a 9 speed Shimano cassette and get a new 9 speed chain.
I was wondering if my existing chain ring (FSA Tempo) will cope with this, seeing as the new chain will be a tad narrower than the 8 speed chain that's on it at the moment.
Also, will I need to do anything with the shifters to accommodate the new gear (after adjusting cable tension).
Cheers.

Cassette here:
https://www.biketiresdirect.com/prod...speed-cassette
You need to replace your 8 sp shifters with 9 sp shifters.
Maybe rethink this idea.

Every hub I know of that takes an 8 speed cassette will also take a 9 speed. The main thing is whether you have indexed 8-speed shifters--they only have 8 clicks. If it can be set to friction (some older mtb/hybrid thumb shifters and bar end shifters will let you switch between indexed and friction) then it shouldn't be a problem. If not, you'll have all sorts of problems shifting.

Are you also changing shifters? Unless you are using a friction setup, 8 speed shifters will not properly index a 9 speed cassette.
Edit: I see everyone answered at the same time.

8 speed shifters will not work with a 9 speed cassette no matter what adjustments you make

8-speed shifters have 7 clicks/8positions. I agree that the shifter positions need to correspond with the number of cogs.

Jinx!

Thanks for your help guys. I was hoping that wasn't going to be the answer but you've saved me a lot of pain.

Thanks for your help and the link. I'm in the UK so will need to find something here, I'm sure I will. I take it a Sram cassette will fit my Shimano hub no bother?

Jenson USA has both Shimano and SRAM 8-speed cassettes available and they both work on the same hub and with the same shifters as your Sunrace. Look here: https://www.jensonusa.com/search?q=8-speed+cassettes

Maybe you should bite the bullet and change to 9/10/11? 8 speed is the red-headed stepchild of the bike industry as you are finding out. I guess though you'll always find someone here on the forum pointing you to an obscure manufacturer of one-off parts. I just prefer to stay with Campy and Shimano, especially for drive train parts.

Replace the chain

I usually run Sram PG-850. Unfortunately a bit heavier than ye olde 105 and up cassettes, but work fine.

Make sure you select the 11-28, 12-23, or 12-26. Some road derailleurs can't handle the other combos shown.

https://www.amazon.co.uk/Sram-Casset...65&sr=8-8&th=1

I find tons of 8-speed cassettes. They have standard mounting and you can swap sizes and manufacturers. There may be some retuning required when the cassette is not exactly the same as previous, but, if the changes are not extreme, it tends to be slight.