Replacing an 8-Speed Cassette with 9-Speed

[chr15b]

I have a Boardman road bike with a 9-Speed Sora setup. I'd been given a new Shimano R500 wheelset but it had an 8-Speed cassette on it. I've removed this and fitted a 9-Speed (12-23) cassette but there is a short gap between the lockring and 12T cog. Do I need to fit a spacer before the first set of cogs, or after the final cog? Photo for information - you can see the gap between the lockring and 12T cog.

Unfortunately I don't have the original 9-Speed cassette and/or the original wheels.

 

[Cycleops]

Yes, you need a spacer, not sure if it needs to go under the lock ring or at the back of the cassette.
Must be the back, found this on Amazon:
Amazon product ASIN B001GSKSNYView: https://www.amazon.com/SHIMANO-HG-Gear-Cassette-Spacer-2-56-mm/dp/B001GSKSNY

 

[Darius_Jedburgh]

Back of the cassette, surely?(And don't call me Shirley)
First thing to put on.

 

[DCBassman]

I've the same wheels and have not needed spacers going from 8 to 9...

 

[Ajax Bay]

I'd been given a new Shimano R500 wheelset but it had an 8-Speed cassette on it. I've removed this and fitted a 9-Speed (12-23) cassette

A R500 wheel's freehub is specc'ed as 8/9/10sp compatible. Might the freehub not be original? If it had been replaced with an 11sp freehub then when the OP took off the 8sp there'd have been a spacer between the hub flange and the largest sprocket.
If so you'd need the same for the 9sp cassette too (1.8mm). NB this is not the spacer @Cycleops linked to - that's a 'normal' 9sp 2.56mm spacer that goes between sprockets - the image in the OP looks as if all the 'normal' ones are present and correct.

 

[Gunk]

I went the other way, 9 to 8 and it just went on and worked, I am sure I read somewhere 8 and 9 speed cassettes are the same width.

They are the same width. You need a 1.85mm spacer, they’re about £6.

 

[chr15b]

Thank you all for your replies. I can't understand why my 9-Speed cassette is shorter than the 8-Speed one when the 8-Speed had no spacer on it.

I went to the LBS and picked up a new 9-Speed 11-28T and that fit perfectly, didn't even need to re-index the gears.

As for the one that doesn't fit... I suspect it's a 10-Speed cassette with a missing sprocket.

 

[Ajax Bay]

I suspect it's a 10-Speed cassette with a missing sprocket.

Well done sorting it.
Suspect all you like, but a 10sp cassette slid onto the freehub would be one mm narrower than an 8sp or a 9sp cassette. And a 10sp cassette missing a sprocket would be about 5+mm narrower - not what the OP image shows. I guess we'll never know.