The famous 1.8mm spacer is what you need if you intent to use an 8, 9, or 10-speed cassette (any type) or an 11-speed MTB-specific (AKA larger than 34t big cog) cassette with a rear hub that has an 11-speed "road" freehub body. It's real confusing, no good reason for it, sorry.
If you want to measure to be really certain, the narrower freehubs are 35mm and the road-11 are 36.8mm.
Made of aluminium, fits HG cassette bodies.
