The spacers sit on top of the fork spring, between spring and cap. Kev's (and those in the photo) were made out of aluminium, but the can be made from mild steel, stainless or acetal (tough chemical resistant plastic) tubing quite successfully. In fact acetal tubing is the easiest to work with for the DIYer as you don't need access to any specialist tooling. It's cheap too. I have always used this material to make spacers and stand offs.

36mm OD x 20mm ID off the shelf acetal works perfectly for XT spacers:

https://www.metals4u.co.uk/plastic/acetal-tube/36mm-od-20mm-id-natural/p2033

You can get four spacers made out of a 100mm length for less than five quid.

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