| Felix,
When the clutch first went in and all torqued up the hub was loose??? took us a while to figure out (was it the wrong spacer, washers missing etc) but then we removed the spider spring and compared it to a brand new one, it isn't flat but has definitely been squashed down a few mm's thus the spring when in situe wasn't applying enough force to the drum to keep everything locked in place. We replaced the spring with a new one and straight away this cured the issue.
Yesterday when Gizmo was swapping the plates he noticed the spring was moving but everything was tight as expected, seems to me (although like I mentioned ^^ there I didn't hang around) that the spring has done the same and been flattened slightly after some use?? | 
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