I have a similar problem to Simon - the supplied M6 spinner screw doesn't reach or fit the M5 crankshaft thread and adapters are n/a. But I Found High-tensile M5x120 & M5x130 on the modelfixings website at
www.modelfixings.co.uk/high_tensile_steel_caps.htm
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The longest screw reaches the crankshaft for my spinner but I will need to sleeve the nose of the spinner from 6mm to 5mm.