I have assembled and disassembled many Sugino cranks, and never needed the special tool... What causes the problem to need to hold the back of the chainring bolts?
I have always been able to loosen, then just apply pressure with my finger to allow me to remove the bolt, and I have always had success tightening as well.
The reason I ask is, if this is a common issue, I should get a tool to keep on hand.
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Slow Ride Cyclists of NEPA
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