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One simple check you can make is to look at the end of the spindle vis a vis the arm after you have it torqued down.

Withdraw whatever fixing bolt you are using and have a look to see that there is some freeboard between the end of the spindle and the seat for the bolt head and washer in the crank arm.

Alloy cotterless arms can only be mounted so many times before they open up a bit to where the end of the spindle can come up to the seating surface for the fixing bolt.

One simple solution when this occurs is to grind a bit off the end of the spindle.

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