Originally Posted by DannoXYZ

Exact how tight did you install them? Did you install them on dry spindle? Oiled or greased? Without an exact torque-figure, they could've been put on too tight or too loose; both of which will result in the crankarm falling off later.

Here's a list of common torque-specs from Park Tool. Square-taper cranks typically require 25-33 lb*ft of torque. This is impossible to do with little L-shaped allen-keys and really requires an automotive-style ratchet-wrench and socket. Preferably one with an 18" handle or longer to achieve the required torque.

Don't worry, you're not alone:

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