Pedal washers are the washers for your pedals. Your new pedals may have come with them, or your new cranks may have come with them. Either way, you might need them. The thicker kind (~2mm) work better. If you only have the thin type (more like 0.5mm shims than washers) double or triple them up.
Some cranks have threads cut into the arms to accept the pedals. These typically don't require washers. Other crank arms have a big hole with a pressed-in insert. If you have an insert, you will need pedal washers. They are required! If your insert falls out there is no warranty because every manufacturer knows why; you didn't use washers.
Pedal inserts are found on many high-end cranksets. Why? You are less likely to strips threads when installing pedals and the threads will withstand more abuse since the insert is steel. We've seen aluminum threads directly cut into the arms get worn and destroyed before. Threads on pedals are steel or titanium which are harder than aluminum; that means the aluminum will give way and wear faster than the pedal threads.
Have you ever had the insert fall out the back of the cranks arm with the pedal still attached? Remove the pedal, install the washer, and put the pedal back on. The insert, washer, and pedal squish together and you shouldn't have any problems going forward. If that doesn't fix it you might try a thin bit of aluminum weld epoxy as a last resort. Pedal inserts can be replaced, but they are difficult to find.
