The washer has two sides. Make sure that the thru-bolt head is resting in the recessed side of the washer.
If the cap will not fit over top of the washer, use needle nose pliers and gently bend the edges of the cap outward, slide the cap over the washer and use the pliers to gently squeeze the edges of the cap over the washer.
The cap cam be stretched over the thru-bolt screw. If you are having issues, you can warm it up a little bit with a hair dryer to get the cap to be a little bit more pliable.
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