I already checked with original size of servo's bolt. (which are inside servo bag if you buy it online.)
[length is exclude bolt head)
Horn's axis x1 : ØM2.5 , length= 3.5mm
Shoulder mounting x2 : ØM2 , length= 8mm
So, if anyone already have a fancy anodized bolt or nut or washer from RC parts, you can add extra total length of bolt you need when add washer, and replace it on Nybble if you like.
Caution : Don't forget to check any clearance of parts.
Just a note on what zetathix is talking about:
Horn's axis x1 is screw A
Shoulder mounting x2 is screw D
A washer will help prevent screw A from loosing. For that purpose, a locking washer will be more helpful. A locking washer and washer are usually used in pairs. the locking washer locks the screw, while the washer protects the softer plastic.
My prefered order(correct me) is:
screw -> locking washer -> washer -> servo arm -> servo shaft.
That will add about 3mm extra length.
Thank you for mentioning it!