Does anyone know for sure if this servo arm (<span id="prodTitleBrand">3D XL 1/2 Servo Arm 4-40: JR</span> <span id="brandName" sizset="378" sizcache="16">byHangar 9</span>)will fit onto a Hitec HS5645MG

Probably not. You shouldn't use a servo arm thats not made specifically for your servo, they may look like they fit but will slip under flight loads.

The reason I say probably not, Dubro has high strength servo arms and they are 3 different part numbers, one for each Futaba, JR, and Hitec splines

Don't do it.
John

Thanks, now if i could just find a parts list for this build I would be set.