Finding parts within a given radius/diameter?

One way of going about it would be to weld a sphere or cylinder to the part, like so:

-- Assume 'Part' is the part you want to check.
local RadiusPart = Instance.new("Part", workspace)
RadiusPart.FormFactor = "Ball"
RadiusPart.CanCollide = false
RadiusPart.Transparency = 1
RadiusPart.Size = Vector3.new(0, 0, 0) -- Change as necessary.
local Weld = Instance.new("Weld", Part)
Weld.Part0 = Part
Weld.Part1 = RadiusPart

Then, creating a table to hold the parts:

local PartsInRadius = {}

And hooking up events to RadiusPart causing it to:

• Add a part to the table when touched.
• Remove a part from the table when it stops being touched. Of course, you would want to check if the part was already in the table, so you don't add the same part twice.

Unfortunately, this method only works with parts which have the script in, and results in many more parts, but hopefully it is helpful.