Gram Scale Suggestions for weight matching pistons
is a thread on the subject.
Things to look for:
A platform large enough to put the part on and still be able to read the display.
A capacity that puts the weight of the part in the middle range of the scale capacity. Like for a 500g piston you'd ideally want a scale w/a 1000g capacity, give or take.
A resolution of </= +/-0.1g
A stainless steel platform is nice to have, but not absolutely necessary if you're careful. All the digital scales have a tare feature that lets you zero out the display after you put a container on the platform to keep the platform clean. Avoid things like getting carb spray on plastic parts of the scales- especially the cheaper models.
A good triple beam scale will work just fine for this, need no batteries, and are accurate. But they take a LOT longer to use than a digital scale.
If you are going to do this a lot, getting a good scale from the start would be my advice. But expect to pay a couple hundred for one.