What is weight of stock 340 piston

I think that the problem we're having here scale's quality and how it is constructed, and how any external side load or tilt on the scale's surface can throw it off. And IMHO, pretty repeatable and accurate numbers can be had. It is all in the test setup, equipment, and calibration. So I think a job within a few grams can be had with a consistent home-shop setup, and that any decent professional shop should be able to easily hit that mark and better.

I think I now see why my flat plate jig is consistent with this particular cheap scale: the weight on the scale rests on a single point; with the weight passing through that single point, it isn't able to tilt the scale's surface. (The jig's stability improved quite a bit when I changed it from 2 points resting on the scale to just one. I'll get some pix of it.) But, for the vertical hanging setup, with the rod's side or big end resting on the scale, it can tilt the scale's surface with any slight movement of the far end of the rod and thus change the cheap scale's readings.

But, to get to 1/2 inch-ounce error (I've read that productions limits were up to +/-2 inch-ounces), then, I agree, somewhere in the 5-10 grams range would stay within that limit. Of course the more accurately the piston-rod part is done, then the better the crank can be made to match.