Hi all.
I helped a friend take apart his 340 last sunday, adn I was a bit surprised at the piston shape and size (see pic). That was my first small-block experience by the way, I only have big-blocks myself. Pistons are free-floating (as stock) .40 over (now could that be stock?). Four identical valve reliefs with a small indentation towards the front. only external marking is the .40 stamped on the top.
Underside, the area between the pin bosses is checkered, and I can see what looks like a 'D' (Diamond?). On the inside of the piston skirt is what could be a casting date : 80 12 75.

the piston is also really deep in the hole (.100), although I've read it should not be the case with small-blocks (pistons popping out and requiring open-chamber heads).
The piston skirts are only attached to the piston top by the pin boss area, and they're completely cut away from the ring land almost all around the piston. I've sen one picture of such a piston in 'How to Rebuild SB Mopar Engines', but no description about this.

This is a 1970 block, with 1972 heads, so at one time this engine was rebuilt, I just don't know to what extent. I hope the block will clear out OK at .40 over, since I wouldn't like having to bore it further.

What I'd like to do is have an idea of the approximate date at which this engine was rebuilt. Did it happen eons ago (as I'm tempted to think with the '75' part of what seems to be the casting date, although the engine itself was pretty clean, with honing marks still visible on the cylinder bores).

Any thoughts?

Sounds like replacement 8.5:1 slugs. 10.5 pistons had a slight dome (well, a slightly raised platform).

bearings are usually dated; your best bet. Who cares anyway, those pistons are junk, check the bores and rebuild as required

Any dates on the back of the rod and main bearings?

I'll check for rod bearing dates tonight, as I've taken one piston and rod home to take this picture and weigh them.

These pistons are not going back in, that's for sure (nor are the rods either).

The block is 1970, and the heads 1972, so there's no question that the engine was rebuilt at some point.

Here's the info on the rod bearing :
2130 CP
FT
020 US

I don't know if there's an indication of date (30th week of 1921 sounds unlikely), anyone knows how to decode that?

I suppose 020 indicates an 0.020 undersize rod journal?

yes 20 under & 2130CP is a modern era sealed power/federal mogul part #. I just stripped a 1974 400 BB & the rod bearings said 1973. So we know yours has been rebuilt at some point. Good luck.