Two pistons broke on my 396, so I bought custom ones from Ross for $1000. I put the Eagle H beam rods back in the same place they were before and all of the new pistons are within one gram of each other. The engine was internally balanced before I had to replace the pistons. The pistons that came off were JE customs. The compression ratio before was 9.2:1 I think, and with the new ones, it's supposed to be 8.75. I didn't weigh the old pistons, but before I put the engine back together I was advised that I should be fine with the balance.
The engine feels fine below 3200, even going up a hill under full throttle. The vibration is most noticeable when it's in neutral just revving the engine. I hope it's a miss, maybe the opti since I've had one act up proportionately as RPM's increased, but I'm afraid it's the engine out of balance. Could a rod have bent? When the second piston cracked, the piece that broke off the piston was banged on for a few minutes against the combustion chamber in the head. None of the rods looked like they were bent.
I will probably be taking it to a speed shop so they can hook it to an analyzer and see if it's missing. (I can't look at misses now with the scan tool since I had to take the reluctor wheel off the crank to put in the double roller timing chain; i.e., now the crank sensor doesn't work.)

What do you think?

 

If the new pistons were not balanced to the crank, would it not be potentially out of balance?

 

I would have thought so, but I got the two replies below when I asked about this before I put the engine back together:

And I haven't weighed the old pistons, but the new ones were for a slightly lower compression ratio, so they should be a little lighter. I probably won't be able to get any answers until I get a diagnosis on whether it's missing or not, but I wanted to post anyway.

 

Hmm...it sounds like you already had your answer(s). Personally, I think I would have compared the weight of the two pistons before putting it together. Seems logical.

 

Yea. The thing seems to be out of balance. I noticed that if I rev it to 2000 then let off, it vibrates from 1900 to 1600. It isn't noticeable when I rev it to 3000 and let off. It doesn't seem to be missing so the whole engine will probably have to come out again. It's a Spec 4 flywheel, clutch and pressure plate. I assume I can reliably say they are neutral?
Does anyone think it might just be the cam thumping and the stock damper isn't good enough? It's a big cam, about 238/245. I know it's unlikely but I'm looking at another 80 hours of work and $500 if I pull the engine out again.