My new pistons are about 25 grams lighter than the old set. Is rebalancing necessary? These are extremely heavy slugs, about 860 g, so 25 g is less than 3% and I've heard going lighter is better than heavier.

 

I would get it balanced. Cheaper now than having to do it after put together.

 

You may have heard that lighter pistons are best, and that part is true as far as it goes. A 25 g difference is significant and will stress the bearings equally, whether it is plus or minus. And it's cheaper to balance for lighter pistons than for heavier pistons.

The balance cost is minimal compared to your current cost for rebuilding the engine. Don't skip this.

 

Something tells me the crank was not rebalanced on the previous build. I don't see any drilled holes that look fresh on crank or flexplate. And my rod bearings looked hammered.
All 8 look like this on the top.

 

If it’s balanced on the light side, it tends to beat the wrist pins and bearings out of it, kinda like your picture. Have it rebalanced. It may not have been done correctly in the first place. If it’s not done now, you could be making it even worse.

 

25 grams is a lot especially where you dont know what was done in the past.

By the way you mentioned drill marks in the flex plate ? That is not good practice in my opinion. If you mess with the balance on the flexplate then it is sort of married to that assembly and if you ever strip the starter teeth or have to replace it for some other reason it can be a hassle. Same goes with the damper.

If you rod bearings are worn on the top side like that it can also be detonation. In my opinion balance issues are going to show up more in the main bearings than the rods.