In a LO3, why can't one use 6.0 rods or longer with the stock pistons (for the stock 5.7 rods) ?

 

You can, just gotta move the head out of the way.

There's only so much space from the crank centerline to the top of the deck/head mounting surface. Add in 1/2 the stroke, rod length, piston height... better be close to that number and not much over it or the cylinder head and bolts are going to be unhappy with your bottom end and fight back.

 

So it's an "alignment" thing ? Just trying to picture it

 

It's not an alignment thing, it's a physical impossibility thing. If you make the rod 0.3" longer, the piston has to move up in the bore by the same mount. Since there's only about .025" between the top of the piston and the deck surface on an unmolested block, the pistons would be sticking up about .275" above the deck.

If you want to use a rod .3" longer than the stock 5.7", you need to use a piston that has its wrist pin .3" higher than stock.

 

OK yeah I didn't have the actual figures in my head. I just thought hey longer rod, would result in higher compression.

 

It would, if you had some kind of hugenormous combustion chamber that would let you bring the pistons up that high without hitting the heads.

 

The day where combustion chambers are measured in cubic inches....

 

My combustion chambers are 4.4 cubic inches, what are yours?