The reason it doesn’t vibrate with the 6bbl pistons is....... they aren’t really that heavy.
In fact, the Speed-Pro catalog shows a .030 over L2355 is actually 4 grams lighter than a .030 over L2266 piston, which is .070” shorter, but has no valve pockets.
It’s the big rods used in the 70-up 440-6 that makes the motor externally balanced....... not the pistons.
I balanced my factory 1968 440 rotating assy many years ago.
The stock std bore pistons and pins weighed 1086 grams.
Std bore L2355’s and pins are listed at 1083 grams.
