I fat-fingered a key and erased a long post- - - -let's try again. I haven't encountered any vibration problems when swapping to aluminum pistons. An inline six crankshaft is inherently well-balanced due to the exact 120 degree spacing of the counterweights, and stovebolts don't spin fast enough for any real reciprocating weight concerns to develop. The cylinder head design pretty much restricts RPM to slightly over 4000, even on engines with dual carbs and Fenton headers. It's like joining two pieces of heavy chain together with a paper clip.
I'm working up a stroker 235 engine, using Carrillo NASCAR tool steel connecting rods and a big stroke increase, from 3 15/16" to 4 3/8", for a total displacement of 266 cubic inches. Since the connecting rods are much lighter weight than original, I'm planning to set up the crankshaft in my lathe and turn about 1/2" off all the counterweights, and then have my buddy with a crankshaft balancer spin it up to check for vibration. I'm doing a bunch of other modifications to that engine, all hidden inside. It's going to look bone stock from the outside. I'm also working on a version of the same rotating assembly that will fit into a 216 block if everything goes right. That will require changing the width of the thrust bearing area of the 216 block and main bearing cap to accept the 235 main bearings. That one's still on the drawing board, but a 266 with full pressure lubrication that looks like a 216 might be a real sleeper!
Lots of 216's have been modified with 235 pistons, with no problems with vibration. Stock-bore aluminum pistons instead of 60 over should give no trouble at all. Have fun!
Jerry
