Okay,Need some help. I have a vibration on my 66'. It is a 428 motor. When driving at say 40+ you get a vibration from front of car,notice it when getting applying gas and rpm's climb. Can get this vibration in park and raising rpm's. Read much how 428 is external balanced and 390 internal & how mismatch of parts can be leading cause of this problem. Verified engine is 428 block,"A" scribed, with "1U" marked 428 crank. The flexplate is a 2 peice setup with large holes bored on outer crank gear,flexplate mounted has diamond shaped cutouts, NO WEIGHTS. I am told this is correct Flexplate for this 428.Although new 428 flex plates have external weight from what I am told. Had new solid driveshaft built with u-joints,transmision torn apart new torque convertor.This motor&tranny were rebuilt at some point in cars life. Believe problem still to be engine balance issue. Thinking I need counterweight on front sleeve,behind dampner. The later 428SCJ mootors had this weight. Not sure if earlier 428 had any weight. Thoughts? My $$$'s to this problem adding up very quickly!!! Tks,Greg

Gregory: It sounds as though some parts and pieces are mis-matched. The 428 and 410 are the only FE motors that are externally balanced. Your flex plate should have a prominent counter weight welded to it near the starter ring gear on the transmission side. The harmonic damper and spacer are not counter weighted. The Super Cobra Jet 428 did have supplimental weight added to the crank to accomodate the "Lemans" rods in that motor only. The Thunderbird 428 does not recquire that.

This message has been edited. Last edited by: kandiapple66,

I have seen pictures posted of the standard 428 flexplate and it shows a heavier starter ring with the 4 large holes drilled in ring. Was told no weight on this, holes are balacing. From pictures of the 428CJ,SCJ they have different starter rings and yes I note weight on these. Just trying to clarify. Thanks again.

The OE flexplate for the 428 has a counter weight welded to it because of the amount of bob weight needed to achieve balance. Holes by themselves are simply not enough to bring the rotating assembly into balance. If your flexplate has no weight welded to it, I doubt that the rotating assembly is correctly balanced. It COULD be done by modifying the crankshaft itself but on a non-racing build up, I would tend to think not. JMO