New long block engine runs well, but has some significant vibration!

[Jim S]

So, I've squashed nearly all my issues so far ( over heating, steering gear leak etc) But this new engine seems to have some pretty significant vibration.  It's bolted up to the same flexplate, torque converter and transmission as the old engine.  No vibration when coasting (engine off) so I thing the u joints are OK.  One mechanic I know listened to it yesterday and said to replace the motor mounts.  I did that today and it made no difference. I'm stumped on this one.  Any ideas, i'm all ears.  Thanks in advance!

[mfg]

34 minutes ago, Jim S said:

So, I've squashed nearly all my issues so far ( over heating, steering gear leak etc) But this new engine seems to have some pretty significant vibration.  It's bolted up to the same flexplate, torque converter and transmission as the old engine.  No vibration when coasting (engine off) so I thing the u joints are OK.  One mechanic I know listened to it yesterday and said to replace the motor mounts.  I did that today and it made no difference. I'm stumped on this one.  Any ideas, i'm all ears.  Thanks in advance!

Is the new engine one of the external balance units?… If so, you may not be using the correct flex plate.

[Jim S]

I don't know about that.  I purchased it from Powertrain Products. I was just a professionally rebuilt Chevy 350 long block minus only the manifolds, water pump and harmonic balancer.  I bolted the crankshaft to my flexplate and that to the transmission assy.

 

[A0136]

Jim 

Did your 72 Avanti still have the 400 Chevy engine. If it did the 400 is externally balanced using a different front dampener and flex plate than the 350 used.  You will need to install the 350 dampener and flex plate.  
Steve.