I just picked up a little jet boat to use this summer until I find what I really want. It's a little 18' Omega with a 396 and a Jacuzzi WJ. I'm a Ford guy and I've never owned a boat with a Chevy in it (Fords, one Pontiac, and one evil Olds 455).

So, this boat was cheap and has issues. I've fixed many of them but one of the last is a vibration issue. The block is a 396 (3902406) and has Passenger car heads, Performer RPM intake, Holley carb, and HEI ignition. All of the cylinders are making heat now (It had a 850 Holley that was messed up when I got it and was running on 5 cylinders). The boat has a vibration that very much feels like a balancer problem to me (I have experience with SBF engines that people have mismatched in the past). Checking online, it seems that 396 and 454 were balanced differently. One external, one internal. Since the boat is a 1975 and the engine is a 1967 out of a Chevelle, I'm thinking that the engine has either a 454 balancer or 454 flexplate/flywheel because it's unlikely it came with a 396.

So, what I need help with is, how exactly do I tell? I figure I have to pull the rear plate to check the flexplate/flywheel for a part number or maybe balance weights? What about the balancer it self? Can you visually tell the difference?


*** EDIT *** The balancer measures 7" and appears to be OEM/OEM style.

Unfortunately the starter is on the block side in the stock location so I can't get it out without removing the engine.

I am going to have to pull the rear engine plate to inspect it .

Nice boat not real sure about plumbing to the headers.

BBCs and SBCs follow the same basic balancing convention... all are internal except the biggest one.  IE: 454 BBC and 400 SBC are externally balanced.

I've seen people replace a 396 with a 454 and reuse the 396 damper and flexplate.  Major vibrations! 

If you're sure it's a 396 definitely check both the damper and flexplate/flywheel for a weight.  There should be none on either.

Flywheel comparison:




Flexplate comparison (size isn't always different, it was the only pic I could find):




If the balance of the flywheel and damper are OK you could have bad U-Joints or a misaligned drive shaft.  There are threads on that topic here, search should bring some up.

Oh, and thanks to everyone that replied to help me!

Damper appears correct. It's 7" and does not have a weight. I may replace it however as the rubber in it is in bad shape. I started a project thread on the boat...