Well my '78 400 engine's stock balancer wore out on me. The outer ring slid back on the elastomer and it started to rub the timing chain cover. I bought a brand new balancer from my Chev dealer, installed it, and now my engine vibrates like crazy right at 1200 RPM, then again but not as bad at about 2500, and agian at 3900 or so.

It had a vibration before I changed the balancer, but it wasn't bad, and I didn't notice it at all untill 2500 RPM but now the shake at 1200 is absurd! My doors both rattle and the dash pad moves about 1/4" it is shaking so badly. The engine has a neutral balanced flywheel AND a cheapo summit "400 couterweight". I think that is what was causing the slight vibration before I changed the balancer -the cheap summit couter weight. But then I thought it might have been the balancer, since the outer ring had slid back, I thought there was a good chance that the ring had also rotated, throwing the engine slightly out of balance. But now with the new one it shakes WAY worse. Oddly, at the RPMS between the ones I mentioned above, there is no decernable vibration. I'm sure there are still out of balance forces at work, but you just can't feel them. Help

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'83 Trans Am: 400 CID oil burning junkyard long block, 224/234 crane cam, Summit aluminum roller rockers, hand ported intake, home bored 2.09" (53mm) throttle bodies, MSD 454 injectors(75 lb/hr), Holley 255LPH fuel pump, custum modded FPR, Edelbrock TES headers, SLP 3" stainless steel cat back, stock ECM & chip. Borg/Warner T-5 World Class, 12" Corvette rotors and clipers, GTA 16" wheels, South Side Machine subframe connectors, 1LE sway bars, 3.45 ring and pinion, Alpine sound.
Best E.T. 13.532
Best MPH 102.24

 

It sure sounds like they didn't give you the right balancer! Check the back of it for the wgts-add. metal-for a 400 balancer.



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86 406

 

Yea it seems like that. The new balancer and the old one look the same though, with the exception of the old one having slipped.

 

Does anyone else have any ideas about this? I looked at both balancers again and it looks like the new one (the one on my car) has less of a counter weight than the old on.(?) The difference is slight -if any- and hard to really tell by eye. GM only made one harmonic balancer for the 400...right?

I would have just pressed my old one back together, but I thought that it may have slipped radially on the hub, throwing the engine out of balance. Soooo, instead of gambling with the old one and ending up with my engine out of balance, I bought a new one and now my engine is out of balance!

Any ideas? The new balancer definitely HAS a counter weight on it. It is not a 350 or smaller balancer.

[This message has been edited by Tom 400 CFI (edited August 29, 2001).]

[This message has been edited by Tom 400 CFI (edited August 29, 2001).]

 

I'm confussed about your flexplate. A neutral flexplate would require the balancer to be neutral and the crank to be internally balanced. Wouldn't it?

 

Yeah, we're missing something here. A neutral balance flexplate would not work unless the engine was balanced internally.

Yes, there was only one balancer made for the 400. An 8" jobbie. The imbalace was the same year to year, too, so that's not the problem.

Is the woodruff key that indexes the balancer to the crank torn up? It should have perfectly square corners and so should the slot that it fits in.

Other than that, you got me. If you took off one balancer and put on an idketical one (with exception of slippage) it should run the same.

 

What he has on the flexplate is a "pork chop" weight, the simulate the stock "unbalance".

The balancer shouldn't have a "counterweight" on it, it should be missing a big slab of material about halfway around the outer edge nest to the timing cover. If your new one doesn't look the same as the old one, then there's something wrong; AFAIK they all look about the same.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Thanks for responding.
RB is correct. I have a neutral balanced flywheel (it's 5 speed) and a little plate that has a counter weight on it that sandwiches between the flywheel and the crank shaft. Cheap, I know, but effective.

The balancer/crank key is in fine condition as is the end of the crank. I'm totally stumped here. I called my dealer and he is clueless too. He is adament that there is only 1 part # for the 400 balancer, but there IS a difference between the new one and the one I have. Also, the one I have has the casting # 3896906 cast into the back side of the hub. This number does not match anything in his (dealers)system.

RB, counter weight/missing weight... same thing. I was just making the point that it is NOT a neutral balanced balancer, just so everyone understood that fact. I didn't want a wasted post suggesting that I have a 350/305etc. balancer. The amount of metal that is "missing" from the 1/2 is more shallow on the new on. In other words, there is less metal "missing" in the new unit. They are bolth 8" dia. I don't knwow what to do here. I'm about ready to press my old one back together and slap it back on my car!

 

Isn't it possible they didn't balance your new dampner correctly from the factory?

I'm going off memory here but, aren't there balance drillings in most dampners?

I'd return it and get another.