I recently purchased a #s matching, SS/RS 1968 coupe , 396/325, 4 speed car with 64k miles. I am the third owner. The only engine work that has been done was a new cam and lifters put in approximately 15 years ago. There is a very noticeable engine vibration between 3500 and 5200 rpm. After 5200 rpm it smooths out somewhat. I initially had the driveshaft balanced but it did not solve the problem. I have isolated the problem to be in the engine (I put the car in neutral and accelerated the engine and the vibration is still there).

I'm not sure if the harmonic balancer could be going bad or could it be that the replacement cam has thrown off the engine balance? How can you tell if a harmonic balancer is bad? Any help is appreciated.

Thanks Team Camaro.

I would suspect the pilot bushing. If it's bad it will let the clutch disc be off center and cause a vibration. I've experienced this on several cars but keep in mind this is just a guess and it could be totally something else.

Instead of putting the car in netural, put the car in gear. Then push in the clutch and rev the engine. I think that should remove the pilot bushing from the equation.


I've never heard of a cam throwing off engine balance, but the harmonic balancer could.

Ck. engine compression and read your spark plugs. The best way to ck. the balancer (if not obviously bad) is to install another. Could be trans or clutch related. Besides vibration, how does it run? I ask because... vac leak, carb problem, high speed misfire could give sense of vibration also. The only way to fully remove pilot or trans from equation is to remove trans, as the input shaft is always touching the pilot. The clutch fork only disengages the clutch disc.

Instead of putting the car in netural, put the car in gear. Then push in the clutch and rev the engine. I think that should remove the pilot bushing from the equation.

Good idea. Clutch stays still.

I just went through something like this. If the balancer is bad, you should feel it at idle slightly. Certainly by 1700 rpm or so. By about 2000 rpm I was afraid of damaging something it shook so bad.

I put the wrong balancer on a BBC. I needed an externally balanced one and put a stainless(appears stainless) 7" zero balancer on it.
Wrong one.

I had to pull it and replace it. The parts store wouldn't take it back because it had little impressions on it from the puller. So now I have a balancer to sell that has little impressions on it and about 10 seconds of run time. :confused:

Anyway, I went through this and if your balancer is bad you'd feel it in a very pronounced fashion. The shake on mine was alarmingly bad.

I would suspect the Harmonic Balancer to be the culprit. Check the rubber seal separating the inside and outer part to the balancer. A good way of doing this is to check it with a timing light. If it is off the scale, the outer part has slipped on the balancer and may causing the vibration.

Thanks for all of the replies and ideas. As far as how the engine runs, it runs very well. No missing and pulls hard all the way to redline. I can only hope it is the harmonic balancer. I will check that out first and see. How hard is it to find an original BB balancer? Can you find them any other plalce than Ebay? Thanks.

OEM balancers can be bought from the dealer through their GM Performance Parts catalog, or through the parts book.

Aftermarket OE is available through the local parts store. SFI certified are available through Summit, Jeg's, or any parts house.

I had a similar problem once upon a time when a friend introduced me to engine rebuilding. It was a 396 and he felt a forged truck crank was the way to go. I had the motor down to a ring gear (to eliminate the flywheel) to convince myself it was in the motor. Took it in to be balanced and they identified the wrong crank issue. A lot of holes in the crank counterweights later, I had a balanced motor. I hope it's your harmonic balancer as a lot have suggested. It sounds a lot easier and less expensive than my route. Since you don't really know how long the problem has existed, it could be interesting. Good luck.

Thanks for all of the replies and ideas. As far as how the engine runs, it runs very well. No missing and pulls hard all the way to redline. I can only hope it is the harmonic balancer. I will check that out first and see. How hard is it to find an original BB balancer? Can you find them any other plalce than Ebay? Thanks.

As I said before, I just happen to have a zero balance harmonic balancer that is a 7".
Zero balance for 396-427.
It looks nice. Its shiny......

BTW- it has been run for about 10 seconds.

Shall I put it in the classified ads on this site?

A freind has just put his 70 on the road...didnt rebuild the engine, that was do some time ago by the orginal owner but just added a few bits in pieces like manifold..
It pulls well, idles nice BUT it is not a smooth engine...
We belive when it was rebuilt, it was not balanced.

Took me THREE times to get BB 454 engine balanced correctly. Had vibrations as you describe, but mine would come in at lower rpm. Machine shop couldn't believe it. They thought I was nuts to keep bringing it back, but it was always out. On the third time, they were in a state of denial and I said I wanted to actually be present as they weighed the pieces and prepared the balancer and bobweights.

Well, on this third time they were pretty meticulous, especially as I was standing right there. I had been around balance shops before, so I knew most of the lingo and the protocol. The guy's jaw dropped when he started spinning the crank/bobweight assembly, the machine was shaking like crazy. He absolutely couldn't believe it, as he himself had previously balanced the engine and it was way out.

Anyway, the third time was a charm and my engine finally runs smooth.

Moral of the story, it could be many things, as stated above. Hopefully, you get lucky and it's simply a harmonic balancer or crank bushing. Don't give up. Keep looking and experimenting.

Try taking belts off. Maybe water pump or a pulley is out of whack.

Alternatively, try taking trans and clutch out and see if better.

Just keep trying...............

Thanks for all of the replies and ideas. As far as how the engine runs, it runs very well. No missing and pulls hard all the way to redline. I can only hope it is the harmonic balancer. I will check that out first and see. How hard is it to find an original BB balancer? Can you find them any other plalce than Ebay? Thanks.

What did you ever find out about this?

Balancer or what?