Not sure what to do next on my Rat motor. I have a slight vibration that you can feel through the shifter and seat of your pants. It starts at about 4000 rpms and stays until redline. My mechanic pulled the trany this week to check out hard shifting and discovered I got F@#$#$ by the guy who sold it to me, supposed to be new parts, instead I got worn out used stuff! Thats another story. He went farther with the motor, pulled the belhousing, pressure plate, clutch etc. All of that is balanced fine, the problem he is telling me is I have a 400 flywheel not a 454. The vibration also is there when you are just sitting still in neutral and rev the motor. The motor was balanced with the crank, flywheel and balancer as one unit, but they did not add any weight to the crank to make up the difference or so they told me. My options seem to be drive it and wait for something bad to happen because of the out of balance problem or tear it down completely. Any ideas or suggestions at this point would be much appreciated! :U

 

The 400 flywheel is not neutral balanced. So maybe getting the correct neutral balanced flywheel would solve the problem. If the oil pressure was still good ide give it a try without going in any further.

 

I am not sure if its possible to change flywheels as mine was balanced to the crank and front balancer. If we put a 454 one on now it would not match the balance job of the crank?

 

You said your mechanic pulled the clutch , pressure plate and flywheel and they were balanced fine. How do you tell this? YOu can not look at it and say it is balanced fine unless he has access to a balancer and even then I would be very sceptical he could tell it was balanced fine. As for the flywheel does a 400 flywheel fit on the bigblock and if so , so what. When the motor was balanced it was balanced for that particular crank/flywheel combination and wouldn't matter.
Is the vibration just a shutter going through the motor or gets really bad as the rpm climbs.
If it is more then a shutter then the motor should come apart before you loose a bearing and destroy the crank.

 

Thats one thing I don't understand, the crank, flywheel and harmonic balancer were are balanced together. One guy told me that because I have the smaller 400 flywheel that the shop would have to add weight to the crank to make it balanced? I know this was not done because I asked them about that.

 

The crank, flywheel and harmonic balancer are always balanced as a unit along with weights to simulate the pistons assemblies. I still would like to know how your mechanic can tell the flywheel, clutch and pressure plate are fine?
There is a way to fine tune the balance on the car if you want.
You can drill holes in the flywheel front the front on the passengers side, under the starter motor.
With the car on a hoist or in the air you can drill and tap 1/4 x 20 holes evenly in the flywheel. I remove the plugs for this.
Start at one spot and mark a hole accurately in from the starter gear. There is room to drill with a hand drill and drill with a #7 drill bit then tap with a 1/4 tap. If the flywheel is 168 tooth count 42 teeth from this hole slowly marking and counting the teeth while turning the motor over and drill, tap hole #2 then #3 and finally #4 always keeping to the 42 tooth count.
Now you can take a peice of whatever, in my case I used a brass block, round about 3/8th thick 3/4 inch diameter with a 1/4 inch hole in the center with the head countersunk for an allen screw.
Install the weight in #1 position, start the car and bring the revs up slowly feeling for vibration, repeat in $2 position then #3 and finally #4 taking note where the vibration seemed the less. This is where the flywheel wants more wieght,
While it takes a little time you can help remove the vibration, in is definitely doable and it works.
I have done this myself to fine tune a slight vibration at a certain rpm.

 

Did this just start happening or is it something that has been there since day one?
Some harmonic ballancers have a bolt-in weight that may have fallen off.

Did you check for a miss in the ignition? Big blocks throw a great deal of weight and any miss or cyl lean will be felt as a vib. Might also want to check the cam lobe lift on each cylinder.
Good luck.