BBFan

Got a 73 Fastback with C6 and 351 Cleveland few months ago. The engine has a severe vibration. The seller said it was the driveline. He had no other information. I bought it knowing I had a problem. The vibration is in the engine or transmission. The vibration is present when the transmission is in neutral and Starts to get real bad above 1000 RPM. The vibration is the same as if you put an external balanced flywheel on an internal balanced engine. After several months of research I started to work. I approached the problem like somebody had put the wrong crank in the engine. The engine has a twenty eight ounce imbalance harmonic balancer and a twenty eight ounce imbalance flex plate. I made a weight identical to the one on the flexplate and tacked it on the flex palte opposite the factory weight on the flex plate. Vibration was worse. Then I cut the weight in half and welded it on either side of the flexplate weight. Vibration was worse. I have a new harmonic balancer to put on. Now I am going to try and slide the torque convertor back and run the engine like that. Are there any precautions to take witht the torque convertor to keep it away from the flexplate? Any other ideas to try?
Right now:
Vibrates in neutral and gets sever as RPMs are increased.
Vibrates worse with weight welded opposite stock weight.
Vibrates worse with extra weight welded on either side of stock weight.

Any help would be greatly appreciated. Dying to drive the car to a Saturday Night Rod gathering.

Thanks for reading and for any advice. BBFan

Al Newman

Try changing out the torque converter.

Stangatic

Since it sounds like you have the trans out, see if you can push or pull the crank. If it moves, it's the thrust bearing. Had the same exact issue and I thought it was my fly wheel. I took the trans out and the went to take the fly wheel off. i notice that when I put some pressure on the fly wheel with the impact gun, the whole crank moved about a 1/4" . The I notice that I could push or pull the the whole crank in and out of the engine which = time for a new engine.

Have you tried to start the engine with the trans off and torque converter off? You'll need an old bell housing or use the one you have on your trans. That would help write off anything in the trans or torque converter.

rmodel65

ok i see you have a balancer you claim is 28ounces...how is the rubber ring sandwiched between the inner and outer layers? they are know to dry rot and then the outer ring slips and everything really gets out of balance..honestly id buy a new stock balancers and see if its fixes your problem...also take a pic of your current balancer and post i here

BBFan

The new Balancer has the holes in the outer ring close to the timing marks. The balancer on the car has the holes much further away. Physically in other ways the balancers are very similar. The thickness of the outer ring is the same on both units. From what I can tell the 50 ounce imbalance has a different thickness outer ring. I am going to change the balancer tomorrow. Thanks

BTW the imblalance term and the imbalance weights have posed a question. What determines the position and size of the weights on the flywheel/flexplate? I always thought that each engine had to have the parts weighed.

rmodel65

the thickness of the rings really dont matter..if its an original ford balancer that if on your engine it will have an E1 or earlier part number IE: E0,D9,D8,D7 etc for it to be 28 ounce balance

a factory built engine isnt blueprinted ie weighing all the parts normally..the parts are usually close enough for acceptable smoothness

BBFan

Installed new 28 ounce imbalance harmonic balancer. No change in vibration. The engine is in the car with the transmission attached. I want to move torque convertor back and run the engine. I have found different ideas on being able to run the engine with the torque convertor slid back. Are there any special precautions to running the engine with the torque convertor disconnected from the flex plate and slid back?

rmodel65

you are not gonna be able to slide the converter back far enough to run the engine..the only way i see running the engine without a converter is if you have a C4 trans you could unbolt the bell housing and bolt it and the starter up..have you made sure the fly wheel you are running is 28 ounce?

67mustang302

Are you sure you have the right balancer? Was the engine rebuilt with a non stock balance factor? The crank, flex/fly and balancer all have to be the same "balance."

And FYI, the weight of the imbalance factor isn't the actual weight of the weights....it's the total force generated by the weights at a certain measurement factor. It's determined by the engineers, so for our purposes we just need to know that they all need it match.

Also, don't **** with balance stuff....it's vastly more complex than 99.9% of people realize, and balance failures causes crankshafts to explode; though typically only at higher rpm.

http://www.epi-eng.com/piston_engine...on_engines.htm

http://www.epi-eng.com/piston_engine..._absorbers.htm

BBFan

The weight on the flex plate on my flex plate has the same physical dimensions of the weight on a known 28 ounce imbalance flex plate. Additionally my flex plate does not have the slot cut into it opposite the imbalance weight. The 50 ounce plates I have looked up have that material cut out opposite the imbalance weight so the actual 50 ounce imbalance weight can be smaller by the amount of the cut out material.

Since rmodel65 says the toque converter cannot be unbolted and be slid back far enough to run the engine then the next step is to pull the engine. Then I can check the crank like stagnatic says. Plus have the torgue converter checked. The crank seems to be a candidate for causing the problem also. I have read that a 50 ounce crank is interchangeable with a 28 ounce crank and that may be the problem. Does anybody know how to tell the diffference between the 50 0unce and 28 ounce cranks ?

Just put the new 28 ounce balancer on last night. Flex plate and balancer are definitely 28 ounce units.