Chasing a heavy rotating viberation

[orre_s86]

Hallo,
I am chasing a heavy rotating viberation in my 390 fe.
The viberation is best felt somwhere around 1000-1500 rpm, with gear and without gear. You can feel it both in upprew and downrew, almost more felt when your releasing gas pedal and the engine is downrewing. It feels like a heavy rotating viberation, and can be felt in the whole body of the car.

The engine was rebuilt last vinter with a new block(old was cracked) and changed pistons for more compression. Rods where weight matched.. Dident have a viberation before the rebuilt.

I have changed the viberation damper for a new one allready without any change.
I am thinking that the problem would most likely be behind the engine in the flexplate or converter as the viberation feels so heavy!?

Had enourmus problems removing the flexplate when the motor was taken out, had to be bent of as the center was way to tight on the crank. So before mounting them together again i enlarged the hole slightly with a rotating grinder.. Dont know if this changed anything!?

Messured the crank, flexplate and converter yesterday with a dial indikator while rotating the engine and have this numbers:

0.0011 on the crank snout
0.016 on the viberation damper

0.027 upp and down on the flexplate and 0.031 sideways chucked on the center in a late. But 0.079 in side movement when the flexplate is mounted to the converter..

What do you think of these numbers? Can you se annything that could couse these viberation? How do i tell if the converter has developed any problems?

Thank you/ Oskar

[jayb]

Welcome to the board, and hope we can help.  I would suggest looking at what has changed in the engine, rather than assuming that an existing part like the transmission or torque converter went bad.  A couple questions:

-You said the rods were weight matched, but was the reciprocating assembly also balanced?  The new pistons probably don't weigh the same as the old pistons, and the crankshaft would have to be re-balanced to match the new piston+rod weight. 

- The nose of the converter has to fit into the crank rather precisely.  Was there any change in the crankshaft that would make the rear hole of the crank larger, so that the converter wouldn't fit into the back of the crank precisely?

I don't think that the runout measurements on the flywheel or crankshaft are a problem, although of course you would like to see those as small as possible.  It seems that you are bending the flexplate a little when you bolt on the converter, you could try to use some shims between the converter and the flexplate to keep the flexplate straight.

[shady]

is it a 428 crank or flexplate by chance? 428s are externally balanced.

[orre_s86]

Thank you,
Yes i have been thinking that way to but.. No the replicating assembly was not balanced. As there where only a couple of grams difference between the rods so the only big difference where the pistons and my machinist thought that it would be okey for a low rpm engine without balancing as it ran good before. Dont really know the weight difference between forged seal pro 410 pistons and Keith black 390 hypo... Do you think that this could be the problem?

I have been told by a guy very familiar with old fords and way more experienced than me that the viberation is to heavy to have to do with the pistons!? That's the main reason that i started looking elsewhere.. How do i tell the difference?

No, not that i could think of.. Only idee that i have is that i removed material on the flexplate unevenly. And that this caused some sort of unbalance!?

Can i tell somehow if i have an unbalance in the converter? I have always had an moving/rateling sound when driving at higher rpm, thougt that it maybe where from the converter?

No not a 428 crank or flexplate...

[orre_s86]

Could some sort of crack on/in the crank cause viberations like this, or would it snap when put under load then? The engine runs strong!

[shady]

you could pull the tranny & converter off the engine & run the engine to see if it goes away, at least that way you would know if it is in the engine or not. double check to make sure your plug wires & firing order is correct. easy to overlook the simple things. other than that, I have no other ideas.

[WConley]

My money is on the converter.  You mentioned that you had opened up the pilot hole by hand with a grinder, and those runout numbers you posted for the converter look HUGE! 

Remember that the converter is filled with several pounds (a few kilograms) of fluid, and if it is not accurately located to the centerline of the crankshaft, you will get a tremendous shake.  The rpm you describe make sense as a significant natural frequency of the engine / transmission assembly on the engine mounts.  That kind of unbalance will definitely shake the whole car.

Engine unbalance (pistons / rods) will be more of a vibration over a big rpm range, instead of a big shake in the car like you're seeing.

As mentioned above, take the converter off and run the engine in the car.  I bet you notice a big difference...

[Drew Pojedinec]

Pretty common that the convertor pilot doesn't want to fit.
I've never even considered altering the crankshaft tho, I always took material off the convertor pilot.  It's considerably easier to do so evenly, even with simple hand tools.

I'm with Bill and others, pull the convertor off, bolt everything else up and run it.

[orre_s86]

The engine is out of the car, and i think that i will change both the flexplate, converter and trans mount before bolting it back together again. But it would be nice to find an obvious reason that causes this.. Really dont want to have to take it out again.

I will check the alligement between converter and crank, havent tougth of that before..

Dont get me wrong, the viberations are there all the time.. But are felt the most in that range. A huge diffrent between 2000 and 1500 rpm.

[Drew Pojedinec]

Oskar,
It's always a good policy to test fit the convertor to the engine.
Making sure it sits tight to the engine but the studs can still slide back and forth.

If the convertor binds at any time, pull everything apart and find out why.