vibes

will a cast crank torque converter fit with a steel crank, if so will it cause vibration, i have a 67 belvedere, it has a vibration at around 2900 rpm it goes away at 3000, but will come and go, i have replaced the u-joints, tail shaft bushing even a new tail stock,the rearend is good, pinion is solid drive shaft is straight but not balanced, wheel bearings are good. it used to vibrate really bad, but with the new u-joints bearings bushings etc.. it has improved greatly.  

I have no idea what engine you are refering to,but torq converters are individually balanced,as are flywheels.If the converter's hub,and the crankshaft pilot hole are in toerance to proper fit of each other,you should have no extra vib. caused.  

it's 383, the vibration isn't terrible, it just irritates me, some folks think i'm being to sensitive to it and that all these old cars vibrate somewhat, but i think it could be better.  

And it is definately rpm related and not speed related?  If it is speed related it could be a slightly off drive shaft or tire balance or a wrong sized u-joint chattering in the drive line (When I had a drive shaft shortened for my Firebird it had a slight vibration around 35mph that would go away by 40mph, two years later it threw the u-joint cap, the chatter marks were evident.  A proper size u-joint was put in and it was vibration free. The old u-joint looked snug, but it wasn't).  If it is definately rpm related it could be a bad balance on the flywheel, I never kept tabs on the SBC's balancing which engine's is internal or external.  A bad motor mount could also be allowing engine vibration to transfer to the frame of the car (this I think would be a come and go symptom, how loaded up the engine is, temperature, etc).  That's my 2 cents.  
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The Silver Buick- '77 Skylark coupe w/ a Fuel Injected Buick 455&TKO-600, '72 Centurion Conv't - 455w/TH400, '67 T-bird 4Dr (suicide) w/428&C6. Needing to replace a '69 Firebird 400.

http://www.bangshift.com/forum/index.php?topic=6189.0

Get the driveshaft balanced,check the tranny mount.  
Power is bliss Torque devine

I would also check pullies ? ,belts and vibration dampner. I had a simaliar problem w/ a fan clutch that went bad between towns and ened up going through the radiator.  
75' El Camino
97' Suburban
77' Buick Regal
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I need more info 2 help you here.See sbc 400 were known 4 internal harmonics being extreme,this was why they were bad for main bearing wear.If you have a STOCK 400 sbc rotating assembly,your crankshaft may b slightly out,or youre vib dampner could b shot.You need the print sheet 4 the thing to find out what your crank's center was.If the center balance wasnt perfect the compensation level of the dampener may not b enough or could b working against the flywheel if it was recentered.These things can b hard to get right.The bigger the stroke the more this is so.The center of harmoics or T.H.D.(total harmonic distortion)as its often called in a 3.75' stroke,piston & rod weights pend,is usually around 2,500rpm.So having a custom engine like that u are starting in the right spot.It may b uncureable with that engine.  

Firstly you say the driveshaft is'nt balanced? its the first thing I would do is have it balanced.Next is the motor and transmission original?or is it a Bitzza?you know a mix n' match deal? Harmonic balancers and torque convertors are related to the crankshaft in terms of balance,being either internally or externally balanced.You cant mix the two even though the parts will bolt on.....................Another place I'd look is all the tyres,spindles and wheel bearings........If vibration is coming directly from the motor,check that something is'nt loose like a flywheel or covertor bolt, and all external bearings and pulleys for wear and alignment respectively.Also check differential crown bolts as these can come loose,especially wth roll backs and burnouts.