[75chev4x4]

I have a vibration on my new crate chev 350 turbo 400 trans. Didn't really think about it until I noticed it on this motor, but i had the same vibration on my old 350 so now I know it was not the motor. It will do it in the upper rpm range( tach not installed yet) 2800 plus I would guess. It does this while driving in that rpm range or Idle if I bring it up to that rpm and hold it for any period of time. My question is, and I didn't even think of this until today, but does the flex plate have to bolt to the torque converter in a certain location? I didn't think it would matter when installing the motor because the motor is internally balanced. I'm thinking this could be where the vibration is, but it could come from something else too.

[DoubleVision]

If the converter bolted up it`s in the right location as it doesn`t have a certain spot it has to be. When you replaced engines did you use any pieces off the older engine? Have you checked all mounts? Driveshaft U joint wear? Tire balance/out of round?

[75chev4x4]

converter bolted up on the 3 holes that it fits on the flex plate. I think it has to be the engine double vision because it will do it when the truck is parked and running just the motor at 2800 rpm or so. Got a gm crate motor so I reused the sfi flexplate (new a year ago) same torque converter, same motor mounts (new a year ago), fuel pump, wp, ps pump, headers, carb and intake. Could a belt being to tight say on the power steering pump cause this? Noticed this belt fits a bit snug before being adjusted down tight. Thanks.

[familyguy]

did your crate motor come with a new harmonic balancer or did you reuse the old one ? good luck.

[DoubleVision]

Thanks for the list. Okay here`s the beans. When you replaced the flex plate for the one you have now, was the vibration there with it? If not then it points at the flexplate and I don`t care what it costs or how good a unit it is someone could have goofed on it`s balance. I used to have a `79 Z28. Going down the road to 50 Mph it was fine, when you got to 55 it would start a pulsating vibration. I changed all kinds of stuff chasing that vibration down to no avail. Finally, I sold the car to a friend of mine (big mistake) and he later changed out the flexplate and the vibration disappeared. Unbolt the torque converter and rev the engine up and see if the vibration is still present, if it is, then it only leaves left the flexplate, balancer or belt pulleys/accessory drives.

[oldschool hero]

The fastest way to tell if it's a flex plate/ torque converter problem is ,take the bolts out from the converter and index it to the next hole and see if there is a change in the vibration.If no change or little change do it again to the next hole and check it.
Sounds like you have a definate problem in that area if I understood your post correctly. Ai around 2,500 + RPM driving or stationary? If it's vibrating stationary at that RPM then it eliminates the drive shaft.

[75chev4x4]

Not sure about the vibration before the previous small block because the truck had a 454 in it and I had to change flex plates to use the small block. Although thinking now, the 454 was a newer crate and a 1 piece rear seal is why I had to change flex plates. Could that have something to do with it? Would the converter be different for a 1 piece rear seal to a 2 piece like the small blocks I have had in it? Otherwise I will try what you guys said about unbolting the converter from the flex plate. Thanks everyone.

[DoubleVision]

The converter doesn`t know or care what crank is in front of it so it has no effect on engine balance.
The fluid in the converter acts as it`s balance and unless the converter is bad internally they don`t cause any kind of vibration. However if you unbolt it and the vibration stops, either the converter is bad or the front pump/drive assembly of the tranny is bad.

[75chev4x4]

Gotcha, great Doublevision, thanks!

[DLux]

I Know this Thread is a couple years old, I was wondering if you found the source of the vibration, I have the same issue going on with my 1985 Chev PU, Had the vibration before I installed a new motor, used a different flex plate, still had the vibration, rebuilt the trans, new torque converter, still have the vibration. If I disconnect the converter the vibration goes away. I'm going to try moving the bolt locations on the converter/flex plate.

[hcompton]

Flexplate broken? They can be crack and hard to tell if its cracked. They are also cheap and do where out. So might be best to slap a new one in it and see. You used all the right bolts and not some from a hardware store. Just one regular bolt and the rest oem will cause some vibration.

But hands down i would double check the balancer and maybe replace with a 8" 13 pound balancer will smooth out your 350. But if something else is wrong it will only help a little.

Also what is the condition of the trans? A bad trans bearing can vibrate also a bad tq converter will cause some issues. Drive shaft is also likely if the problem is only when driving.

[DLux]

I replaced the balancer and flex plate with the new engine, I did use correct hardware. I'm hoping that re clocking the fastener locations on the flex plate will help.

Thanks