454 Flexplate question

[Hair]

71 GMC with Inline 6 and power-glide tranny - I pulled the I6 and put in a 1989 454 from an RV. I have not started the engine yet, still waiting on a carb and a starter. The flex plate on the 454 would not bolt to the torque converter on the power-glide, so I just used the flex plate that was on the I6. But now I've learned that the 454 is externally balanced and uses a special flex plate.
Here are my questions:
1. Do I need to take the I6 Flex plate off and get the correct flex plate?
2. What will happen if I run the I6 Flex plate?
3. Can I buy a 454 flex plate with a hole pattern that will fit the PG torque converter?
4. Will a 454 flex plate fit in the bell housing of the PG?

[kwmech]

Yes you do.
It will create an engine vibration being out of balance.
A new 454 plate from Summit should have a dual bolt pattern
It should clear but if it does rub, minor grinding can make it clear the aluminum nubs inside the bell

[Ol Blue K20]

If you run it with 6 cyl flex. You will shake the main and thrust bearings out of it in minutes. Probably ruin pump in the trans too.

[Hair]

Ole Blue,
Within minutes??!! You got to be messing with me? I figured it would cause issues over time? If you're serious, I'm glad I haven't tried to fire it up!

[Ol Blue K20]

It took about 10 minutes to screw up a fresh small block 400 / turbo 350.

[MARKDTN]

Originally Posted by Ol Blue K20

If you run it with 6 cyl flex. You will shake the main and thrust bearings out of it in minutes. Probably ruin pump in the trans too.

Agree. 400SB and 454 have unique flywheels/flexplates. You do not want to run external balance style on them.

[LockDoc]

-
Don't they make a separate bolt on balancer plate that will compensate for that?

LockDoc

[Wrenchbender Ret]

If they are the same diameter, drill holes in the 454 plate to match the converter.
It will have a horrible vibration if you use it.
I had a customer onetime that had put a sb400 plate on a 350. It didn't damage anything but had a bad vibration. I went up through the opening on the bottom & knocked the weight off. It saved tearing it down & worked fine.

[68Gold/white]

Originally Posted by Hair

Ole Blue,
Within minutes??!! You got to be messing with me? I figured it would cause issues over time? If you're serious, I'm glad I haven't tried to fire it up!

I've heard of this "mistake" by a couple of shade tree mechanics...had to start all over again with build...

Concerning the good olden days of Chevrolet V-8 engines, the largest displacements of the small block (400) and the big block (454) were externally balanced at the harmonic dampener and at the fly wheel/flex plate, AND I believe the 454 and 400 fly wheels are different..

There's nothing to buy to add to a neutral balanced flywheel... A person COULD have a weight added to a neutral balanced flywheel. It would have to be the exact precise weight and in the exact precise location...all this would cost more than buying the correct needed fly wheel/flexplate...

[Mike C]

Most aftermarket flexplates are dual drilled for both patterns. $60 gets you an SFI certified one from Summit, Jegs, Amazon, etc. that is externally balanced for your 454.