454 Flexplate question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Hair · 03-11-2020, 11:15 PM

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** · 03-12-2020, 12:51 AM

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** · 03-12-2020, 09:16 AM

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.

MARKDTN · 03-12-2020, 01:47 PM

Quote:

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** · 03-12-2020, 02:30 PM

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

LockDoc

Hair · 03-12-2020, 10:20 AM

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!

68Gold/white · 03-12-2020, 04:46 PM

Quote:

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 · 03-13-2020, 07:33 AM

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.

**Ol Blue K20** · 03-12-2020, 01:00 PM

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

Wrenchbender Ret · 03-12-2020, 03:28 PM

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.

03-11-2020, 11:15 PM	#1
Hair Registered User Join Date: Mar 2008 Location: Tullahoma, TN Posts: 406	454 Flexplate question 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?

03-12-2020, 12:51 AM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,488	Re: 454 Flexplate question 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

03-12-2020, 09:16 AM	#3
Ol Blue K20 Proprietor of Dale's Corner Join Date: Nov 2012 Location: Vacaville , CA Posts: 15,986	Re: 454 Flexplate question 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. __________________ "Some Days Chickens And Some Days Feathers" Dale XNGH ECV Sam Brannan 1004 R.I.P. 67ChevyRedneck R.I.P. Grumpy Old Man

03-12-2020, 02:30 PM	#5
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,404	Re: 454 Flexplate question - Don't they make a separate bolt on balancer plate that will compensate for that? LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

03-12-2020, 10:20 AM	#6
Hair Registered User Join Date: Mar 2008 Location: Tullahoma, TN Posts: 406	Re: 454 Flexplate question 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!

03-13-2020, 07:33 AM	#8
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,715	Re: 454 Flexplate question 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. __________________ 44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax

03-12-2020, 01:00 PM	#9
Ol Blue K20 Proprietor of Dale's Corner Join Date: Nov 2012 Location: Vacaville , CA Posts: 15,986	Re: 454 Flexplate question It took about 10 minutes to screw up a fresh small block 400 / turbo 350. __________________ "Some Days Chickens And Some Days Feathers" Dale XNGH ECV Sam Brannan 1004 R.I.P. 67ChevyRedneck R.I.P. Grumpy Old Man

03-12-2020, 03:28 PM	#10
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,207	Re: 454 Flexplate question 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.



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