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been checking out flywheel selections for an upcoming 351m to 400 conversion and getting conflicting info on the balance.
searching hasn't been very definitive.
retailers sell neutral, 28, and 50 flex plates, and the flywheels mostly just say "balanced" ???
internals will be mostly stock, bumping compression on the pistons.
the 351m with C6 is in remote storage, the 400 donor didn't have the balancer or flywheel so I cannot easily check what it is.
been told to go 180 teeth.
already have a ZF for the swap and want the 12" clutch setup.
I know all the 289-351 windsors are 28 oz balance, not certain on the 400
what say 'ye 400 experts??
Last edited by 78Broncoinpieces; Mar 5, 2025 at 12:10 PM.
Reason: add more info
All 351M and 400 engines are externally balanced using the 28oz hardware. This never changes and the only engine that uses the 50oz is the 5.0 from about 1983 up. 351W also uses the 28oz with or without a 1 piece rear main seal.
ordered and received a new Centerforce flywheel.
huge for a 12" clutch and 40 lbs. should smooth out the idle.
drilled for the 28 oz imbalance.
they ain't cheap.
ordered and received a new Centerforce flywheel.
huge for a 12" clutch and 40 lbs. should smooth out the idle.
drilled for the 28 oz imbalance.
they ain't cheap.
Pretty too.
Had a '65 GTO, 400 RA engine swapped, 4spd, bumpy Lunati cam, good solid 12 sec car. Put in a 40 lb steel Hays FW with Lakewood scatter shield, it smoothed the rough idle. Hell on the transmission and Chevy 12 bolt rear though .... or was it just how I drove it those times when they busted.
Seen a flywheel bust on a Chevelle, stock flywheel, but guy was reving it hard by hand on linkage, leaning in over fender. Flywheel let go, cleaned the firewall, he was lucky .... like "real lucky".
What about an automatic flex plate? Seems like the weights are not that big. I know the converter takes care of adding mass but not engine balancing..
Just a curiousity
That's right, the weight on the flexplate takes care of the assembly balance and the torque converter adds mass just like a flywheel does. Have you ever started even a stock engine with just a flexplate? It really makes it sound snappy and sort of racy.
One time a long time ago a circle track customer of mine called me after he picked up his engine after running it on the dyno. We had done about 100 pulls worth of work so we had run it a lot. When it got it home and put it in the race car he couldn't get it to idle. On the dyno I run a flexplate with an aluminum drive plate so the mass is pretty low but in his race car he had a transmission from Lane that used a flexplate for starting only and when you let out on the clutch that flexplate didn't turn with the engine. It had so much less inertia that it had to idle about 3 or 400 RPM higher than on the dyno.
the imbalance is described as ounce-inches.
similar to torque ft lbs the farther out the force is applied the less force is needed.
the weights on a flex plate are way out by the teeth so its actually a lot less than 28 oz.
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