Flywheel balance

[sfm6s]

I am rebuilding a 390gt engine and I noticed that the flywheel (std cast iron) has a slight imbalance built in - a raised section on the back face, 2.5" wide in a 90 deg quadrant. Given that the engine is internal/neutral balance why is there any flywheel imbalance at all?

|Cheers
Colin

[PaulProe]

Suspect it is a flywheel from a 428, which is externally balanced. They won't play well together

Paul

[olddog]

Originally Posted by sfm6s

I am rebuilding a 390gt engine and I noticed that the flywheel (std cast iron) has a slight imbalance built in - a raised section on the back face, 2.5" wide in a 90 deg quadrant. Given that the engine is internal/neutral balance why is there any flywheel imbalance at all?

|Cheers
Colin

Are you calling it a 390gt engine based off of casting numbers? Have you actually measured the stroke? It is possible someone has dropped a 428 crank in a 390 block, in which case that crank would be externally balanced.

[sfm6s]

Yes I am thinking this is a 428 flywheel - strange it didn't feel too badly balanced before. Its definitely a 390 and it has a neutral front damper.

[olddog]

Well a great many things are possible, but most don't make sense to do. Unless you know the history of the engine build, you never know what might have been done.

1) It is possible that someone took a 428 flywheel and neutral balance it. Hang it on a low friction bearing and see if the heave spot rotates to 6 O'clock. If it will set still at any position it is neutral.

2) It is possible that someone had the crankshaft and rotating assembly balanced with that flywheel and harmonic balancer. No idea why anyone would do such a thing.

I just do not think an imbalance would run smooth at all rpm. There would have to have been some vibration at some rpm, if there was a miss-match. Now I seem to fuzzily remember that some 428 Cobra Jet engines had the hatchet on the front of the engine, but maybe not all of them did. If some did not have the hatchet,maybe there was more than one offset imbalance flywheel made.

Here is a link to some different 428/410 flywheel casting numbers. I hope this helps.
https://www.428cobrajet.org/id-flywheel

[sfm6s]

Well it's a neutral balance flywheel now that it has paid a visit to the milling machine. Off to the balancing shop tomorrow along with the rest of the reciprocating assy.