[Mr427]

The FOUR flywheels:
Guys; the 428 has AT LEAST four different balance specs for its flywheels, possibly as many as six. Why?

First, you had the 1966-68 Q-code Thunderbird / Marauder / 7- litre XL engines. That package came with a C-6, and it MIGHT have the same balance specs as the std. Cobra Jet engine.

Then there was the CJ, both manual and automatic. Each had its different flywheels and probably different piston weights (682 grams)

The SCJ has yet another balance specification, since it has the "LeMans", capscrew connecting rods and heavier pistons at 712 grams.

There is a third wieght figure, but I can not state what it was or which model it was. I think it was a version of the CJ engine, w. piston weights between 690 and 700 grams.

So, ANY mix and match of flywheels on these engines may result in failed rear main bearings and in the end ruined cranks and blocks. I have seen it happen, more than once. One particular case involved a ´67 Gt-500, that was converted from auto to manual. The part number on the flywheel was correct, still the engine would not not balance out correctly. At between 2200 and 4500 rpms it would vibrate and you could feel it up through the floors and finally the engine failed.

This is my main cause for my allergic reactions to externally balanced engines.

Have it balanced if you install a new flywheel. You will be glad you did!