Because it is matched to the motor.
I'm sure there is a balancer in your area that can match a new one if needed. Hell they can even match a flexplate to a flywheel if needed.

just fyi,they are not balanced with the engine,you would be suprised if you knew how gm does their balancing,it is not like a quality balance job done in a quality machine shop.the 454 and 502 flywheel,s are bolted on the already assembled engine,not done on a balancing machine.i have seen bbc engines that had all the pistons&rods so mismatched it would scare you but they still ship them out and they run forever at the rpm level they were designed to run at.

Not just Chevy. Took apart a Ford 302 one time that had 3 rods for a 289 in it. Now how does that happen and how do you balance that?

do the GM 454 and 496 wheels share same part number? I would think with the different stroke, they would be weighted differently. also, a truck 496 goes to 4k rpm, and driver (or trans) shifts to higher gear. then the motor runs along 95% of the time or more at less than 50% throttle at 2600 or so. 496 in a perf boat, engine hits 4k...sometimes driver throttles back and holds it there at cruise. mosta time he is heading up to 5k or more. get it balanced. really balanced. not assembled with GM parts that are in spec.

NO, they do not share the same part number. Actually, there are not many things that they do share. People assume a 496 8.1L GM/MERC engine is a stroked 454. It is not.

I personally don't have experience with the merc 496 engine. However, I would not be surprised if the flywheel, was identical to a GM truck flywheel. I highly doubt mercury got these engines shipped in from GM, disassembled them, custom balanced them, etc, and reassembled. Best guy to talk to, is Raylar. He would know.

aren't the 8.1 496 metric sizing in some way? in the prev. links posted they say metric.

I know the 496 manifold studs and bolts are.

Well so far the 496 is a unique engine.out of the 4 I have disassembled 1 of them being a junkyard truck motor with 215,000 miles. None of them have any cylinder wear. Manual says compression second ring end gap .0177 min. These motors old ring set measured .0180 in old bore that to me is freaking amazing. Rod and main bearings almost untouched where as in the surface looked to be "like new" shape. Coated piston skirts slight to no wear.
I am really excited about rebuilding these engines and can't wait to see how they perform in my 43ft 15ft beam boat.

I am havering the rotating assembly re-balanced for my peace of mind.

The 3 merc motors had bad blockage in the coolant crossover tube with a pinkish orange paste. Highly recommend flushing the coolant systems on these closed cooling units. I understand why merc. Recommends coolant flush and change

well they have several flywheels used on the same engines if you look in the GM parts, so if you can use the ones that came with that engine originally you are better off having the "correct" one...