I need to replace my 168t Mercruiser Flywheel on my 5.7L 2001 Mercruiser. Can I use any GM compatable 168tooth flywheel as long as it is for a one piece rear main seal? Theres nodular iron, cast iron, steel and many more plus many different weights...Also replacing the triangular 3 bolt engine coupler.... My flywheel was rusted away, teeth brittle thin and slivers of rusted material falling off it.
Suggestions please....need to get her back on the water

I believe the only diff on sbc between a one piece seal and two is the bolt pattern on the mating surface of cranks . Unless its stroked any 168t for a one piece seal should work fine .

If the flywheel is good and just the teeth are bad you maybe could just put on a new ring. I have changed them before.

Teeth are thin and agree on changing the ring gear but the flywheel is shot! rust chunks coming off in slivers / sheets...
Wondering what would be best as far as flywheel material vs. cost......this motors stock and no plans on doing anything to it other than maybe....maaaybe doing an overhaul on it while its out of boat.

80 bucks at summitt. probably 25 to 50 at boneyard...any one piece big dia wheel will do.

the 5.7 [350] is a neutral balance engine,make sure you get a flywheel without a counterweight.a 454 or 502 flywheel is externally balanced,so those flywheels wont work,even though they might bolt up.

My question would be, why is it rusting so badly????????

Well i have the same questions.....and no real answers. The motor has quite a bit of rust...wondering if the bilge was full of water for an extended time...or.... was the bilge pump no working and while riding around, it was sitting in salt water..... i dont know.
I havent owned the boat for a long time but i do know the bilge pump works and since ive owned it, its always washed / flushed every time. Lots of rust back side of block between flywheel and block, spots along the lower sides of block and metal accessory brackets have some rust. I dont see any water damage anywhere else in the boat and everything else looks in excellent condition......pulled engine mount bolts and runners feel solid as can be.
Not sure how well the guy before me took care of it (looks good!)...but i do have lots of documentation of all the services/oil changes and any repairs.
Anyone have any ideas?

Depending on the climate where you live it may have been caused by the natural heating and cooling cycles causing the engine to sweat . When storing i try to keep the engine bay ventilated by opening the hatch a bit and also a coat of WD40 works wonders to keep rust at bay and moisture from infiltrating your plug wires and cap .

Better check for rusted out freeze plugs at the rear of the block. Over time automotive engines find their way into older boats. Many DIY boaters in fresh water areas won't bother to change the freeze plugs or head gaskets. It becomes a matter of when ...not if. Salt water won't wait but a few months...then anything that's not marine will be like an Alka-seltzer in a glass of water.