I just had new big blocks built for my NINJA. The old blocks were 4 bolt mains with steel cranks . 1 crank was craked and both blocks head bolts had been heli-coiled. The new blocks are 2 bolt mains. I went with 2 brand new cast cranks and A R P studs and also A R P head bolt studs. I have been told that I would be better off with the cast cranks because they will flex. I was also told that 2 bolt mains will work fine because I used A R P studs. I was just wanting to know if anyone could let me know if this was right or wrong.
Thanks

go with the forged crank if you want to spin the engine more than 5000rpm. just my .02 worth.

4bolt mains are always stronger. There are aftermarket 4bolt conversions that are available to help hold that crank in at 5000rpm. You can't go wrong with extra support when you're running a motor at those RPM's all day as opposed to a car that runs 1500RPM due to a transmission.

Not to start a pissing battle, but thast not totally true, 2 bolt blocks can handle as much as a 4 bolt block.. Most people aould go with a 4 bolt if they are rebuilding, but there isn't anything bad with a 2 bolt..

Cast cranks will inded flex, but a stock forged gm steel
BB crank is good to about 700hp...

I just went through this myself- I spent a lot of time and money finding a Mark IV four bolt main block- they are not as plentiful as they once were- and all the engine builders I talked to said 2 bolt mains were good to 600 HP. For what most of us on this site are doing with our boats 2 bolt mains and cast cranks work just fine. I think it is more important to run forged pistons, balance everything and run the good head gaskets- just my opinion- Scott

I did buy .030 over 100th dome alum. fordge .Everyone has told me as long as i used ARP studs Id be fine. These motors will be red lining at 5000 rpm so its not like they will be stressed to hard

It’s a boat motor; you can’t stress a motor any harder than in a boat.

Mercury uses steel cranks and 4 bolt blocks on anything over 365 hp for reason.

The 330 hp motors were 4 bolt cast cranks. But not meant to rev past 4800 rpm

600 hp in a car motor is not the same as a boat. Imagine your car driving up a 45 * hill at 5000 rpm towing a8000 lbs trailer for hours at a time. That’s what your boat is doing every time you go out. This is why they use heavy duty parts in boat motors.


I would go on eBay and look for a set of 4 bolt main caps. Have your machinist fit them on your block. And if you’re building anything over 400 hp I would look for a steel crank and dimple rods.




Just my 2 cents

The 2bolt main block should be fine. The cast crank may not do as well if you plan to run it wot very often. Had a cast crank in a 475hp 454,and it broke in front of 7-8 rod journal. It was run hard, but broke at about 150hrs. Max rpm was 5200.

Thanks for the input. I dont know if it was such a good idea but the price was right. Sounds like it could cost more in the long run.