7.4L merc rebuild questions HELP!
 

Ok so the first trip to the lake this season my boat started fine held 40 lbs of oil pressure, ran 160 degrees things looked good. She idled for about 10 minutes at dock then eased out of no wake zone and throttled in slowly.. Boated all day, only to start it up about 10 p.m the go back to trailer as I was almost on plane I heard an awful knocking from engine bay so I immediately killed it. Got a pull back to dock, loaded and went home... I pulled engine next day drained and pulled pan to find the crank was broken :/ this engine only had about 40 hrs on it it was built by sky tower performance in somerset ky. My question is..... I already have crane came freshly honed cylinders can I replace mains and crank, or would you line bore it first? What should I do... Time isn't an issue because I just bought another 454 and got the boat back on the water but I want an engine that will jerk it out of the water and run about 5200-5500 Rpms safely.... No constantly but for a few minutes at a time ya know.... Thanks!!

Just have a shop measure everything for you, that will tell you is anything is tweaked. Dont put another cast 7.4 crank back in it. get a new decent crank and rebalance the whole thing.

Steel crank? So I just need to take the block and whole rotating assembly to them to have them check rods and line bore? Do you have a rough estimate on what line boring typically costs?

one or two peice rear main seal? what gen is the motor? if your a 1 piece gen 5 or 6 and budget minded, look for a take out crank from a 502 that someone is building a 540 etc... there are plenty out there. 454 mag cranks are good, 7.4 330 has cast, not good.

How do u tell what gen the 454 is I'm new to the big block world....

Line bore/hone isnt expensive. But its just a small part of building a solid shortblock assembly. If you are unfamiliar with any of this, I highly recommend taking the entire engine to a reputable machine shop, and letting them machine it, balance it, assemble it, etc. If you bolt $hit together, without precise measurments and tolerances, you'll be doing it over in a very short time.

Mark IV will have a fuel pump boss on the block, with a 10 bolt timing cover. Gen V, no fuel pump boss, 10 bolt cover. Gen VI, had fuel pump boss, but a 6 bolt timing cover.

I've built performance Diesel engines I'm familiar with the engine building but not so much the 454 itself.... That's or performance gas engines as a whole... I mean the bottom end of a diesel and gas are the same with longer stroke essentially.... So I'm ASSUMING I can do the job... It's a winter project mainly so I can have a faster boat in the spring lol

The block with broke crank has no fuel pump boss.

Dale, pull the oil filter and have a machine shop cut it open for you. If it is full of debris, you can figure on pretty much a complete rebuild, because that $hit will go everywhere. Plan on replacing the oil filter too. I would also make sure the correct balancer and flywheel was installed on this engine too. It's not common for cranks to just break without other mitigating factors. You need to do a proper "post mortem" on this engine to try to determine the cause, so history does not repeat itself with your new engine.

When you get it out check the flange on the crank. Sounds like you have a Gen V block, so it should have a round flange and a one-piece rear main seal. The Gen V and Gen VI take the same crank, and 502 and 454 cranks share the same stroke, so you can use either for your rebuild if you are rebalancing it anyway. If you are going to romp on it, best to upgrade the rest of the reciprocating assy - forged pistons, possibly better rods (or at least upgrade the weak 3/8 rod bolts to ARP), etc. You will have lots of questions - first search through this forum. There is a lot of good info without rehashing everything, but the members on this forum are happy to help. I built a stoked 454 (489) myself with the help of many on this forum. Sounds like you are capable o the same if you have the proper tools. Get ready to open that checkbook - it's a slippery slope! :)