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! :)

BTW, I'm going to be in your neck of the woods at Green River Lake this weekend.

sounds like crank was cracked, or just a fluke, and a cast crank is fine for a setup like this, raced cast ones many time at way more rpm than this will ever see..

For about $200 you can buy a 4" stroke cast steel crank from Scat. These are actually a pretty decent crank for a mild to marine build - they are rated to around 650 HP. That would be an upgrade over the stock GM cast crank - the steel used is a higher grade alloy. If you elect to go for more of a performance build, might as well go forged if for no other reason the peace of mind, resale, and the ability to handle more power if you elect to go with a supercharger. If you decide to go the stroker route, prepare to open a can of worms - you will find yourself upgrading everything else too! ;)

There was very little debris in filter, best friend owns a shop and he looked through it... Only debris I've found was in bottom of pan... Oil galleys and filter lines have been flushed since then as well... I used oil cooler and lines on new engine so I had to make extra sure it was clean. It has forged dome pistons in it already about 9.0:1.. H beam rods, I don't know about main and rod bolts tho I didn't assemble engine... But as I mentioned above u can still see the honing "X" on the cylinder walls it's practically bran new... Only thing we can come up with was a bad cast in crank, or a poor line bore? What I was going to do is have rods checked to make sure nothing bent or stretched, have line bore checked, have cylinders checked, buy a good crank, clevite bearings unless you recommend something else, new oil pump, new main and rod bolts, and go to town.... Or should I not?

And have it balanced....

Was at green last weekend... We go to Cumberland more, watch the fish and wildlife, got a new one over there!!!

I'd find a nice takeout gm forged crank for 200 bucks over a new cast scat or eagle crank myself. Teague sells takeout dimple 7/16 rods cheap.

I've been pounding on chevy steel cranks for years ..Also , a diesel engine is nothing like a big block chevy. Yes they both have crankshafts , yes they both have rods and pistons. Things like piston to wall clearance , rod and main clearances , rod side clearances , ring gaps, cylinder wall finish, lifter bore clearances, and so on, are application specific. For many decades guys who built drag race big block chevy engines daily have a hard time making a MARINE big block chevy stay together.

I understand that, just hoping to save some money, by doing lots of research so I'll accept any tips and help I can get! Right now there is just a fresh race engine with different cam and carb in my boat.... It does ok, but I'm scared of it not holding together so I'm very easy on it.... I want what I had, which was a little power and dependability, until crank snapped obviously, hoping I can get it... But u are just suggesting take it all to machine shop and let them go at it? Or is it possible with research and knowledge to DIY.... I have a good friend who builds 454 pulling truck engines also... Which I understand he turns 8500 for 20 seconds .... I'll be turning 5000 for an extended period..,. But different applications, different parts...???

And surely I can find specs and clearances online???

What did the main bearings and main journals look like? If line hone/bore was off, I would expect to see uneven wear on the main bearings. I had to have my line hone done over by a different shop because the first guy had it tapering from .0025 in the #1 main all the way to .0045 in the rear main. Even something like that would not typically break a crank. What kind of balancer was on the engine? Might consider replacing it with a good new SFI approved unit to be safe. Harmonics can break cranks too. Examine the area where the crank broke to see if there is evidence that it was an old crack. However, forged cranks can run for a while with a small crack, while cast cranks usually fail catastrophically when a crack develops.

Thanks for the heads up on that. Will have the family with me, so won't be getting too crazy. Still, we will wrap the "adult sodas" in can wraps and huggies. I make can wraps by grinding the top and bottom off an aluminum soda can with a belt sander and then cutting it down the side. Slips right over a beer can, and makes it look like you are drinking a Mountain Dew, Pepsi, whatever. Stealth mode. :D

Dale, when you get your engine built over the winter, I would recommend running it on a dyno to break it in and get your fuel / timing dialed in. I had mine donw by Dale Meers in Buffalo, KY, right outside Hodgenville. He does a lot of MOPARS, but does BBC stuff too. $400 to run a full day on the dyno. Money well spent. Another guy to talk to is Eddie Young in Tenn. He posts on here frequently, and specializes in performance marine builds. Good luck.

Journals look fine however I haven't plastigauged anything the #1and 2bearing were spun but not demolished makes me feel like it spun when crank broke??? Everything else looked virtually new

Everyone says dale meers... I guess that's a good thing.... He does machine work too right?

I would stay away from using plastigauge on a performance marine build. Needs to be done with a good set of micrometers. Most folks are aiming for .0030 on the mains and .0027 on the rods. Lots of other critical clearances that should be checked too.

Sounds like a balance problem to me.

Oops - wrong thread!

Thanks! I'll definitely take that into consideration any other tips are much appreciated

Here's a big one - if you have a Gen V / VI motor, make sure you have the correct oil bypass valve installed in the filter pad on the block. Car / truck applications used an 11 lb valve; boats require a 30 lb valve, due to the restriction from the extra lines for the oil cooler and remote filter. If you don't change this out, you will kick the bypass and send hot dirty oil to your bearings. Alternately you can plug it alltogether. Do a search on the General section under the Technical subforum.

Hmmm........I've never heard of Skytower, but safe to say you probably wouldn't recommend them for a build? :lolhit:

Actually strangely enough I would... Always heard good things... I don't think my problem was their fault, more then likely a cracked crank and it finally broke... That seems to be the common thoughts anyway...