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Old 04-16-2020, 12:19 AM   #1
teeteeplz
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Cracked 350 Crank, build 383?

Hey hoping to get some suggestions as to an economical yet fun solution to my recent dilemma.
Ive got a 1972 C-10 with a jasper reman 350SBC (quadrajet, stock manifolds, stock build i believe) installed in 2014 and ive put about 20k miles on it since the install.

Last week or so I noticed a knocking of a two tones (hi,lo) near the motor and noticed the harmonic balancer was hitting the timing tab, and had more wobble to it than normal. I bent the tab up a bit hoping for a quick fix, until i got the time and $ to replace the original harmonic balancer. it took care of the pinging of hitting that timing tab, but a deep knocking remained, faint at idle until hot, and noticeable with the vehicle at 0mph and the truck in gear (auto th350 trans). oil pressures seem normal

I dropped the truck off at my local repair shop for further investigation, and possible harmonic balancer replacement. when they popped it off, they noticed (and I later verified) a large amount of play on the snout? of the crankshaft. They claim that it is cracked, and others have told me that it has broken in a way that is allowing the motor to run but is on very borrowed time. The motor has never been overheated to my knowledge, but was installed by a rather inept mechanic prior to my ownership.

Is it safe to assume the Crankshaft is toast at this point and requires engine removal from the truck, crank replacement, and Main bearings done while in there? How much would this cost for a minimal fix with new parts? Is this a smart solution?
Would I be better off Building a 383 stroker out of this lightly used 350 core,reman 350SB? As the Crank sounds like its in need of a replacement anyhow. Or should I buy a crate of some sort and dump this 350?

I believe the rear end gear ratio on this truck is a 3.08(3.07?) as my tach is just about 3k if im doing 85mph with 265/75/15. I dont have known any issues with the TH350.

-true dual exhaust flowmaster 40's

The truck has been in my family since around 94 and has a lot of sentimental value. I appreciate any recommendations on what I can do to add some fun with out a second mortgage

the only other modification I really want for this "rebuild/repair/replacement etc" Is switching to Posi-trac. One wheel burn-outs just arent cutting it!
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Old 04-16-2020, 12:41 AM   #2
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Re: Cracked 350 Crank, build 383?

I noticed my last post on this truck was from 2007, as I was anticipating driving this truck at age 16 (over a decade ago). I did end up having to pay for it to get it from my pops a couple years ago, but I'm glad they made they decided to hold off on me. I prolly would have wrecked it in my youth. Anyhow, here are some updated photos, mostly the same, just a few minor changes thankfully!
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Old 04-16-2020, 12:51 AM   #3
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Re: Cracked 350 Crank, build 383?

nice!
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Old 04-16-2020, 06:16 AM   #4
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Re: Cracked 350 Crank, build 383?

Very nice looking truck. As for the engine you could remove the flywheel cover to watch the fore and aft movement of the crankshaft on both ends of the engine. Wobble of the damper is not good. Or it's yank the engine to pull the pan and give it a good look over. Lower end noise is never good. A fella used to be able to buy a stock reman'd crank kit for less than 200 for an affordable repair. A stroker crank kit runs the cost up. How deep is your pocket?

Loose torque converter bolts make a knocking noise too.
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Old 04-16-2020, 08:26 AM   #5
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Re: Cracked 350 Crank, build 383?

If the rest of the motor is in good shape, I would disassemble and get the bore size. I would then buy an Eagle internally balanced 383 kit from someone like Competition Products.

Complete rotating assembly, balanced for about $800.
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Old 04-16-2020, 02:17 PM   #6
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Thanks for the replyís! I think the motor is in good shape besides the alleged broken crankshaft. I like the idea of the 383 kit plus some supporting mods (intake heads cam headers carb(if required, havenít had any Quadrajet issues yet, but hear they can be tough to tune to the un-acquainted). Iíd like to make more than 400hp/tq or so with the motor. Budget is flexible, Iíd rather wait and save and do it right than to skimp out and wish I had done it differently in the future. Iím hoping to get it done with 5k or less including positrac upgrade.!
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Old 04-16-2020, 04:56 PM   #7
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Re: Cracked 350 Crank, build 383?

Nice set up! sorry about the noise...

Old saying..."there is not replacement for displacement".

Stroke it! so long as you make sure the crank is junk, or stroke it anyway.

I can't help with costs as I have only stroked a F@rd, and P@ntiacs are born stroked. Both are expensives motors to build, but the Tried & True 350 ought to be budget friendly.

Think about a rear gear change since you're swapping guts out. Next thing you know, you'll need wider wheels and stickier tires...It's fun spending your money.
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Old 04-16-2020, 05:14 PM   #8
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Re: Cracked 350 Crank, build 383?

My two cents:

Its virtually impossible to know the extent of the damage without opening it up. I wouldn't consider anything short of a full tear down. Take the block to get checked for cracks in the webbing. Irregular vibrations at high rpms can cause all sorts of problems.

If the block gets an OK from a reputable shop, you've got a clean slate. At that point, if you're looking for 400hp/400tq you may as well go with a longer stroke rotating assembly. You could even get a 3.875" crank and build a 390 (std bore) or a 395 (@ 4.030" bore).

You'll likely want new heads and valve train to provide better flow. Personally I like Holley carbs, but they're are several good options out there. You may even consider an EFI system. My brother put a Holley Sniper kit on his '68 Firebird and he loves it. It's self-tuning and the car runs much better in the cold weather.

I'd have a hard time sleeping if I rebuilt a motor without having it checked for cracks first.
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Old 04-16-2020, 06:50 PM   #9
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Re: Cracked 350 Crank, build 383?

Quote:
Originally Posted by crazyjeenyus View Post
My two cents:

Its virtually impossible to know the extent of the damage without opening it up. I wouldn't consider anything short of a full tear down. Take the block to get checked for cracks in the webbing. Irregular vibrations at high rpms can cause all sorts of problems.

If the block gets an OK from a reputable shop, you've got a clean slate. At that point, if you're looking for 400hp/400tq you may as well go with a longer stroke rotating assembly. You could even get a 3.875" crank and build a 390 (std bore) or a 395 (@ 4.030" bore).

You'll likely want new heads and valve train to provide better flow. Personally I like Holley carbs, but they're are several good options out there. You may even consider an EFI system. My brother put a Holley Sniper kit on his '68 Firebird and he loves it. It's self-tuning and the car runs much better in the cold weather.

I'd have a hard time sleeping if I rebuilt a motor without having it checked for cracks first.
I agree in principle, but if you're going that far, I'd consider an L31 crate. The roller cam and Vortec heads will make a huge difference in how it makes power.
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Old 04-16-2020, 10:16 PM   #10
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Re: Cracked 350 Crank, build 383?

Makes me kinda wonder if the thrust bearing isn’t out of it. I guess it’s possible to be cracked and flexing but I would expect it would only flex once or twice before it broke all the way thru and the balancer wouldnt be turning if it even ran at all. Thrust bearings seem a lot more likely in my mind. They can and will make all kinds of noise with the crank walking back and forth. I guess that internet diagnoses also depends on the direction of the play, if it’s in and out thrust bearing is my bet, up and down... not as likely to be the thrust...
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Old 04-17-2020, 10:14 AM   #11
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Re: Cracked 350 Crank, build 383?

Quote:
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I agree in principle, but if you're going that far, I'd consider an L31 crate. The roller cam and Vortec heads will make a huge difference in how it makes power.
L31 is a far cry from 400/400. Getting the block machined, purchasing a good rotating assembly and some decent heads shouldn't put you much higher than L31 would cost. And this way you'll end up with a lot more than 250hp.

Going custom allows you to choose all your engine parts. Use your desired horsepower/torque output as the basis for your design and go from there.
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Old 04-17-2020, 02:36 PM   #12
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Re: Cracked 350 Crank, build 383?

The L31 with a HT 383 cam in it will put out 330 HP and 400 ft-lbs. I didn't read the part where he said he wanted 400/400. Torque is what you want in a heavy truck anyway.
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Old 04-17-2020, 06:42 PM   #13
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Re: Cracked 350 Crank, build 383?

I might agree with the L31 advice, or if you have time yank it out and have a shop check it out. I went with a 4-bolt main 350 195-249hp crate motor (hp rating varies depending on the web site) but am adding a Comp Cams roller cam & lifter set with Brodix IK-180 alumimum heads. Had I took more time to investigate, I may have gone with a Vortec L31 and maybe a different cam. Or had the timing worked out, my cousin just gave my dad a later 350 with a 1-piece rear seal and provisions for a factory roller cam ('95-'99, casting 10243880), which would be a great block to build on.
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Old 04-18-2020, 12:17 AM   #14
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Re: Cracked 350 Crank, build 383?

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Originally Posted by 57taskforce View Post
Makes me kinda wonder if the thrust bearing isnít out of it. I guess itís possible to be cracked and flexing but I would expect it would only flex once or twice before it broke all the way thru and the balancer wouldnt be turning if it even ran at all. Thrust bearings seem a lot more likely in my mind. They can and will make all kinds of noise with the crank walking back and forth. I guess that internet diagnoses also depends on the direction of the play, if itís in and out thrust bearing is my bet, up and down... not as likely to be the thrust...
This was my first thoughts. It kinda hard to imagine that a crank which is cracked all the way through, enough to allow wobble at the dampener is going to get very far without some serious problems. Thrust bearing seems more likely. Still, building a 383 is fun work if you're not pressed for the motor.
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Old 04-18-2020, 01:17 AM   #15
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Re: Cracked 350 Crank, build 383?

Best Bet Pull It And Check Crank end Play Might Need To Be Line Board Seeing Crankshaft Is Wobbling in a block< Then All The Metal Shavings And Maybe Thurst And Bearing Shavings! might Be Cheaper To buy A GM 383 3 Year 50k Miles Unless You Know a Great Machine Shop My 2 Cents
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Old 04-19-2020, 06:31 PM   #16
teeteeplz
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Re: Cracked 350 Crank, build 383?

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Makes me kinda wonder if the thrust bearing isnít out of it. I guess itís possible to be cracked and flexing but I would expect it would only flex once or twice before it broke all the way thru and the balancer wouldnt be turning if it even ran at all. Thrust bearings seem a lot more likely in my mind. They can and will make all kinds of noise with the crank walking back and forth. I guess that internet diagnoses also depends on the direction of the play, if itís in and out thrust bearing is my bet, up and down... not as likely to be the thrust...
it definitely is easily movable up and down sadly
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Old 04-19-2020, 07:03 PM   #17
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Re: Cracked 350 Crank, build 383?

any suggestions for a crate motor? https://blueprintengines.com/product...ock-bp38318ct1 4,550$ after rebate+tax. vs https://blueprintengines.com/product...or-bp38318ctc1 5300$ +tax

I like that these are new cast blocks vs "seasoned" blocks. is this an important factor?

I am tempted to find a motor with a forged crank after this one took such an early crap out on me. I imagine supplemental modifications would need to be made to the TH350 to make it survive awhile.

https://blueprintengines.com/product...etor-bp3961ctc 5,500$+tax

The 396 is more power than I currently am planning for, but its so close in price to the fully dressed 383, I dont want to kick myself in the pants later wishing i would have done this project differently.

I dont have any intentions of drag racing with the truck, just want tire smoking adrenaline goosebumps and the classic lopey idle.

also, project cost creep is definitely going to be difficult to manage on this, but hey when things are done right, and properly taken care of, they tend to last a long time. thanks everyone for their insight.
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Old 04-19-2020, 07:14 PM   #18
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Re: Cracked 350 Crank, build 383?

Quote:
Originally Posted by teeteeplz View Post
it definitely is easily movable up and down sadly
Itíll be interesting to see what you find.
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