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Old 03-21-2014, 02:19 AM
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My crank twisted in half, could you guess why?

500ish miles after installing a RMVB th400 behind a stock high mileage buick 3.8l v6..... This happens.
Now i'm scared to use the thing for my 383..



Almost everyone I have asked about this say it couldn't be the tranny's fault but, I am just not sure..
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Old 03-21-2014, 04:32 AM
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When did this happen, seems to awfully rusty in there ?
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Old 03-21-2014, 05:38 AM
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Malc - you would be amazed at how fast rust happens. I washed my block and between putting the hose away and rolling it back in the shop - <3-5 minutes and I had a film of rust on the entire block, inside and out.

OP - As far as the broken crank - there are lots of ways but a soft coupled engine(torque converter) being broken by a transmission wouldn't be anywhere near the top of the list. Most cranks are cast iron which means a shock or many back to back like pre-ignition (ping), can break one, especially if it had a flaw - slag inclusion during manufacture, dropped and a crack started, wreck with the snout tweaked. If it was the transmission - spinning wheels in snow, water, mud, sand then hitting dry pavement .......big time shock load and about the only way I can think of that a transmission could transfer load - but again, there's that converter to lessen it some.
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Old 03-21-2014, 06:31 AM
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That's a somth'in. Can't believe it's the trans's fault, it would have to be the weak link in the chain and take the hit. Even the drive shaft would have to suffer damage before the crank, huh? So many other weaker parts to go first.
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Old 03-21-2014, 08:07 AM
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My take.. the th400 shifted HARDER.. forcing the engine to drop rpm faster.. by way of slowing the crank.. and the weak crank. now let go..
it most likely was on it's way out maybe with cracks or just a defect that the softer shifting trans didn't bring to light..
that's my guess.. it's not the trans.. fault.. just the added load changes made the weak link break..
the old trans and a wet rail road track that it looses traction then grabs when back on pavement would've most likely done the same thing..
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Old 03-21-2014, 10:40 AM
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NOT the transmissions fault.

Somewhat common occurrence with the standard low performance(non-turbo) Buick V-6.

The reverse pattern manual valve body hits no harder than a full-on shift kit like the B&M Manual Pack valvebody mod kits.

Hard shifting trans will break things AFTER it, like u-joints, driveshafts, rear gears, axles, and twist axle tubes.
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Old 03-21-2014, 10:53 AM
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Quote:
Originally Posted by malc View Post
When did this happen, seems to awfully rusty in there ?

About 9 months ago.
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Old 03-21-2014, 11:07 AM
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Quote:
Originally Posted by gearheadslife View Post
My take.. the th400 shifted HARDER.. forcing the engine to drop rpm faster.. by way of slowing the crank.. and the weak crank. now let go..
it most likely was on it's way out maybe with cracks or just a defect that the softer shifting trans didn't bring to light..
that's my guess.. it's not the trans.. fault.. just the added load changes made the weak link break..
the old trans and a wet rail road track that it looses traction then grabs when back on pavement would've most likely done the same thing..
This is what I am thinking also. The th400 def. shifted harder and faster, dropping rpms very, very quickly.
Is the basic purpose of a clutch? To lighten the load on the crank during shifting?


Is it possible that the torque converter failed somehow and "locked up" transferring the force to the crank?


I should also state that I was simply cruising along at 70mph when the INTENSE vibration started. Towed it home, pulled the trans and the flywheel was just spinning freely. Crank in half.

I thought cranks usually failed at the snout not in the middle like this?
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Old 03-21-2014, 11:17 AM
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I am also concerned about the line pressure of this trans. It would always squirt tranny fluid out of the vent tube at high rpm (3000 is high for this 3.8)

I actually had to run an extension hose from the vent tube up back into the dip stick tube.

Line pressure puts extra load on the crankshaft right? Or just the thrust bearing?
I am not sure if I will ever be comfortable installing this trans behind my 650$ crank..

Since I need a serpentine setup anyways(best I have found is like 300$ ), my plan is to buy a junk yard vortec and run it for awhile. If the crank doesn't break after some mileage, then I will feel more comfortable with the trans.
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Old 03-21-2014, 11:41 AM
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I've seen cast 302 Ford non-HiPo cranks, 351W cast, cast SBC cranks cast BBC cranks all broken just as yours is. About half were broke in cruise or very low speed situations. The 351W crank broke while the car was being driven up onto a trailer- after being run hard 6 years of street/strip use and 2 seasons in a circle track car, about 425 hp...but it broke at near idle speed and hurt nothing in the engine, broke at a main bearing journal.

One 400 SBC crank in a 3-ton cube delivery truck that was 30 years old, broke at 20-25 miles per hour right in the middle of the crank at the 3rd main bearing, backed by a stock TH-400. Probably never run hard, but fairly heavy loads its whole life. Didn't hurt the block, just stopped running.

I know lots of people running reverse manual tranny's in street/strip cars as fast as 10 second rides with both cast and forged cranks, myself included...nobody has broken a crank and blamed the transmission....the 351W example above was backed by a shift kitted C-4 auto trans and 3500 stall converter when it broke.

Stop blaming the transmission/valvebody....it ISN'T an issue, folks all acrossed the country are running Street/strip/Drag cars/desert racers/rock buggies with manual valvebody's - forward or reverse pattern - and crank breakage is not rampant because they use these transmissions. You are scared for no reason.

Line pressure doesn't affect the thrust bearing...but too high cooler circuit/converter charge circuit pressure will. Sounds like your trans may not be built right...or you are running the fluid level too high, it will cause the vent spewing problem you described due to frothing
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Old 03-21-2014, 12:45 PM
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Thanks eric

I trust you guys on this site..lots of help over the years.

Your helping me get my brain over this. There has just been alot of un-easiness over this tranny because I really got it for a steal and have always been worried the guy just screwed me over and knew there was something wrong with the tranny.
He acted as if he didn't even know what type of tranny it was just knew it was for a chevy engine. I had no idea about the valve body being manual when I bought it and I tried for longer than I want to admit to get the thing to shift automatically.




After thinking and being told I broke the pump gear from installing the torque converter wrong, I had a guy "repair" it on my tailgate. Turns out the pump gear was not broken I think the converter just slipped off or something from not being installed all the way. After the repair is when the fluid started squirting from the vent tube with decent rpm.
So I think he may have installed the pump seal incorrectly? Miss-aligned?

After the "repair" the trans was shifting VERY rough with a HUGE bang every shift. Felt weak in reverse.. Pull the pan to find A valve body bolt hanging out in there.. They are all loose. He forgot to tighten the valve body. After I torqued the bolts it started shifting a whole lot smoother. I am thinking this is when the crank could have been compromised...

I watched the guy pull the front pump and am pretty confident that I could do it for an inspection.


Sorry I am still rambling on about my transmission... I know this is the engine section.. I did post engine pictures to be fair.

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Old 03-21-2014, 12:51 PM
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[quote=0trbo4myCHEVUICK;2209682]500ish miles after installing a RMVB th400 behind a stock high mileage buick 3.8l v6..... This happens.
Now i'm scared to use the thing for my 383..



That is a classic fatigue fracture, look at the beach marks on the upper right side. That failure has been cooking in there for a long time. I'd say the tranny's contribution was it eliminated the slippage of whatever transmisison was in there before it such that when you got on the throttle and the tires hooked up what was left of the crank that was not already cracked then failed as seen as the continuous smooth, beach less section of the break of lower right to lower left.

I would suspect that the break actually started inside toward the center and radiated outward toward the left of the picture. Note the gray rough area to either side of the center that shows the edges of that crack were connected till the last fracture. Looking from the left upward and around toward the upper right you can just make out long and slow to develop beach marks. Then as you observe going into the upper left the beach marks come more often and severely as there is less and less resistance of the remaining structure to contain the forces. Then it lets go which the zone is sweeping from the upper right around back to the lower left.

Center break is odd as they usually start at a surface and work inward. This probably is a defect in the noduralization of the cast iron where the carbon remained in flakes and did not form noduals. This would have left a weak spot deep within the casting. A mixture of alloying elements and or a heat treat problem where the deep metal didn't become hot enough or didn't quench right to form carbon noduals all the way through the structure.

Bogie

Last edited by oldbogie; 03-21-2014 at 01:07 PM.
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Old 03-21-2014, 04:57 PM
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Got a picture of the piston tops? Curious if it was detonated hard or not.
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Old 03-21-2014, 05:58 PM
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Got a picture of the piston tops? Curious if it was detonated hard or not.
I do not have a picture but the pistons did not show any signs of detonation. I'm believing this as a fatigue situation. That crank prbly had over 300 K on it.
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Old 03-21-2014, 06:02 PM
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I will probably still buy a Junk yard vortec and run it for awhile so I can get my exhuast and mounts set up.
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