Breaking flex plates

Wayne

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I've got a customer with an F550 that keeps breaking flexplates. '08 F550 bucket truck we recently ******d with a stock power tune. It's killed 3 flex plates within the last 3 months / 500 miles, and I can't figure out why. The engine runs great and the trans shifts good. This last round, we replaced the torque converter with a new factory one, just for good measure, but it lasted the shortest round of all at less than 50 miles. Crankshaft thrust was right within spec as well. Any recommendations on what to check?
 

webb06

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I was thinking crankshaft thrust at first

Actually have a truck we bought at our shop that supposedly had a bad engine, after a few minutes messing with it we noticed the flex plate had the center tore out of it. Suspecting crank thrust on this truck. Interested to know what you find.


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DEEZUZ

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Hmm. I'd think the torque converter would be forgiving with in/out motion?
 

taterthedog

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Put a crank in a 6.0 once that had been dropped. Sent it to machine shop and they said it was good, but I think all they did was straighten the tone ring. I didn’t want to put it in, but I wasn’t the boss so in it went. Truck ran flawless but kept breaking flex plates.


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Call me crazy but I've seen this on a Duramax. Are the alignment dowels for the trans snug in the block? If they are loose or missing, it'll eat flexplates like fuel
 

Wayne

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Yeah, I'll triple check trans alignment, that the bell housing is tight. and probably throw a harmonic balancer in it for good measure. Funny thing is I've never had this problem before on anything, and out of the blue, this problem child of one of my good customers suddenly starts eating flex plates. Last major thing we did was a new fuel system when we found metal in the filters, and multiple cylinder misfires a few months before this saga began. I'll be sure to update you guys on what we end up doing. Thanks for the input.
 

webb06

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I was thinking crankshaft thrust at first

Actually have a truck we bought at our shop that supposedly had a bad engine, after a few minutes messing with it we noticed the flex plate had the center tore out of it. Suspecting crank thrust on this truck. Interested to know what you find.


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Pulled trans on this truck today. Can literally use a pry bar and move the flex plate up and down with prybar 1/4”

Time to put an engine in it.


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KTM300

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When I kept breaking flex plates on my 7.3 I talked to Brian at BTS and he said to replace the starter. He said he had a few that fixed. I totaled the truck before I got a lot of miles on it after the starter replacement, so I can't say for sure if it solved mine.
 

Wayne

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Well, it got a billet Suncoast flexplate and new OE harmonic balancer. The flexplate is still good, but it blew the pump seal after about 3 weeks of use. It's had delayed reverse, so I've got the trans on the bench now to correct that, or replace the whole thing if it's too badly damaged, and see if we discover any other signs of the cause of failure.
 

Wayne

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**Final update** ...theoretically. pump bushing was bad enough to allow fluid to blow past the still good pump seal. It's getting a full trans rebuild so I suspect we're at the end of this saga.
 
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This might be silly but did you make sure that the torque converter fully seated into the transmission? There was one time I was sure it seated fully and when installed it noticed there seemed to be more pressure on the flexplate. I removed the transmission and kept turn and pushing the converter and there was a finally clunk. After that I kept turn and pushing and jiggling to make sure it was 100% seated.
 

sootie

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This might be silly but did you make sure that the torque converter fully seated into the transmission? There was one time I was sure it seated fully and when installed it noticed there seemed to be more pressure on the flexplate. I removed the transmission and kept turn and pushing the converter and there was a finally clunk. After that I kept turn and pushing and jiggling to make sure it was 100% seated.

I feel 100% certain Wayne knows how to seat a TC LOL LOL
 

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