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Old 01-21-2014, 06:23 PM
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When to upgrade flexplate. Bends under load

Hey everyone. I recently got done installing a new engine and trans into my 1986 olds cutlass. I was doing some pulls up the driveway just to get a feel for the brakes, suspension, response, etc when I noticed that there was a slight chirping. I got on the brakes hard, stalled the converter and the noise was very bad. I had my dad look at it from the side and he said he could see some sparks (not many) from the bellhousing area, and the outer ring on the flexplate seemed to be flexing a lot. Almost seems that its hitting the block... and yes it is on the correct direction. First thing I checked. Its a GM factory replacement flexplate for a 1 pc late model. Everything seemed fine during assembly.

Should I have upgraded to a SFI flexplate? What is the normal limit for normal flexplates where they flex too much? Motor is a 350 sbc. Dyno corrected numbers were around 480hp (seemed high should be around 380-400hp).

Thanks
Shane

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Old 01-22-2014, 06:55 AM
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It kind of hard to imagine an flex plate flexing that much, I guess that's why its called a flex plate? I bought a Jegs brand SFI flex plate, its much thicker. Trans is bolted to the engine tight? Flex plate isnt loose? Is it bent? Never had that problem before.
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Old 01-22-2014, 10:39 AM
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Quote:
Originally Posted by dogwater View Post
It kind of hard to imagine an flex plate flexing that much, I guess that's why its called a flex plate? I bought a Jegs brand SFI flex plate, its much thicker. Trans is bolted to the engine tight? Flex plate isnt loose? Is it bent? Never had that problem before.
It is supposed to flex some... but not as much as it is.. Maybe if the temp gets above freezing I will go out and start it up and take some videos from under the car while its running. Transmission is tight. I know I torque the bolts on the crank to the ARP recommended spec. I talked to my machinist about it. He said he has seen a SBC do that before and it just needed a stronger flexplate. I had never heard of it before and just wanted to get some more input on it before I go and buy a flexplate and separate the engine and transmission again.
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Old 01-22-2014, 11:00 AM
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Could be a junk converter or plugged cooler return line. Converters can balloon (either poor quality or over-pressuring) and force the flexplate out of whack, along with trashing the engine's thrust bearing.

Russ
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Old 01-22-2014, 11:36 AM
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Originally Posted by S10xGN View Post
Could be a junk converter or plugged cooler return line. Converters can balloon (either poor quality or over-pressuring) and force the flexplate out of whack, along with trashing the engine's thrust bearing.

Russ

It does look like the converter is being pushed out of the trans and causing the flexplate to bend... and I did just have the trans rebuilt. I wonder if I should call the guy that built it and see if he can test the pressure on it.

The converter is a used redneck racing converter... nothing high end by any means but has a 3000 stall and an anti ballooning plate. The trans builder looked at the fluid that came out of the converter as well as used some pliers to spin I believe the stator inside the converter to check for wear without having it cut apart. He did not seem concerned about its condition.
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Old 01-22-2014, 04:14 PM
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I've seen them bend enough to hit the starter drive before, but not the block. I've some issues with certain starters.
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Old 01-22-2014, 06:36 PM
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Quote:
Originally Posted by lmsport View Post
I've seen them bend enough to hit the starter drive before, but not the block. I've some issues with certain starters.
It's most likely the nose of the starter that it is making contact with. In your application was it a stock flexplate that was the issue or something wrong with Trans or converter.
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Old 01-26-2014, 05:16 PM
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Can anyone else weigh in on possibilities?
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Old 01-26-2014, 07:32 PM
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I don't see how the convertor could be pushing it, the convertor lines up with the crank. If it has enough pressure to balloon the convertor, it should be leaking fluid past the front seal everywhere. Just my opinion though.
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Old 01-27-2014, 01:42 PM
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One of favorite vids:


Russ
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Old 01-27-2014, 05:56 PM
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Never seen that video before but it's interesting. I wonder how good the crankshaft thrust bearing was in this video?
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Old 01-27-2014, 10:42 PM
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Originally Posted by S10 Racer View Post
Never seen that video before but it's interesting. I wonder how good the crankshaft thrust bearing was in this video?
Or how the OP's engine is?
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Old 01-28-2014, 04:44 AM
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Or how the OP's engine is?
Crankshaft trust was within spec when I put the motor together this past summer. It's only been ran on the Dyno since and then just a couple test pulls in the drive way.
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Old 01-30-2014, 06:44 PM
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That video doesnt show what the OP is decsribing. The video's flexplate isnt flexing at all, the TC is simply pullung out of te trans to accomodate the entire assembly being pulled forward by the crank.


OP Have you tried backing out the TC bolts, pushing the TC back, starting the engine and taking a peak at the flexplate as she spins?

That'll show any installed runout of the flexplate.

peace
Hog
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Old 01-31-2014, 09:13 AM
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Quote:
Originally Posted by Hogg View Post
That video doesnt show what the OP is decsribing. The video's flexplate isnt flexing at all, the TC is simply pullung out of te trans to accomodate the entire assembly being pulled forward by the crank.


OP Have you tried backing out the TC bolts, pushing the TC back, starting the engine and taking a peak at the flexplate as she spins?

That'll show any installed runout of the flexplate.

peace
Hog
Don't think the OP was describing anything other than a strange noise he's trying to track down. Take a closer look at the vid, the flexplate is definitely moving as the converter expands and pushes it forward. It could conceivably be making contact on it's (ever so small) journey forward...

Russ
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