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post #1 of (permalink) Old 03-01-2004, 05:29 AM Thread Starter
 
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Cracking flexplates. Need help!

I've got a quadratrac CJ7. The 304 has Hedman headers, Edelbrock intake, and TBI. I rebuilt the TH400 in November and installed a new 304 flexplate from Krage motorsports.

Last Saturday (one week ago) I cracked the new flexplate while the Jeep was sitting in the garage. Two of the three spokes were cracked completely in two. Since I still had the old flexplate and there was nothing visibily wrong with it, I put it back in.

This morning on the way to work that flexplate cracked too.

Any ideas what could be causing me to break flexplates? I haven't changed anything in the drivetrain to change angles, etc. since I rebuilt the Jeep last winter, and the EFI I installed late last summer was the last thing I did to the engine. I rebuilt the transmission myself (first one I've ever done), but it's been shifting fine and I've had no indications of trouble.

I really need help figuring this out. This is my DD, so I can't wait too long to get it straightened out. Any ideas would be appreciated!

Thanks,

Clay
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post #2 of (permalink) Old 03-01-2004, 07:12 AM
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Re: Cracking flexplates. Need help!

The short answer is that it's flexing and shouldn't. Do they look like fatigue cracks; no displacement of the metal around the cracks except for one small area where it finally tore loose?

The only thing that will cause that is misalignment. I'm not familiar with automatical transmissions, so I can't be more specific. Someone should come on soon with some good suggestions.

Or you could install a flywheel instead; they won't do that. [img]images/graemlins/laugh.gif[/img]
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post #3 of (permalink) Old 03-01-2004, 08:43 AM
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Re: Cracking flexplates. Need help!

Like Jim said, it sounds like you didn't get the transmission bell housing completely against the engine and the flex plate is flexing. I had one flex plate crack like that but it was due to the torque converter bolts loosening up. Check your torque converter bolts, crank bolts and bellhousing fit. It's got to be one of those.
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post #4 of (permalink) Old 03-01-2004, 09:14 PM Thread Starter
 
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Re: Cracking flexplates. Need help!

Well, I had to have a Ford tow me home from work today. That's almost worse than walking!

I didn't get home until a little after dark so I didn't crawl under to see how bad the failure is, but the first one that broke didn't really show signs of fatigue failure. That's what I immediately suspected, even though I didn't notice excessive vibration, so I thought putting the original one back in would take care of it. It obviously didn't.

I have a 360 sitting in the garage that I was going to rebuild over time and swap out, so I think I'll accelerate the rebuild and swap it now. Since I'm doing that, I'll pull the engine and transmission out as one unit so I can inspect it all closely and see if I can find the misalignment. I know I got everything snug and tight the second time around, so I'm not sure what the problem might be.

I did have one thought today, maybe you guys can tell me. My transfer case mounts to the crossmember with a two bolt mount, and then another mount farther towards the drivers side. I lost the nut to the drivers side mount back in November and have never gotten around to replacing it. I figured it would be no big deal because the engine torques in that direction and it would provide support without the nut holding it tight to the cross member. Was I wrong and that's the cause of my problems? The bushing is still in place, it's just not torqued down.

Thanks for any help,

Clay
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post #5 of (permalink) Old 03-02-2004, 07:57 AM
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Re: Cracking flexplates. Need help!

I can't see any way that the missing nut on the torque arm could be contributing to your flekplate problem.

Since you're sure that the bellhousing is bolted properly to the engine, I'd suggest checking the torque converter bolts as Will suggested.
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post #6 of (permalink) Old 03-02-2004, 08:19 AM
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Re: Cracking flexplates. Need help!

[img]images/graemlins/tongue.gif[/img] The three things that could cause the flexplate to crack are: Angular misalignment; Centerline misalignment; and stress applied due to a longer-than-stock torque converter. When you installed the tranny, did the torque converter have just a tad of free movement front to back before you bolted it up? Were the torque converter bolts easy to get started? Did the bolts get hearder as you ran them deeper in the threaded holes? Were the bellhousing bolts loose when you took the Jeep apart the very first time? Is the engine plate clean and does it fit nice and flat against the engine? VERTICAL DISPLACEMENT is dynamite, and could shatter a flex plate in no time at all. Be sure that the tranny does not have worn mounting holes which would allow it to "slump" down before it is bolted up. Try using studs in some of the mounting holes to get a more precise fit. Make sure the tranny slides onto the non-threaded part of the studs. [img]images/graemlins/tongue.gif[/img]
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post #7 of (permalink) Old 03-02-2004, 09:23 AM Thread Starter
 
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Re: Cracking flexplates. Need help!

Thanks for your input, Dave.

I can't answer all those questions for sure right now, so I'll take your list when I pull the whole assembly and check it all very carefully. I'm really hoping I'll find something obvious so I'll have confidence it's fixed.

Thanks again,

Clay
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post #8 of (permalink) Old 03-02-2004, 09:43 AM
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Re: Cracking flexplates. Need help!

</font><blockquote><font class="small">In reply to:</font><hr />
Be sure that the tranny does not have worn mounting holes which would allow it to "slump" down before it is bolted up.

[/ QUOTE ]

There are two locating "tubes" at the 3 and 9 o'clock position of the bell housing similar to the head to block locators that are a tight fit to align it. Make sure that they seated all the way too.
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post #9 of (permalink) Old 03-02-2004, 10:19 AM
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Re: Cracking flexplates. Need help!

On that note, I've heard about some sort of "centering button" for the front of the convertor....similar to the tranny input going into the pilot bushing, but I didn't have/find one to use when I put it together. It's held up fine so far...

Any chance the flexplates are incorrect for your 304...aka for a 360 or 401?
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post #10 of (permalink) Old 03-02-2004, 12:42 PM Thread Starter
 
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Re: Cracking flexplates. Need help!

Will,

I know the tubes you're talking about and I'm sure they were mated correctly the second time I did it because I had one catching and had to pry the transmission up just a bit to engage it properly. Since I was under the Jeep at the time and fighting the transmission by myself, I can't really swear that nothing was caught between the case and the aluminum plate or the engine. I don't think there could be anything there, but can't be absolutely sure.

Dave,

I wondered about a "centering pin" like that too. I never had one, but figured if the torque converter was bolted tightly to the flexplate it shouldn't really need one. When I ordered the new flexplate, I ordered it for a 304. After it broke, I noticed it wasn't exactly the same shape as the old one, so I thought that may be the cause if it didn't balance right. Putting the old one back in obviously didn't fix it.

I have another thought. On top of the misalignment issues we're discussing I see out-of-balance as a possible cause. Could I have had a failure in the torque converter that causes it to be out of balance without making continuous noise that I would notice? What's the harmonic balancer construction on these engines (I'm at work and can't look)? Does it have a rubber ring in it like a Chevy so the outer ring can spin and throw the whole engine out of balance?

I appreciate the brain power going into this one,

Clay
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