cracked flexplate

[drgracr]

hi all, a friend of mine has a 6.2 pickup, problem is it keeps cracking flywheels. he is now on his 3rd and can't seem to find a problem as to why. I'm wondering if there is something wrong with the torque convertor.
If anyone has any ideas, your help would be appreciated.. thanks Paul

[dieselbegreat]

Something tells me that a main thrust bearing surface is gone. The only thing that locates the crank in the block, I believe, is the thrust surface. Improper preloading of the flexplate at the flywheel will force the crank forward, and this back and forth movement is too much for the flexplates and, over time, can't be good for the motor in general.

Maybe you can verify this by pulling the oil pan and checking the thrust clearance, and then just replace the thrust bearing without disassembling everything.

[Salemone]

Does the flexplate always crack in the same quadrant?

[waorth]

I have to bring this old topic up.

My truck (6.2L TH400 4wd) keeps eating flexplates. I discovered the 3rd cracked plate in between the last 3000mls.

Does the failure come from the front (engine) or from the back (torque converter & transmission)?

The cracks show up on the inner circle, where the plate bolts to the crank.

There's a minor oil leak from around the rear main seal - one or two drops a day.

The transmission is doing well.

If anyone could put me in the right direction please.

Thanks in advance.

Walter

[john8662]

waorth,

Actually most of the flexplate failures that crack around the crankshaft are due to the installation. I've seen a few that cracked mainly due to excessive torque on the bolts. The book calls for 65 ft. lbs of torque, I stand by this. I don't install anything with an impact wrench. Anyways, the best method to this install is taking a little loc-tite and applying it to the threads on the bolts. Then tighten the bolts a little just to get them all started and somewhat snug. Then tighten them all down to 65 ft. lbs. in a cross pattern like you would installing lug nuts on a rim. You might go as far as stepping the torque on them up to 65, starting at 35 or so. This procedure will keep from compressing the thin metal of the flexplate, and help keep the structure without warping it.

[waorth]

Thanks John for the answer.
I suspected the bolts to be too loose the first time it had cracked.
I torqued them to 90 newton meters, thats close to your 65 ft.lbs.

The bolt on process however could be done the wrong way. Remembering the last time we did the job - in the middle of nowhere 2am in ICELAND (yes the island in the north atlantic) - it was kind of a hurry ;)

Btw, what size are the bolts (plate to crankcase)?

Could it be the torque converter fixings (the welded plates with the threaded holes)? If they aren't level (not sitting flat on the plate), would this cause the plate to crack?

Walter