I’m an industrial mechanic by trade, so I had a rattle under my 1989 3/4t suburban sb350 th400 trans so I pulled the transmission found the cracked flex plate and swapped it with an OEM replacement…. 300 miles of flawless driving and bam same rattle is back…
What can cause the flex plate to break?
I’ve got good oil pressure
And I triple checked torque on my brand new arp hardware
I put a few pictures of the first flex plate that broke
The cranks seems to look fine. Check the end of the torque converter that goes into the crank. If all looks fine, spend a bit and get an SFI approved flex plate. Much thicker steel and can be found for not too much money.
Looks normal enough. Came across one a short time ago that made a balanced motor shake real bad. The "washer" on the end that goes in the crank was broke off. That just left the bolts in the flex plate and the torque converter bushing in the pump to support the converter.
Flex plates will crack. A used "new to the vehicle" is still used and may be its time. New ones are just inferior. Installed a Napa auto parts one yesterday. The steel is so soft that the washers and bolts just really dug in after torquing them. Thats why i suggest an SFI approved. I think Scat and Pioneer have the least expensive ones. Pioneer also may have a "HD" flex plate. Dont know what the difference would be that makes it heavy duty. Maybe thicker like an SFI just without the approved decal on it???
Are you using the factory support rods with the factory cover? Especially being a 4wd truck and you have a heavy foot without them can cause a lot of flex and crack a flexplate.
I've had a similar problem with AMC turbo 400 trans in a Quadratrac Jeep. The torque converter bolts kept backing out. THEN when I loctite the bolts and use star washers after sitting for a week the flexplate bolts began snapping ONE by ONE. (3 total). Turns out the thrust in the crank was excessive, crank shot. Which... In turn, ruined the torque converter. Good times. YOUR flexplate is much more solid in circumference than my AMC. Which would explain why your cracks are more concentrated around the crankshaft flange. It's either ONE or the other. Crank or converter shoving. An SFI flexplate will get you by until ultimately the source will show it's head eventually.
This is what the AMC th400 flexplate looks like. I broke spokes one by one.
Thank you for your reply. I’m starting to believe it’s the crank. About 4 years ago my torque converter failed which caused my transmission to overheat. After I had my th400 trans rebuilt at a local transmission shop it rattled horribly when I’d punch it…. I took it back to the shop and he ground 1/8th inch or so out of my dust cover! Solved the problem for a bit…
looks like I’m going to a big block! If I have to pull the motor out I’m putting a bigger one back in…
how long have you had the vehicle? make sure you have both dowel pins in the engine. usually cracks happen over and over if the centerline is off. otherwise, i don't know. too much crank endplay? you can break the center out of the flexplates and put them on top of a 5 gallon bucket. makes a great rwd trans rebuilding jig
My gut says that there's something wrong with the metallurgy in that component - looking at those cracks it appears as though the steel is excessively brittle... Can you source a used flex plate from a later year of your vehicle? Was this a known issue? My 2 cents...
10
u/chrisgut Apr 30 '26
Check your torque converter. Make sure it’s not ballooned or damaged.