Brought car home to go over it, in preparation for Bracket Final at Lebanon Valley, New York, in 2 weeks.

Found the flex plate cracked around 3 of the 4 torque converter bolt holes. Also cracked ring gear. I will be pulling it apart. I have an extra converter, and 1 extra flex plate. I use the mopar performance 8 bolt crank flex plates, SFI. They are no longer available!!!! What can I use instead??

I used to use B&M flex plates but they cracked way to often. B&M flexplates cracked about twice a year. Cracked from torque converter bolt holes out. The Mopar Performance flex plates last longer. I saw that Maziere has a Billet 8 bolt flex plate, for around $419 . Very expensive. But if they last, it would be worth it.

Will post pics in a few minutes..

pics

Nice catch.
You are very fortunate to have found those issues before they found you.

Are you making too much power for those parts or do you have something else going on with the car such as tire shake or a driveline vibration? I don't think I've ever heard of someone cracking flexplates on a regular basis.

No vibration, and no tire shake. I didn't have 1 crack for the last 3 years. Then this last winter we freshened up the motor, adding a little more power. I removed a flex plate that was in for 3 years, was still good, and installed a new flex plate with newer SFI numbers.

When I used the B&M flex plates, they would crack from the torque converter bolt holes out. There is only about 1/8" of metal from the torque converter holes to the outside edge. They always cracked there. I would get a rattle when you shut the car off. No rattle when shutting of with this cracked flex plate..

The Mopar performance Flex plats have more material from the torque converter bolt holes to the edge of flex plate. This is the 1st Mopar performance flex plate that I had crack. Now they quit making the mopar performance 8 bolt flex plates, about 2 years ago. I put around 130 passes on this fresh engine and flex plate this year.


I did have the engine kick back a few times when going to start it the last few weeks. I think that was when it cracked. I have the timing locked out at 36*. I get engine spinning 1st and then flip the ignition toggle switch ON. I even installed a MSD starter saver , that is suppose to pull 10* of timing out under 800 rpm to help with kick back. Doesn't help, I even cut the wire so it will retard timing 20 *.


Are the Mazeire 8 bolt billet flex plates any good?? They are expensive enough....

SUM-TING-WONG! You seem to have that issue more than the norm as Andy said. Must be the trans brake and good hook!

To bad it wasn’t a 6 bolt plate. I have a brand new one (old stock) sitting here I was going to list for sale

I would check your flexplate runout.

Perhaps?

transmission pump bore and crank checked for run out ?

That engine kick-back can’t be good for anything. I would address that issue and fix it before it happens again.

The tougher the flex plate, the less it flexes; thus it isn't actually a flex plate, its a drive plate i.e. won't flex when there's misalignment.

The B&M flex plates used to crack when I ran Super pro , from 2005 until 2010, launching with the trans brake. I have 3 or 4 sitting in back garage. I know I threw a few away. No sense in saving junk.

Then I switched to running in Pro- Foot brake racing, No trans brake, except for reverse. Fresh motor and flex plate installed March of 2011, 505 cu. in. used until block cracked around August 2013 . we built another 505 cu .in engine , used same flex plate for 3 more years, until that block cracked at the end of 2015, flex plate was still good, No cracks. I used the same flex plate up until April 2016, when I replaced flex plate with new flex plate, and a New 540 cu. in. , using an aluminum block. . No cracks, still in good shape. No trans brake, foot brake racing. I used that motor and flex plate for 3 years, until the end of 2018. We freshened engine up, ported heads, and installed dominator intake for the beginning of this year.. Gained 57 horse power.

This year we were allowed to use transbrake. Now I am back to cracking Flex plates.

Have you ever checked your runout? Its a pain coz you have to gut your trans and check with that case. Are you using a mid plate? Any noticeable ware on the converter snout?

Follow the procedure in the service manual under Clutch using the pump bore and face of your auto trans.

Take a grinding stone and place a small chamfer on the converter bolt holes then send the plate out to have it shot peened.

We had to shot peen all the production flex plates because of breakage on the V/6 fwd applications.

Me thinks the motor, trans or converter are talking to you saying something ain't happy
I have drag raced Mopars since the early 1960s, your not the only one who has had this problem with both (Mopar and B&M brands) stock type flex plates. The others did get their problems fixed but not all of them had the same problems or used the same fixes

If I recall correctly, mine was off about .015. I used offset dowels from McRobb. The Lakewood ones don't quite fit even though they say they're for Mopar.

I had to check mine and max was .020.I had a small amount of loc tite buildup on the crank flange that didn't help but I think it was the tightening pattern on the crank that I messed up also.

I will be pulling the trans tomorrow. Will do some checking then...

I know you have been at racing for a while but you may not check the vert fore aft play before you separate the engine/tans.I think if the vert is tall on any end it may force the flexplate in or out to stress the bolt holes.just and idea.
ps:I checked my fp runout against the back of the engine but a plate can be bolted to the trans bell to measure in to the plate.