Mystery rattle help

[Dusty Dude]

Hello All,

Really need some help here folks. I built a 383 based 496" stroker for my '69 Coronet which is bolted up to a '75 727 filled with John Cope and A&A parts. The converter is a Hughes 3,000 stall unit. The flex plate is a stock Mopar part that came with the car and at the time of assembly looked to be in good condition.

440 Source stroker kit.
Stealth CNC ported heads.
Howards hyd roller cam and lifters.
Harland Sharp roller rockers.
Mopar M1 single plane with ProForm 1050.

Engine has been broken in and runs great. It has all the usual clatter that goes along with the roller stuff at idle. The last time I ran the engine after changing out the break-in oil I noticed a deep sounding rattle that is audibly the same as you walk around the front of the car. I have a stethoscope and I have listened everywhere I can and all I hear is what I would call normal mechanical noise so at this point I am stumped.

My question is, is it possible that the flex plate has broken around the crank bolts? The noise is very noticeable inside the car and also while driving it into my garage. I have never experienced a broken flex plate so I have no idea what that sounds like. The fit-up of the converter to the flex plate was good when I mated the engine and trans together before installation so I'm sure I didn't shoot myself in the foot on that one. I have no idea what kind of life the flex plate had before we got the car so wouldn't be surprised if it died.

Also gave all the pulleys and accessories a good looking over and listening to and all seems well there.

Any help would be greatly appreciated.

Thanks All

 

[PurpleBeeper]

That is possible. I broke a flywheel/flex plate on a Mustang that acted like that. Maybe try to better isolate the noise if you can. A bolt backing off would do that too. Maybe just pull the inspection cover and look (?)

 

[dvw]

I've broke more than a few flex plates. Everyone that was a Mopar, was broken around the conerter bolt holes. When you intstalled the converter did you check that the converter slid in to the crankshaft all the way until it contacted the flex plate? It should. Quite a few in the last few years that need some clearancing.
Doug

 

[Dusty Dude]

That is possible. I broke a flywheel/flex plate on a Mustang that acted like that. Maybe try to better isolate the noise if you can. A bolt backing off would do that too. Maybe just pull the inspection cover and look (?)

Thanks for the input. So, I put the car up on blocks (no hoist) , hooked up my remote start switch and my buddy and I crawled underneath, pulled the dust shield and did a visual inspection. Everything looked perfect. I crawled out and fired it up and then crawled back underneath to what I could see. Sounded like a coffee can with a half dozen ball bearings rattling around in it. Shut it down and grabbed a wrench to check the four converter bolts found every one of them SLIGHTLY loose. WTH! I spent quite a bit of time making sure that these ARP bolts would not bottom out in the converter lugs and I also took about .040" off the top of the heads to make damn sure they would not hit the block. They were installed with blue thread locker and torqued to 270 in/lbs per the FSM. So this time I took them all out, cleaned them and used red thread locker upon installation. Torqued them and buttoned up everything. I let the car sit for a couple of days then finally went for a spin for an hour or so and all is well. Noise gone. I am so glad that it was that simple to fix as at 77 I was not looking forward to lying on my garage floor wrestling that 727!

 

[khryslerkid]

Thanks for the update!

 

[oldbee]

Thinking I would keep an ear out for a re-occurrence. Just thinking of what ‘dvw’ previously said.

 

[threewood]

I hope that was the trick. I have had to shorten ARP converter bolts because they were bottoming out.