View Full Version : What does a cracked flexplate sound like?

06-19-2007, 04:41 PM
Well, the Hudson has a Ford 460 / C6 combo.

I've been working on this car a long time, and jsut got the car running 2 weeks ago, and am still trying to shakedown a few items...

The trans I had for the car turned out to have a bad pump, so I went and bought a used C6 from a junkyard and installed it last week.

I took the car for a drive last Thursday, and heard what i thought to be valve clatter at idle. I thought this strange as the 460 has positive stop rockers and Hydraulic lifters.

Friday night I took the car for about a 40 mile round trip. I noticed that under acceleration, there is a very loud and distinct clicking sound coming from the front of the trans area. Almost sounds like a rod knock, or perhaps a loose torque converter nut hitting the bellhousing or something.

I also noticed that the car will "clunk" when it shifts into gear, and from first to second, and second to third. It's not a super loud noise, but sounds similar to a bad U joint.

I know the Ujoints are good, I just had the driveshaft reconditioned and shortened.

I pulled the valve covers off and double checked all the rockers, they're fine.

I dropped the pan on the tranny, looking for any sign of metal shavings, dirt that might clog the pump, or anything...there's none. ATF is clean and the screen looks brand new.

I'm beginning to think I might have a cracked flexplate. Has anyone ever had a flexplate crack, and if so, does it sound like what I am describing? Would a cracked flexplate cause sloppy shifts too?

Any ideas? I've about exhausted every other possibility (I think), and I'm just wondering if there is anything I'm missing before I pull the f@$&ing transmission out of the car again...


06-19-2007, 04:45 PM
When I got the 56 project, the flywheel/flexplate was loose and made a nasty sound when you cranked it and at idle, but it would smooth out when you gave it gas, yours sounds like it does the opposite? Guess it still could be the flexplate since they're totally different engine/transmission...

06-19-2007, 04:50 PM
Oh, another thing, if I put the car in park or neutral, there is no loud noise, just the "lifter" noise. So, the noise only happens when the trans/engine is under strain.

06-19-2007, 04:52 PM
That "lifter noise" when unloaded and at idle is what we usually see in our shop from cracked flywheels.

Bishop Welding
06-19-2007, 04:54 PM
It sounds like this.

KkHhhhhhhhhhhkkkkkkkkkkkkkkkKKKKKhhhhhHHHHHHHHKhhh hhh.

06-19-2007, 04:55 PM
Did you install the torque converter with the tranny, or did you bolt the converter to the flexplate first, then slip the tranny on the converter.

06-19-2007, 05:07 PM
Did you install the tourqe converter with the tranny, or did you bolt the converter to the flexplate first, then slip the tranny on the converter.

Converter with the tranny.

Slag Kustom
06-19-2007, 05:12 PM
cracked flexplate sounds very close to a rod knock. I have seen a bunch where motor was pulled for said rod knock and broken flex plate was found.

06-19-2007, 05:13 PM
Had one on a 390 fe ford once.
It sounded like a big end knock at idle, went quiet when put into drive.
When stripped down, the plate had split around all 6 bolt holes. it was a sort of hex drive, which rattled when under no load, but quietened down as the load came on because it was always against just the one side of the hex.
We welded it up, due to a lack of spares and a tight timeframe.

06-19-2007, 05:15 PM
cracked flexplate sounds very close to a rod knock. I have seen a bunch where motor was pulled for said rod knock and broken flex plate was found. True That!

06-19-2007, 05:22 PM
Every time I put a tranny in somthin it gets a new flexplate and torque converter, if it looks good or not. Its worth the $$ not to have to pull one out twice. When I was a kid I did alot of work 2 and 3 times.

06-19-2007, 05:26 PM
i bought a real cherry diesel powered 79 coupe deville one time because the guy that owned it was certain it was going to need diesel engine number 3 ...i gave him 500 bucks and pulled the tranny back after he left...sure enough the rod knock was a busted flywheel !i ended up transplanting a gas v-8 into it after my friend out accelerated me in his 1948 flathead powered cad.

06-19-2007, 05:35 PM
chinga chigna chinga chinga....

06-19-2007, 05:55 PM
hmmm. It sounds like I may have located the problem. I'm really not excited about pulling the trans out of the car again, but I guess that's just in the cards.

Could a cracked flexplate be causing the sloppy shifts between gears?

06-19-2007, 08:29 PM
It sounds to me like your torque converter may be on the way out.

They make a klanging or tinging sound as the motor spins.

A bad torque converter will have a rythmatic clang that corresponds with the engine rpm. The good thing is that torque converters can be cut open and repaired buy a tranny specialst. While your torque converters open, get them to bend the (stators?) finns a bit. You can take your stock 1400 rpm converter and bump it up to 1800 or so which allows you to put more hp to the ground a little sooner. 1800 rpm isn't to high of an rpm so your converter will still lock up properly at 60 mph. I had a C4 converter cut open and bumped up to about 2100rpm. It made a huge differance off the line with a 4500 lb truck and a freshly built 5 litre.

Good luck on your exploratory surgery.


06-19-2007, 08:48 PM
Make sure the torque converter bolts aren't loose. It will rattle like crazy if they are.

06-19-2007, 08:51 PM
I've had to replace a couple flex plates and they never affected shifting. On a couple 350's all they did was a bit of a ting, ting, ting. Not nearly as deep as a rod knock. Very similiar in rythym though.

As noted above, could be the T.C.

06-19-2007, 09:26 PM
I have to agree with wvenfield, when I cracked one it was more "tingy" - the way mine cracked was 360 degrees. When I took it apart it fell in two pieces. It was amazing to me that I could even drive if after I saw that.

06-19-2007, 09:58 PM
Yeah the ting ting ting noise sounds about right I had a 302 with a broken flexplate, it was cracked between each bolt hole and fell apart in two pieces when I pulled it out.

06-19-2007, 10:27 PM
Well, I pulled the transmission out of the car just now. The converter bolts were tight.

The flexplate is not cracked.

If I take my converter to a trans shop, can they test it to see if it checks out ok?

Rusty Kustoms
06-19-2007, 10:48 PM
what you explained earlier sounded just like a cracked flexplate but you just may have a bad tranny, i have had torque converters go before and never get a knocking/clanging noise from them

02-14-2012, 09:26 PM
I got a brain teaser for ya, I'm thinking broken flex plate in my 73 Blazer however when I'm in gear low rpm's and rolling down the street I hear the consistent pinging, but if i put the truck in neutral while still moving the noise quits. I'm at a loss. please help a new member

02-15-2012, 03:05 PM
Can't help you with that eightyfive, but for anyone interested, my issue wound up being a rod's been resolved for a few years now.

02-16-2012, 10:14 AM
When they crack near the crank flange, they sound rod-knock-ish. When you crack them out where the converter bolts are, they have a very tinny, almost ringing sound under load, and the cracks are very hard to see. I cracked two flexplates on a 350 olds I drove balls out all the time, until I threw away the torque converter and got a good one.

02-16-2012, 10:49 AM
Did you check the converter pilot that goes into the crank,ford had different depths and size pilots,also if converter was not totally seated and you pull the tanny up with the bolts you will damage the pump and flex plate

02-16-2012, 11:00 AM
Had a noise under load that I thought for sure was a valve or a lifter.
Pulled the covers , everything was fine, same noise.
Turned out it was loose exhaust manifold bolts.