Broke another flexplate, what am I missing?

[79dentside]

Hey guys, need some advice. I bought my truck with a broken flexplate so shortly after buying it, I put a new one in with the assistance of a trans tech friend to make sure that it was done right. I went ahead and put a new converter in since it was going to be separated anyways, and had no issues. Here I am a bout a year later and it started knocking again last night. I cracked another flippin flexplate.

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Not really sure what’s going on, it’s a fairly mild setup, but well over stock. 350 crate motor with 24,000 miles, TH-350 w/ 2,700 stall, shift kit and then 4.10 gears. I’m not super easy on it, some of this could be self induced.

I am inclined to think that I have more severe problems going on, but I have no idea where to start. I put a generic 168 tooth flexplate in, I suppose it could have been a defective part, but the fact that this is the second one makes me wonder if there isn’t an alignment issue to the trans or something. Any ideas? Or should I just stick a higher quality flexplate; maybe an SFI approved one, in it and not think anything of it?

 

[shiftpro]

Hey guys, need some advice. I bought my truck with a broken flexplate so shortly after buying it, I put a new one in with the assistance of a trans tech friend to make sure that it was done right. I went ahead and put a new converter in since it was going to be separated anyways, and had no issues. Here I am a bout a year later and it started knocking again last night. I cracked another flippin flexplate.

You must be registered for see images attach

Not really sure what’s going on, it’s a fairly mild setup, but well over stock. 350 crate motor with 24,000 miles, TH-350 w/ 2,700 stall, shift kit and then 4.10 gears. I’m not super easy on it, some of this could be self induced.

I am inclined to think that I have more severe problems going on, but I have no idea where to start. I put a generic 168 tooth flexplate in, I suppose it could have been a defective part, but the fact that this is the second one makes me wonder if there isn’t an alignment issue to the trans or something. Any ideas? Or should I just stick a higher quality flexplate; maybe an SFI approved one, in it and not think anything of it?

Absolutely go with better quality parts, if you can get them. If there was an alignment issue you would feel it. I think you got the luck of junk tbh..

 

[79dentside]

Absolutely go with better quality parts, if you can get them. If there was an alignment issue you would feel it. I think you got the luck of junk tbh..

See that was my first assumption too...

But I have to correct myself, looks like I did purchase an SFI flexplate... I didn’t remember seeing that, but this is the one that I bought. Not sure if $53 is too cheap for a flexplate or if that is a pretty good quality?

https://www.jegs.com/i/JEGS/555/601071/10002/-1

 

[BigDaddy72]

I had 2 flexplates brake teeth off and after extensive inspection I discovered that the nose of the converter was too big for the pilot in the back of the crank.

Measure both with a dial/digits caliper and check your measurements.

If the nose of the converter is jammed or in a bind it will pull the flexplate back when the bolts are tightened down and will brake/crack it over time and might cause starter alignment issues as well.

There shouldn’t be a gap between the converter and or the flexplate.

Make sure your converter spins/turns in the pilot of the crank.

 

[79dentside]

I had 2 flexplates brake teeth off and after extensive inspection I discovered that the nose of the converter was too big for the pilot in the back of the crank.

Measure both with a dial/digits caliper and check your measurements.

If the nose of the converter is jammed or in a bind it will pull the flexplate back when the bolts are tightened down and will brake/crack it over time and might cause starter alignment issues as well.

There shouldn’t be a gap between the converter and or the flexplate.

Make sure your converter spins/turns in the pilot of the crank.

Good to know. Just curious, what are the torque specs on a flexplate? Just saw an internet spec saying 60 foot pounds (not sure if that is correct or not..), if this is correct, mine are way over tightened...

 

[Ricko1966]

So it's cracked 2 flex plates with 2 different converters, as @big daddy said make sure the torque converter pilot fits the crank, if it does I absolutely would lean towards an alignment issue are your bellhousing dowels there and fit the transmission snug?Also check crankshaft end play excessive end play will crack flex plates. If your dowels are there and tight and the transmission holes aren't wallered out, then I would look into off set dowels to correct the problem. Check clearance from 1 tooth of a new flex plate to 1 alignment pin rotate and Check the other pin. Should be within .006 . You won't feel a minor misalignment because the flex plate flexes until it crystallized then it cracks.

 

[BigDaddy72]

Torque spec sounds correct to me without checking. Did you use blue thread locker?

Are/were the bolts loose? (In the conv/and fp.

The only reasons it would crack is because it’s loose and wobbling around /vibrates if it’s pulled back from pilot being to small for the converter and or being in a bind from alignment issues. There is the outside possibility that you had 2 bad fp. But not likely. Likely Something is in a bind.

Be very careful how you take it apart and pay close attention to everything. When it comes apart. To see if you can find the problem. Everyone is pointing you in the right direction.

Remember if it (and this applies in 99.9% of anything mechanical) or any part of it has to be forced, pried on or hammered into place in anyway you can almost guarantee you are installing it incorrectly or something is out of tolerance/intended position or broken. Especially when it comes to engine parts and components.

 

[crpntr78]

Could it possibly be that he's installing the flexplate wrong side out?

 

[Ricko1966]

I do not know if you are working on a C or a K. This only applies to a C . You can change the flex plate without pulling the transmission. Buy some bolts the same length as the starter bolts, pull your bottom 2 bell housing bolts replace them with the long bolts. Unbolt the transmiision and torque converter and slide them back, you will have just enough R&R the flex plate. Again make sure your transmission locating dowels are in place.

 

[shiftpro]

Very interesting thread here. I have never heard of these issues before. Always learning!

 

[79dentside]

Tell you what guys, you all are awesome. Thank you so much! I WILL be buying a dial indicator, but I have a theory... coincidence? Maybe? But I need to lay the cards on the table.

The truck had been sitting for a few weeks so I had decided to fire it up and run some errands in it (Saturday Morning). I let it warm up and then coast onto the street and heard a strange noise so I pulled over. Couldn’t tell what it was, so I get back on the road and come to a red light and I hear a horrendous whine. It was an absurdly loud whine. Thinking maybe my ps pump is low? I accelerate and get the the next red light and it is still whining like a son of a gun, so I pull into the gas station, pop the hood and see nothing of concern. It this point the noise randomly stopped. What I found weird was that when I was turning, the noise did not get louder. My original assessment was a PS pump which would get louder as I crank the wheel down. This did not, it was all load based. I decided to put it away and stop driving it until I get a new ps pump, but I wanted to take the belt off to make sure the whine wasn’t present. I pull the belt off, first start after all that whining and “KNOCK KNOCK KNOCK KNOCK” on start up... flexplate broke on the first start.

I wrote this whine off as a ps pump, but the timing is uncanny... wonder if I had a failure of the torque converter?

I WILL be putting a new converter in. I have slight suspicions that I have a torque converter issue anyways. Ever since it was new, it always “ticked” at idle and it drove me nuts.

Just some info if it helps...

 

[Bextreme04]

Tell you what guys, you all are awesome. Thank you so much! I WILL be buying a dial indicator, but I have a theory... coincidence? Maybe? But I need to lay the cards on the table.

The truck had been sitting for a few weeks so I had decided to fire it up and run some errands in it (Saturday Morning). I let it warm up and then coast onto the street and heard a strange noise so I pulled over. Couldn’t tell what it was, so I get back on the road and come to a red light and I hear a horrendous whine. It was an absurdly loud whine. Thinking maybe my ps pump is low? I accelerate and get the the next red light and it is still whining like a son of a gun, so I pull into the gas station, pop the hood and see nothing of concern. It this point the noise randomly stopped. What I found weird was that when I was turning, the noise did not get louder. My original assessment was a PS pump which would get louder as I crank the wheel down. This did not, it was all load based. I decided to put it away and stop driving it until I get a new ps pump, but I wanted to take the belt off to make sure the whine wasn’t present. I pull the belt off, first start after all that whining and “KNOCK KNOCK KNOCK KNOCK” on start up... flexplate broke on the first start.

I wrote this whine off as a ps pump, but the timing is uncanny... wonder if I had a failure of the torque converter?

I WILL be putting a new converter in. I have slight suspicions that I have a torque converter issue anyways. Ever since it was new, it always “ticked” at idle and it drove me nuts.

Just some info if it helps...

Could have been the trans pump whining like that. My TH400 whines bad on a cold start if you use the wrong crap paper filter in it as it has a hard time sucking through it. Using the high flow brass filter gets rid of the cold start whine.

 

[79dentside]

Could have been the trans pump whining like that. My TH400 whines bad on a cold start if you use the wrong crap paper filter in it as it has a hard time sucking through it. Using the high flow brass filter gets rid of the cold start whine.

Very possible. The truck was in a warm garage and I’ve never heard this noise ever before, but anything is possible.

 

[Rusty Nail]

This is a good thread.

My initial thought involved big ass convertor bolts with a large flange.
I think they are 18mm heads..
Then you started talking about the whine and I thought "transmission pump" but then you took the steering belt off and it broke?

Huh?..

Are any of these old 400 SBC parts by chance?
I own a B&M brand flywheel that is 7 or 8 pounds heavier than stock.

We're gonna need more pics and Paul Harvey and? - the rest of the story.

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[79dentside]

We will see. I’m tempted to put a transmission pump in while I’m there. The transmission has been gone though at some point, I imagine the pump is okay, but you never know.

I’m looking into flexplates and a lower stall converter. All the 2,700 stall made me want to do is keep my foot in it. Driveway has plenty of burnout marks in it, was more playful than I needed it to be.