Cracked flexplate question

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Tuckerjr

Tuckerjr

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Hi guys,

I have a fairly stock 360 (4006830-360-6) engine and a 904 (PK3681843) transmission in my 74 Dart Sport. I have owned the car for 6 or 7 years now and it has always had a rhythmic vibration between 45 and 65 mph.

A year or so ago I had the stock flex plate break on me. I replaced it and it broke again just the other day. I am fairly sure it the correct torque converter (externally balanced), but I will verify this again.

I put the following parts on in the past few years: New harmonic balancer, new driveshaft, new u-joints, new motor and transmission mounts, and changed from an 8 1/4" to an 8 3/4" rear end. It had the same vibration with the previous rear end.

I have also have several rim/tire configurations over the years. I just had new tires installed and finally got a decent alignment (non computer), and it was driving the best it ever had besides the vibration until the flex plate cracked.

If I have the correct torque converter, and if both dowel pins are present for bellhousing to engine alignment, what would cause the flexplates to fail other than engine to transmission alignment? I thinking that the bellhousing may need to be checked and aligned with offset dowels. What else should I take a look at?

Like I said, I need to verify somehow that its the proper torque converter, because if my research is right than this transmission was originally behind a 318.
 
If it is a nuetral balance convertor (for internally balanced engines like 318) and you're using a stock flex plate that is a problem. Should be causing a noticable vibration idling though. You might want to try a B&M weighted flexplate designed for use with a nuetral convertor and 360. Besides providing the correct balance for the convertor it is much stronger than stock. https://www.summitracing.com/parts/bmm-10239/overview/make/dodge
 
See the thing about flex plates is that they flex (and crack sometimes from it)
You may not have an actual converter problem but just the power going through it cracks it.
The aftermarket solid plates are WAY stronger.
 
see the thing is - if you're going to keep hammering on the thing the way you do (you ain't denying it!!! :D) - you need to go to an upgraded flexplate -- aka:expensive race piece. ~ as mentioned above
 
Tuckerjr said:
Hi guys,

I have a fairly stock 360 (4006830-360-6) engine and a 904 (PK3681843) transmission in my 74 Dart Sport. I have owned the car for 6 or 7 years now and it has always had a rhythmic vibration between 45 and 65 mph.

A year or so ago I had the stock flex plate break on me. I replaced it and it broke again just the other day. I am fairly sure it the correct torque converter (externally balanced), but I will verify this again.

I put the following parts on in the past few years: New harmonic balancer, new driveshaft, new u-joints, new motor and transmission mounts, and changed from an 8 1/4" to an 8 3/4" rear end. It had the same vibration with the previous rear end.

I have also have several rim/tire configurations over the years. I just had new tires installed and finally got a decent alignment (non computer), and it was driving the best it ever had besides the vibration until the flex plate cracked.

If I have the correct torque converter, and if both dowel pins are present for bellhousing to engine alignment, what would cause the flexplates to fail other than engine to transmission alignment? I thinking that the bellhousing may need to be checked and aligned with offset dowels. What else should I take a look at?

Like I said, I need to verify somehow that its the proper torque converter, because if my research is right than this transmission was originally behind a 318.
Click to expand...

Just to make things clear, you think the transmission was originally behind a 318. If the converter you're using came with the 318 transmission, this is the wrong converter for the 360. A 360 converter can have two different converters with weights. One converter will have two rectrangular weights that measure 1.62" x .75" x .380". The other converter will have one triangular weight(120grams) that measures either 1.375" or 1.545" tall. These weights are welded on the engine side of the converter in both cases.
The 318 converter has no weights welded to the engine side of it.
There are to the best of my knowledge only 4 flex plates Chrysler offered. Engines and transmissions make no difference, but rather the crank shaft bolt holes (evenly spaced or unevenly spaced), and in 11" or 12" diameter, depending on the size of the converter, again, 11" diameter or 12" diameter.
Hope this helps.
 
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If it is indeed a 318 converter that's most likely your problem but in addition to a possible converter mismatch something that I noticed is you say you replaced the harmonic balancer. Are you certain it is the correct one? Only a 360 LA (not magnum) balancer will be balanced correctly.
 
Thanks for the replies everyone. I am 99% sure that it has the correct type torque converter (it has two weights) for the engine. It was raining last night when I got off but I will get under there to verify it as soon as possible. I have this harmonic balancer https://www.summitracing.com/parts/sum-163318/overview/make/dodge , which I believe is correct. I would have rather had the ATI harmonic dampener but the funding just wasn't available. There was nothing wrong with the stock balancer, I put this one on troubleshooting the vibration.

The flexplate v8fish mentioned above is for a 360 and a internally balanced converter, which B&M plate is correct for a 360 and an externally balanced converter in a 904?
 
When does it vibrate? If it only does it while driving, its most likely the driveline. If it does it in neutral and park, it's probably in the engine or trans area. Something else to check, is the converter could be bad. I had an ATI converter that wasn't balanced correctly. Also had a B&M that had internal issues and the hub was welded on off center. So possibly a different converter might help.
 
Thanks furyus2, I hadn't thought about the converter being bad. I will check and see if my brother-in-law has a spare at his shop. They used to race Mopars so there is no telling what he has laying around that I can use for troubleshooting.

If he has an externally weighted converter for a 904, I will still need to figure out which B&M flex plate to get.
If he has an internally balanced converter for a 904, I can get the B&M 10239 flex plate as mentioned above.
I guess another option is to take the weights off of an externally balanced converter and still get the 10239 flex plate.

Is all this correct or am I wrong in my thinking? (wouldn't be the first time)
 
Tuckerjr said:
Thanks for the replies everyone. I am 99% sure that it has the correct type torque converter (it has two weights) for the engine. It was raining last night when I got off but I will get under there to verify it as soon as possible. I have this harmonic balancer https://www.summitracing.com/parts/sum-163318/overview/make/dodge , which I believe is correct. I would have rather had the ATI harmonic dampener but the funding just wasn't available. There was nothing wrong with the stock balancer, I put this one on troubleshooting the vibration.

The flexplate v8fish mentioned above is for a 360 and a internally balanced converter, which B&M plate is correct for a 360 and an externally balanced converter in a 904?
Click to expand...
You say it has two weights. Are the weights on the side of the converter or on the frontside(engine side) of the converter? If the weights are on the side, it's a 318 converter as the 360's weights are welded on the front side. For most applications, if everything is correct, the stock Chrysler flex plate works just fine and dandy.
 
Make sure you tighten all bellhousing bolts before the flexplate bolts? Ask me how i know:rolleyes:
 
Does the hub on the converter engage the back of the crankshaft for centering? Does the crankshaft bushing match the converters hub size?
 
First, you need to verify what kind of vibration it is. Is it tire/wheel vibration or is it engine/drive train vibration?

Very easy to differentiate. If you can feel the vibration as a low frequency vibration (vibrations far apart) then it is likely tire and wheel related.

If the vibration is high frequency (vibrations close together) for instance like a heard of pissed off bumble bees, then you have an engine/drive train vibration.

Also, does it vibrate with the engine running and the car sitting still? Run it up in RPM in park and neutral to the RPM where you feel it driving and see. This will tell you if it is between the balancer and torque converter.

If there is no vibration with the engine running and the car static, then you have eliminated the engine and torque converter.

Try all that and report back.
 
RustyRatRod said:
First, you need to verify what kind of vibration it is. Is it tire/wheel vibration or is it engine/drive train vibration?
Click to expand...

Rusty. Very curious on the tire/wheel vibration doing damage to a flexplate. I can't see how that equates but I'm open minded and I'm still not quite to stubborn to learn new things. Thank you, sir.

I went back and re-read the OP, and I see where he has replaced and tried many different things to no avail but continually cracks flexplates. Do you think two problems at once?
 
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Blind Squirrel said:
Does the hub on the converter engage the back of the crankshaft for centering? Does the crankshaft bushing match the converters hub size?
Click to expand...
There should be no issue with the pilot engaging in the crank, as all 904/V8 converters from 1968 up have a 1.810" pilot. I can't say that in all my years in the industry, I ever saw a bushing in the crankshaft of an automatic transmission equipped car.
 
Blind Squirrel said:
Rusty. Very curious on the tire/wheel vibration doing damage to a flexplate. I can't see how that equates but I'm open minded and I'm still not quite to stubborn to learn new things. Thank you, sir.

I went back and re-read the OP, and I see where he has replaced and tried many different things to no avail but continually cracks flexplates. Do you think two problems at once?
Click to expand...

That's probably not your problem, I was simply explaining the difference between the two vibration frequencies. Get it?
 
Looks like I have found at least part of the problem. One of the dowel pins was pushed back into the ear of the transmission and was not engaging on the engine side. Need to test drive it at highway speeds some more to be sure but I didn't hear the droning in and out vibration on the way home that I was hearing before.
 
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