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Balancing 401 flex plate

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chrisinchwk View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chrisinchwk Quote  Post ReplyReply Direct Link To This Post Topic: Balancing 401 flex plate
    Posted: Apr/19/2021 at 2:34pm
First time posting, long time reading.  I have read the sticky in flex plates, but here is my issue.

I have a newly built 72 401, mating to a TF 999, flex plate is from a ?????  (I know it's not from a 401, it's almost completely solid, thinking 80s 258?  It bolts correctly to both motor and torque converter).  Is there any specs any where that would tell me the amount of weight a 401 flex plate has welded to it and where?  I don't have access to a 401 flex plate to have it match balanced.  Searching aftermarket it seems almost all flex plates are "universal" between304-401, which I know is not correct.
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Heavy 488 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/19/2021 at 2:51pm
Guesstimating how many ounces/ grams of weight and where doesn't end well. Do you have a flywheel? It should have been balanced to the rotating assembly when it was built.
Inline 6's are all internally balanced and have no added weight to flex plates.
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chrisinchwk View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chrisinchwk Quote  Post ReplyReply Direct Link To This Post Posted: Apr/19/2021 at 3:58pm
I do not have any fly wheel or flex plate for this motor.  I did not build it.  Buying a used flex plate for a 401 that has been completely rebuilt is guesstimating based on stock weight off set isn't it?   

So do you have a suggestion as to what I should do?
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SC397 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SC397 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/19/2021 at 4:22pm
The machinist should have balanced a flex plate when he machine the engine. 
I and others have noted that each engine was balanced individually at AMC. So even if you have a flex plate from a different 401 it could still very well vibrate.
Having noted all of that, the best you can do now is buy a new one for a 401 or find a used one and hope for the best.  The 401 flex plate has two bolt patterns for the torque converter so is the same for a 727 or 904-998.
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chrisinchwk View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chrisinchwk Quote  Post ReplyReply Direct Link To This Post Posted: Apr/19/2021 at 4:33pm
Thanks.  That's what I am currently trying to do.  New ones are for the most part generic, meaning they fit 304-401 which seems pretty wrong.  I have found a place that sells specific to 401 flex plates but by the time I get it to Canada it will be over 1k.  I was hoping someone would know how much weight was welded to the 401 and where, this should get me pretty close to a stock one.  

I know they were individually balanced from the factory, but being mass produced they couldn't possibly have made individual flex plates for each motor?  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SC397 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/19/2021 at 5:02pm
No, they drilled holes here and there in each flex plate to balance it to the crank and vibration damper  assembly.
There are lots of AMCers in Canada.  It shouldn't be hard to find a used 401 flex plate.
IMG_2940 by Rick Jones, on Flickr
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/19/2021 at 5:17pm
Have you looked for an FRA-305 flex plate? This should be an off the shelf 401/torqueflite
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chrisinchwk Quote  Post ReplyReply Direct Link To This Post Posted: Apr/19/2021 at 7:58pm
No I haven't, wasn't aware of a part number.  Thank you
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