Are all 5.9 Magnum flex plates the same? 904 install issues.

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JoePole1

JoePole1

A dude in a B body
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Hey guys. Having a bear of a time mating my 904 to my 5.9 magnum engine.When I bought the engine I did not get the flex plate with it so I bought a used one. When I installed it on to the back of the crank one of the holes did not line up and one was completely blocked. I tried rotating the plate around the crank and 5 bolts was the best I could get and this is with one hole hogged out. The crank has six threaded holes. Is this normal for the magnum flex plate? I know I have to hog out one hole for the converter to bolt up but havent heard about having to do this at the crank. Also wondering if this is going to cause a balance issue 5 vs 6 bolts.
The second issue is that I cannot get the bottom of the trans mating surface flush with the block. As I draw it in with the bolts it gets really tight to the point I am afraid I will snap off the mounting tab on the block. The upper bolts pull in fine. I tried repositioning the trans a few times. The alignment stud doesn't seem to be the issue because I get this on both sides and there is only 1 stud on the driver's side of the block.I
Any help is appreciated.
 
Here's a picture.

1650143126094-428164311.jpg
 
**** happens, your not the only one to have done this little “Whopsie.”
 
Swapped it around proper and got the 6 bolts in. Tomorrow I'll try to install the trans again. At least the engine and trans are out of the car. Makes it a little easier. Would have really sucked if the trans was still in the car.
Thanks again. SMH.
 
Yes that would have sucked. But like I said above…..
 
JoePole1 said:
Swapped it around proper and got the 6 bolts in. Tomorrow I'll try to install the trans again. At least the engine and trans are out of the car. Makes it a little easier. Would have really sucked if the trans was still in the car.
Thanks again. SMH.
Click to expand...

Before you install that, you need to see if your flexplate is weighted for engine balance or not.

If not, your 904 torque converter will have to have the weights for the 360 magnum to have the proper balance.

5.9 flexplates came both ways from ma mopar, balanced on the flexplate, and some not balanced on it but rather the on the converter. Has to have balance weights on one or the other for a stock engine.

To add to the fun, the old LA 360 balance weights are different than the 5.9 magnum balance weights. If weights are on the converter you have to know which motor it was set up for.
 
mgoblue9798 said:
Before you install that, you need to see if your flexplate is weighted for engine balance or not.

If not, your 904 torque converter will have to have the weights for the 360 magnum to have the proper balance.

5.9 flexplates came both ways from ma mopar, balanced on the flexplate, and some not balanced on it but rather the on the converter. Has to have balance weights on one or the other for a stock engine.

To add to the fun, the old LA 360 balance weights are different than the 5.9 magnum balance weights. If weights are on the converter you have to know which motor it was set up for.
Click to expand...

I am swapping my 904 and converter from my LA 360 to an 01 5.9 Magnum. I am using a Blueprint balancer with the correct magnum weights installed. Should be good to go.
 
The balance weights on the converter if installed on it are different between an LA 360 and 5.9 magnum. LA 360 weights are heavier.
 
So how do I go about getting this thing in balance assuming magnum weighted balancer, magnum weighted flex p!ate and LA weighted converter?
 
JoePole1 said:
So how do I go about getting this thing in balance assuming magnum weighted balancer, magnum weighted flex p!ate and LA weighted converter?
Click to expand...

Provided this is a stock bottom end 5.9, if you have the 5.9 weights on the flexplate (you need to verify as not all of them had them), then you don't need any balance weights on the converter. Cut them off. Otherwise you will have a hell of a vibration and get to take it back apart.

If you want to post some pictures of the torque converter side of your flexplate we can help you figure out your combo.
 
Ok good catch. There it is. Weights on both flex and converter. I'll knock the converter weights off. Wish I would have checked into this sooner, I would have bought a higher stall. This one is 2500 stall. I'll save and mark everything if I re-use. Going to A518 next winter. Mogoblue I owe you an adult beverage!!!

IMG_20220418_083712_hdr.jpg
 
JoePole1 said:
Ok good catch. There it is. Weights on both flex and converter. I'll knock the converter weights off. Wish I would have checked into this sooner, I would have bought a higher stall. This one is 2500 stall. I'll save and mark everything if I re-use. Going to A518 next winter. Mogoblue I owe you an adult beverage!!!

View attachment 1715909933
Click to expand...

Just my two cents, but run what you have and spend the money on a new converter for the 518 since the 904 converter doesn't interchange with it.

Glad to help.
 
The 5.9's use a weighted flexplate, while the 5.2's use a nuetral balance flexplate. If you are running a stock bottom end, use the 5.9 flexplate with a nuetrally balanced torque converter. Be sure to check the for & aft converter freeplay so the torque converter doesn't bind or have too much freeplay. Also, use loctite on all those bolt threads.

200R4 3 (Medium).jpg
 
Yep, got it. I just misread the magnum swap description...I have a 5.9 magnum weighted flex plate and a neutral balance converter.
 
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