Flex plate for 5.9 and 904

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remytherat

remytherat

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Researching parts for my 5.9 swap into my dart which has a 904. Looking to keep the stock converter.

Since the b&m flex plate is out of production, what are my options? I've heard some of the 5.9 magnum flex plates can be made to work (97 and up??) with a neutral balance 904 converter. Looking for cheap.
 
Yes, a stock 5.9 flexplate works. That's what I'm using. Seems like I had to oblong one hole though. Darn memory isn't as good as it once was!
 
When I was running a 904 I used the stock 5.9 flexplate from a 96 and later truck. One hole needs to be opened up maybe an eighth of an inch to allow it to mount to the 904 converter.
 
I have the same question but a slightly different issue. I had a custom torque converter built and the Magnum flex plate will not work.
The lip on the factory flex plate contacts the converter out by the ring gear before the mounting tabs contact at the attachment points.
The new attaching bolt holes are slightly smaller diameter than the converter bolts provided. Yes I can drill them out.I

I bought a used B&M SFI flex plate #10239 not realizing that this is for an LA/904. Won't work on 5.9 Magnum/904. The correct B&M 10241 was discontinued long ago.
Any ideas?
 
JoePole1 said:
I have the same question but a slightly different issue. I had a custom torque converter built and the Magnum flex plate will not work.
The lip on the factory flex plate contacts the converter out by the ring gear before the mounting tabs contact at the attachment points.
The new attaching bolt holes are slightly smaller diameter than the converter bolts provided. Yes I can drill them out.I

I bought a used B&M SFI flex plate #10239 not realizing that this is for an LA/904. Won't work on 5.9 Magnum/904. The correct B&M 10241 was discontinued long ago.
Any ideas?
Click to expand...

Could you get the custom converter balanced for the 5.9? Then you could use a neutral balance flexplate.

The other option might be to shave the flange off the Magnum flexplate until it works. I would guess only an issue if the CPS is used.
 
DionR said:
Could you get the custom converter balanced for the 5.9? Then you could use a neutral balance flexplate.

The other option might be to shave the flange off the Magnum flexplate until it works. I would guess only an issue if the CPS is used.
Click to expand...
i would be afraid to shave the flex plate for balance issues plus I don't have a lathe to do it.
I thought about adding washers in between the flex and converter. Looks like .065 would do it.
I guess I could get it balanced and use my LA flex plate. Unfortunately I would have to ship the converter across the country NY to Cali
and back. My transmission is sitting on the trans jack. Was hoping for a quicker solution.
I am going to call the converter builder and ask about the shims/washers.
 
JoePole1 said:
Converter builder says that I am ok to 0.100 so shim it I will. Thanks.
Click to expand...
That is a lot of shim thickness. I would be afraid of that restricting fluid flow back from the converter. Or maybe to the converter. Not sure. But either way, the easiest thing to do is use the old Magnum flex plate to mark the position of the weight onto the converter itself. Then remove the weight from the stock Magnum flex plate and weld it onto the converter. Now you can run any neutral flex plate that fits your six bolt crank pattern and four bolt converter pattern. This is basically what the old Mopar weight kit was for, but that is long discontinued.

I had a post in another thread showing how to mark a neutral B&M 10230 into a Magnum 10242 but that one also has the larger converter bolt pattern that interferes with some 904 setups because of the diameter of the 10230.

I should also say that when making any real power over a stock 5.9 Magnum, a better flex plate should be used anyway.
 
Bobzilla said:
That is a lot of shim thickness. I would be afraid of that restricting fluid flow back from the converter. Or maybe to the converter. Not sure. But either way, the easiest thing to do is use the old Magnum flex plate to mark the position of the weight onto the converter itself. Then remove the weight from the stock Magnum flex plate and weld it onto the converter. Now you can run any neutral flex plate that fits your six bolt crank pattern and four bolt converter pattern. This is basically what the old Mopar weight kit was for, but that is long discontinued.

I had a post in another thread showing how to mark a neutral B&M 10230 into a Magnum 10242 but that one also has the larger converter bolt pattern that interferes with some 904 setups because of the diameter of the 10230.

I should also say that when making any real power over a stock 5.9 Magnum, a better flex plate should be used anyway.
Click to expand...
I was looking at that post/thread. Good information.
I like the idea of transferring the weights to the new converter but I am going to try the shims first. I had an in depth discussion with the guy who built the converter (also the business owner) and I'm confident with the idea plus he is backing up his converter 100%. I am going with .065 which is just enough to clear the weights and keep the lip from bottoming out. If this does not work I will transfer the Magnum flex plate weights to the converter. In the mean time I am going to search for the elusive 10241. I know...needle in the haystack. Thanks for the info!!!!!
 
Just got everything buttoned up and went for a ride. Working well with the shims/washers. I epoxied them in place on the converter. There isn't much room to hold in place while bolting to the Magnum flex plate. Bottom line is that you need to note the height of the converter mounting stand-offs so that the 4 bolt mounting tabs on the Magnum flex plate contact before the outer lip contacts the converter out by the ring gear. My factory 340 converter (weights removed) mounted with no issues as well as a factory A999 LU so I assume all factory converters are ok. Unfortunately I did not measure the mounting tab height on my custom built converter but I will follow up with Edge Racing Converters. Andre is super nice and owns Mopars. You may want to discuss with your aftermarket builder or you can go with a balanced/weighted converter and zero balance flex plate.

Edit....I also had to drill out the flex plate mounting holes for the larger bolts provided with the new converter.
My converter came with the factory offset and non offset mounting positions.
 
I just assembled a Magnum based 408 and used a B and M 10239 flex plate, with neutral balanced convertor, correct magnum balancer and it is smooth as glass...87 block if matters..
 
Last edited:
Mopar Tim said:
I just assembled a Magnum based 408 and used a B and M 10239 flex plate, with neutral balanced convertor, correct magnum balancer and it is smooth as glass...87 block if matters..
Click to expand...

20240623_093538.jpg
 
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