Flexplate and TC for a BluePrint 408 Magnum


Jim Kueneman

FABO Gold Member
FABO Gold Member
Jul 26, 2016
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Sonoita, Az
Are there any catches to using a flex plate for an LA and LA 360 Torque Converter on a stroked 360 Magnum or will any LA flex plate bolt up to the Magnum and I can choose either the 10" or 11" 360 Torque converter?
IIRC, the 360 mag balance situation
360 mag specific flexplate needed

Either the plate or the converter has weights

My set up,
Neutral converter weighted plate, with the correct magnum harmonic balancer
Yea ok I am getting more confused... I have a question into Blueprint as to what they expect for the plate and TC for their rotating assembly.
If your stroker rotating assembly is internal balance use a neutral balance flexplate,
The 5.2 Magnum is neutral balanced, where the 5.9 Magnum is not. One of the torque converter bolt holes must be offset ground slightly to line up.
Yea ok I am getting more confused... I have a question into Blueprint as to what they expect for the plate and TC for their rotating assembly.
I'd wait and see what @Johnny Mac says. With the Magnum crank out the window, none of that matters anymore. Great question that will be a good learning experience for all of us.

Morning all,

Let me try and clear up any confusion on the matter. Even though our engine uses a Mangum block, it's built to be as close to an LA engine as possible. The only thing "magnum" about it, is the block, the weight on the harmonic balancer, and the intake bolt pattern. The REAR Balance, is as LA360 as it could be.

Aftermarket rotating assemblies can be balanced several ways. It's really just adding, and/or removing weight from various places to achieve balance. Be it internal, or external.

In the case of a BluePrint 408 mopar, it's balanced as any external LA 360 would be on the rear.

The balancer we use is a 1970+ LA style, we simply use the magnum weight on it. This cuts down time and cost to the end user when balancing. That's where anything magnum leaves the discussion when it comes to the rotating assembly.

The title of the thread could be edited to "BluePrint 408 stroker flexplate and Torque converter options" as it's not a "magnum" engine.

Now, let's also understand that today, most aftermarket flexplates sold. Carry the rear engine balance weight. That's because most converter companies just sell a neutral balance converter. (Chrysler was the only company from the factory to weld weights to converters, instead of flexplates, that i know of)

So assuming you're running an aftermarket converter, with no weight, you want this flexplate.
Chrysler SB Steel SFI Flex Plate - LA 360 SB EXTERNAL BALANCE - Chrysler Compatible

There aren't many/any oe converters I'm aware of that would have high enough stall for a cammed stroker, and be balanced for an LA 360. So again, you are looking aftermarket (probably neutral) converter.

Here is our balancer that comes with the magnum weight installed. So problem solved there too. Harmonic Balancer - Chrysler Compatible - 273, 318, 340, 360 and 408

As a notable caveat. Since chrysler did balance converters, and not flexplates, if you did somehow buy or find a stock la360 balanced, weighted converter, you could run that with a stock (neutral) flexplate. Beyond the obvious of being a 50 year old flexplate, That would work, from a balance perspective.

Either the converter or flexpalte need to carry the LA360 weight. One or the other. The oe and aftermarket placement just vary on where they put it.

Hope that clears it up!
Where is your go to place for a converter?
Personally I like Yank torque converters out of Akron ohio. They are owned by gearstar transmissions. They are 100% custom, and will build one specific to the car.

High-Performance Transmissions, Custom-Built - Gearstar

Yank Custom Built Racing & Performance Torque Converters

If you are looking to save some cost on a more "off the shelf" offering. Hughes is decent also.

But I can't say enough for a fully custom, built to order, converter, from a company like gearstar/Yank.

I put our iron headed engine in a dart years back, and with a cheap B&M converter, and the car barely ran 14's. I then had a custom converter built, and it ran 12.20's with ZERO other changes. A good converter vs one that is "close" and off the shelf, can be that big of a difference. My example is extreme, but shows a good example of one mismatched, vs custom.