I know some have used an LA B&M flexplate with a magnum 5.9 and claim not to have any terrible vibration issues but I would not want to do it.

I tried to come up with a fairly easy solution to this problem without doing any welding.

Shown here is an LA 360 to 904 trans B&M flexplate that has a basic radius of 5.42" and is made of .125" steel. Based on these numbers I calculated the size and centroid of a section (directly opposite the existing negative offset) that could be cut out with a band saw to reduce the offset weighting by 5.14 ounce inch. 5.14 oz-in is the difference between factory LA and magnum offset ballance of flexplate/flywheel.

Measure as close as you can a "cord" that is 4.766" and directly opposite existing cutout. Note that the height of this section from cord to radius peak is about .552". It is unlikey that you will hit these numbers exactly with typical tools but I didnt want to round my theoretical numbers.

After cutting this section out the flexplate should match factory 360 magnum flexplate offset weighting.

Note: I have checked my math a few times and am fairly confident of numbers but havent needed to try one out yet. Also I have not had one checked on a balancer. The numbers given only apply to the 904 flexplate, the B&M 727 flexplate has a larger radius and the area to be cut would be different.

Have you had it checked on a crank balancer

can't remember what year.... but all the magnums became neutral balance. All the flexplates and converters between the 5.9 and 5.2 had equal bolt up dimensions and no weights on the converters.

Maybe you could make some paper templates instead of me measuring. I have a 1995 engine.
What is the part no. on that flex plate?

727

http://www.summitracing.com/parts/bmm-10236/overview/make/dodge


904

http://www.summitracing.com/parts/bmm-10239/overview/make/dodge

Jbody, not trying to call you out sir, but later 5.9 magnums are still external balance, they just moved the weights from the converter to the flex plate. Flexplate bolt to crankshaft pattern only allows it to be bolted on in one position to make sure weights are in the right spot. Also the two different harmonic balancers are weighted but interchangeable- old style has bolt on crank belt pulley, new style pulley is cast in as one piece with the balancer.

Not true.

On the 5.9 they either had weights on the converter or flexplate. The engine was never neutral balance. I have owned both version and can confirm.

At some point they did use a 5.2 flexplate with a 5.9 and the weights are on the converter.

Treed!

Question- How about calculating how much would have to take off a common 904 neutral balance flex-plate? A few million of them floating about.

Seems it would be easier with a hole saw, a lot less math.


Either way You could weigh the plate before and after on a postal scale and confirm you removed the correct weight either way.

There isnt enough material on a stock flexplate to remove enough weight.

I considered drilling holes first but if you drill small holes (.5 inch or less) you need to drill a large number of them and I thought it was really undesirable.

If you would rather use a hole saw (with just a single hole) there are only two options.

1) a 1.625" hole centered at a 4.370" radius from center of crankhaft.

2) a 1.750" hole centered at 3.768" radius from crank center.

If you go with the 1.625 hole you end up about .24" away from outer edge of flexplate.

Mr P body, I updated my post to make it clear I have not checked this on a balancer.

I based all my numbers on mopar spec. LA engines require 19.79 in.oz. Magnums 14.65 in.oz. of weight offset.

Also, am relying on the 1/8 inch thickness of B&M steel being fairly consistent.

BTW, steel = 4.537 oz/cubic inch

I've looked at LA vs magnum weighted flexplates and the offset weighting while different appears to be in the same angular position.

I think what I have given here should be able to get someone as close as factory balance tolerance or as close as some of mopar performance book 360 flywheel weighting alterations.

I dont have a template in the works.

thanks

thank you for the clarification.... I just remember at work they changed the weight style and after installing a trans and converter in a v8 equipped grand Cherokee, that it shook after I completed the job. Converter was supposed to come with a new flex plate iirc or something like that.