which flexplate ?? internal VS. external

[Princess Valiant]

I know I read one time that there is a flexplate to adapt a 360 that is externally balanced to a transmission that may be internally balanced.

I have a 1975 318 and 904 ....(no idea if it is external or internal balance) out of a 75 duster.

The 318 is dead so I would like to swap in a running 1977 360.

Not even sure if I can use the existing flexplate or if I need a B&M adaptor thingy.

I am a 4 speed person so I need some help with this auto stuff

 

[lemondana]

You need a B&M 10239 flex plate.

 

[RustyRatRod]

BM. Yeah I need to take one.

 

[moper]

You're close Rainy... I know you tend to want to know the background so here you go...
The transmissions are all neutral balance.
The engines are not all neutral. All 318s (LA and Magnum) are neutral
On Mopars, the cast crank units, and the 440+6 engines are external. External is done two ways with regard to automatics: the harmonic dampener on the front of the crank, and the torque convertor on the back end. The flex plates (all factory versions) are neutral. The weight's welded to the side of the convertor that the flex plate bolts to, between the bolts bosses. So in order to run the factory parts, one would need the factory convertor that is matched to that engine's needs, or the weight package to add weight to whatever neutral convertor you have. The aftermarket makes it easier, by offering a flex plate that includes the right weight bias. This allows any neutral convertor to be used with the given engine.

In your case - you have an internally balanced 318, with a neutral convertor now. To go to the externally balanced 360 with that same convertor you need the aftermarket plate with the offsetting weight.
I didn't double check but I believe Lemon got you set: you need the B&M 10239 flex plate.​

 

[67Dart273]

The transmissions are all neutral balance.

...................Well...................

This might be misleading, let's boil it down.........

It is the CONVERTER on the LA engines which is WEIGHTED for non neutral engines, or NEUTRAL or BALANCED for neutral engines

Magnum engines / LA transmissions / vise versa are a bit more complicated because on Magnums, the FLEX PLATE is weighted or neutral, not the converter

and Magnum engines require different weighting for a 5.9 than does an LA 360

 

[Princess Valiant]

so a 1975 318 and 904 is internally balanced??

I was thinking later 318s were cast crank and possibly externally balanced ???

is it a good assumption to say that cast cranks mean external balanced??

 

[318willrun]

so a 1975 318 and 904 is internally balanced??

I was thinking later 318s were cast crank and possibly externally balanced ???

is it a good assumption to say that cast cranks mean external balanced??

Here is the low down, I've done this 100 times........ make this 101 time

Forget 904/727 tranny. They have NOTHING to do with what motor is in front of them when it comes to internal or externally balanced.

• Now, All 318's are internally balanced. yes, there are some small weights on the 318 torque converter to balance the CONVERTER, but not the engine.
• 360's are externally balanced. They balanced the torque converter (using larger weights) on the 360 for the EXTERNAL balancing. Obviously, the torque converter is outside the motor, making it external. So if you balance the converter, it's externally balanced.

Now, if you remove a 318 and want to install a 360, you need to decide how you are going to externally balance the engine. Two ways it's usually done: Use a 360 torque converter (it will be balanced for the 360), or use a balanced flexplate. The idea of a balanced flex plate is so you can bolt a 360 to a 318/340 torque converter (neutral balanced). You can buy a flex plate balanced or have your existing flex plate balanced.

My 360 in my '76 Duster has a balanced flex plate and bolted to the 318 converter. I took my flex plate to a machine shop and had them balance it. I like the balanced flex plate. It allows me to buy all neutral balanced converters so I can use them behind all small blocks.

• A side note: You don't want to put a 318 in front of a 360 torque converter either, because it will throw the balance off (318 is already balanced). Some will knock the larger weights off the 360 converter and use it behind a 318.

Hope this helps

 

[Princess Valiant]

Thanks everyone who replied, just now ordered a B&M 10239 flexplate from Summit racing that will ship today. The page also said it is for a 360/904 combo. I would have never found it if I wasn't pointed in the right direction first so thanks FABO.

I hope to have this car running by the first of the year but we shall see what the car God's grant LOL

also, @RustyRatRod ...don't fall in the toilet man