Ring gear size

[B5blueGC]

I need to get a new torque converter for a 904 transmission. It is bolting to a 72 340 motor with a cast crank and B&M flex plate. Looking at the specs for a 904 vs. a 727 it shows the ring gear tooth count is different and one is narrow and one is wide.

904 - 122 tooth narrow
727 - 130 tooth wide

I had heard that you can take a starter from any engine ebody and use it. But that tooth count would make me think differently.

Looking for input from those smarter than me.

 

[Mr Cuda]

A 904 converter will only work in a 904 trans, so don't worry about the 727 converter tooth count. Starter fits all.
The b+m flex plate should have dual converter bolt pattern, for a 904 or both sizes of 727 converter.
Make sure you get the flex plate that zeros out the 72 cast crank imbalance, so you can use a zero balance converter. (Your included info is for a lockup 904. Make sure you have a lockup, with that part#)
And yes, I know you can run a 904 behind a 340, but why?

 

[B5blueGC]

A 904 converter will only work in a 904 trans, so don't worry about the 727 converter tooth count. Starter fits all.
The b+m flex plate should have dual converter bolt pattern, for a 904 or both sizes of 727 converter.
Make sure you get the flex plate that zeros out the 72 cast crank imbalance, so you can use a zero balance converter. (Your included info is for a lockup 904. Make sure you have a lockup, with that part#)
And yes, I know you can run a 904 behind a 340, but why?

The flex plate balances the crank so I am getting a neutral balance converter. The engine was balanced using the flex plate.

Thanks for the info. I am using a 904 because that is what I have. Originally had a 318.

 

[Chryco Psycho]

you need to watch the flex plate diameter , there are 2 different diameters , smaller converters use a smaller flex plate

 

[Ricks72Chlgr440]

And yes, I know you can run a 904 behind a 340, but why?

Built properly, a 904 provides less rotating mass so the engine spools up quicker with less loss. This is especially true if you use a smaller diameter converter compared to stock. Depending on the year of the 904, the first gear may be better for launching the car. The 904 also weighs around 130# with converter and the 727 is about 160#. With good clutches, bands, shift kit, and a QUALITY converter, the 904 can actually take quite a bit of torque and survive. I would also add a quality transmission cooler since heat is the enemy of a transmission. I would not run a stock 904 with a high perf engine like a 340 without mods.