SFI Flex Plate differences

[Archeryguy02]

My motor builder sold me a SFI (not sure what brand) flex plate with the cut out for external balance, but he didn't know the brand. The motor is a 360 and it is balanced to this flex plate, but we couldn't positively identify as 904 or 727 or both. If I go by the pictures on the B&M sight it looks like a 360-727 flex plate mainly because it has the clusters of 3 holes near the outside vs the 2 holes on the 904 plates.

Can anyone shed some light on the differences or what issues it may cause if any if it is intended for a 727 and I run it on a 904.

I have had it on the car and run it for a while with no issues beyond the converter bolts getting beat up a little, but that could be a tightness or hole size issue.

Thanks in advance.

 

[Bobzilla]

If you have the one with three converter bolt patterns, it should be the larger diameter flex plate that was originally intended for 727 applications. The smaller bolt hole converter pattern is out further from the center. That is for the really big 727 converters usually in big cars that are like 11 3/4 or 12 inches in diameter.
The 904 intended flex plates are smaller in diameter so there is no room for that larger converter pattern.
If your engine is balanced to your flex plate, you can put any trans and converter package behind it, 904 or 727, as long as you verify the starter will clear everything on a 904 setup, and as long as the converter is neutral balance.

 

[12many]

We’ve got the B&M flexplates for the 904 and the 727, both are for the external balance 360. Each mocked up on a Hughes 11” 904 Converter. The last two pictures are the 727 version plate mocked up showing converter teeth exposure and the clearance and the only case clearance concern: splash shield bosses. Hope this helps you.

 

[A body]

Interesting.... I have the 3 hole plate on my small block and use it with my 727 and my 904. 8” converters in both.

 

[Archeryguy02]

We’ve got the B&M flexplates for the 904 and the 727, both are for the external balance 360. Each mocked up on a Hughes 11” 904 Converter. The last two pictures are the 727 version plate mocked up showing converter teeth exposure and the clearance and the only case clearance concern: splash shield bosses. Hope this helps you.

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Thank you, this was very helpful.

 

[A body]

Well I see why we have the larger plate. All our stuff is internally balanced and the only “904” plate they have listed is for an 8 bolt crankshaft. Both our mills are 6 bolt.

 

[B3422w5]

Well I see why we have the larger plate. All our stuff is internally balanced and the only “904” plate they have listed is for an 8 bolt crankshaft. Both our mills are 6 bolt.

you can still find the “ small” Mopar plate. I found a new one over the winter for mine

 

[A body]

I finally had the car up in the air again looking things over and thought about this post. Appears to me that the larger plate (904/small block combo) is REALLY close to my starter. So much so I see sometime in the past we ground away some material off the starter drive. Now until last year we’d run a 727 and the 904 hadn’t been in the car since the mid 90’s... and at that time we had a cast crank so the small plate was on it. Believe the large plate didn’t come into play until our cast crank cracked and was replaced with a forged unit.

 

[Archeryguy02]

I finally had the car up in the air again looking things over and thought about this post. Appears to me that the larger plate (904/small block combo) is REALLY close to my starter. So much so I see sometime in the past we ground away some material off the starter drive. Now until last year we’d run a 727 and the 904 hadn’t been in the car since the mid 90’s... and at that time we had a cast crank so the small plate was on it. Believe the large plate didn’t come into play until our cast crank cracked and was replaced with a forged unit.

I have the 727 flex plate on my 904 and sometimes the starter will fail to engage and just spin. Where exactly were you grinding down? The OD of the drive gear?

 

[Archeryguy02]

OK so I got the car together and replaced the starter to reduce the chance of having the starter engagement issue. I ended having to clearance the starter case right close to starter gear at the bottom of the opening for the starter gear. Works good.