A-904 - Five Clutches, Anyone Done that?

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i also believe that those 5clutch drums came in 904s behind 360 engines , my 904 had i think cordoba-76 written on it... probably writen on it at some junkyard a long time ago and put in my car in the 80´s and it has a 5 clutch drum
 
I do not believe that the 5 clutch drum from the A999 will fit the 904, I recall we were going to use one in my 904 when we did the low gears swap, but the drum is physicly larger in diameter, I could be mistaken, we did have 4 trannsmissions tore apart at the same time that we were mix and matching parts from, but I am pretty sure thats what it was. As stated though you can have a new c clip/spring clip groove machined in (if your drum has the room for it) to allow you to stack another clutch in there.
 
It was my understanding that the A999 and A904 have the same case. Could be wrong though, it's happened before.
 
Here's where the problem lies and I'll try to explain it to the best of my ability.

Here are the parts that are different from the 4 disc to the 5 disc non lockup 904

input shaft
rear clutch drum
reaction shaft
front clutch drum
front clutch piston

Now you can physically fit a 5 disc front drum in a 4 disc trans by clearancing the reaction shaft to clear the drum as it's deeper. The problem is that the splines that engage the front clutch discs on the rear clutch drum are not deep enough to fully grab the last disc. It may work for awhile but not long. Ask me how I know.

You can't just put the 5 disc rear drum (I don't mean 5 rear discs) in either as the size of the single seal ring that goes into the reaction shaft is different, it's too big. So you need the five disc reaction shaft to match. Great, but now the 5 disc reaction shaft requires 2 seal rings on the input shaft and the 4 disc input shaft only has 1, plus it's smaller. So no big deal you say right, just get a 5 disc non lockup input shaft. And there is the rub, you can't find them readily. They were only made a few years when there where lockup and non lockup trannies available in the same model year. As far as I know I got the last two and my buddy has one of them.

If you are lucky enough to find an input shaft you can make a non lockup reaction shaft from a more readily available lockup R/A shaft by removing one sealing ball used to seal off one port. If you compare a lockup to a non lockup (whether it's for a 4 or 5 disc) you can readily see which ball needs to be removed.

Is it as clear as mud now?
 
In my opinion a 4 disc will hold fine in a 904 or 727. I've even set some trans-brake 727 transmissions up with 4 disc, with no problems on tear down.

I'd focus more on proper assembly and clutch pack clearances to make the unit last .. my 2 cents.
 
I would agree to a point. I try not to second guess the factory. They put that 5th disc in there for a reason. A four disc 727 has a lot more surface area than even a 5 disc 904. I wouldn't be too afraid of a 4 disc 727 for just about any application. A 4 disc 904 while probably more than adequate for the street/strip cars around here isn't, in my opinion, going to stand up to transbrake launches from a 700+ horsepower stroker small block for a long time. It's all about matching the intended application.

How long do I think a 5 disc 904 would last behind that same stroker small block? At least a full season if not more.
 
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