904 Trani

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tiffinohio

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Hello, new to the Forum! I am restoring a 71 Valiant. /6 trying to get 350HP out of ??? Installed a 7 1/4 Surelock rearend to try to stay stock. But my issue is the 904 transmission. I am rebuilding it and I have found the following. The rear clutch pack has only 3 frictions and 2 steels (as torn apart). It is suppose to have 4 frictions and 3 steels. The front clutch has 3 frictions and 3 steels (as torn apart), it is suppose to have 4 frictions and 4 steels. This is all according to the repair manual. I thought maybe it wasn't a 904, but everything measures out including 31" from nose to tail. The front clutch has about 3/16" clearance to the snap ring, with 3 frictions and 3 steels (close to same as pre-teardown). So I am getting the pressure plate milled 0.050 with that I can add the 4th friction and steel and end up with .080 clearance. I think I am OK there. But my rear drum with only 3 frictions and 2 steels has 0.035 clearance. I am replacing whatever was in the 904 with alto red frictions and Kolene steels. I would have to mill >0.125 off the pressure plate to achieve the required clutches and steels. Any guess why both clutch packs were short a steel and a friction, and any recommendations on what to do with the rear clutch pack, can I mill that much off the pressure plate? My boy is going to be a burnout giant so I don't want to skimp on clutches. I am also installing a Manual\reverse pattern valve body. No idea if anyone has been in there before. Picked up the car on the side of the road, my 13 year old fell in love with it and we are doing a COMPLETE restoration. Any help would be greatly appreciated!

Tiffinohio
 
there were different versions depending on the engine in front of the trans. I did a 727 last year, it was originally a 318 station wagon, it also had only 3 clutches in one clutch pack - don´t remember which one. So this could be absolutely ok for your application.

Michael
 
As Michael said there's different versions of 904's. Nearly all slant 6 904's came with the amount of frictions and steels you said yours has. Even 318 904's sometimes didn't have any more than that. Only the heavier duty 998 and 999 (versions of the 904) have 4-5 frictions in the clutch packs. I wouldn't cut .125 off the pressure plate. That'd make it nearly as thin as a steel plate cause their only about .215 thick from the factory. IMO it'd likely bend and fail if you cut it that much. You set the clutch pack clearance via selective snap rings. That's the snap ring that holds the clutch packs together and they come in different thicknesses. By your description of the clearance issues your seeing I wonder if you have the rear clutch and front clutch friction discs mixed up. The friction discs for the rear clutch are a lot thinner than the ones in the front clutch pack. The discs for the front clutch pack measure about .086" thick while the ones in the rear pack measure about .061" thick.

A manual valve body is common to see in a high performance application. Nearly every racer that's running a 904 uses one. Make sure you get a valve body with low gear braking. Without low gear braking you can trash the sprague if your not careful. Leave the "no low gear braking" valve body's for the racers

What manual do you have that claims all 904's have that many clutch plates per carrier? If it has that error I'm not sure I'd trust it for anything else it says.

In addition, I wouldn't worry so much about your slant 6 tearing up a 904. Racers have been using 904's behind very high powered V8's for yrs. If they'll last behind that your slanty isn't going to trash one very easy. Not putting down the slant, it's a great engine but it doesn't produce enough torque to scrap a 904 too easily. That's why slant 904's only have 3 clutch discs per carrier. I'd be more worried about that puny 7-1/4 rearend grenading.
 
That makes sense. I checked my frictions and they all are the same thickness +- .005. I emailed the guys at Oregon Transmission where I got the rebuild kit to be sure if this is the case. I'm a little concerned about the 7 1/4 rear end too. I'm hoping the surelock will save it. Trying to keep things as stock as possible. The manual is from the Automatic Transmission Service Group. It shows for a 225CID 4 frict + 4 steels front and 4 frictions + 3 steels in the rear.
 
With 350hp and a boy who wants to be the burn out king Ide be real concerned about the 7 1/4. Do your self a big favor and replace it now. You said about keeping it stock but the 350hp isn't stock. Just saying [youll be sorry]
 
There really wont be anything stock about it. Just the stock components with upgrades. More super stock I guess. Boring 0.060 over, Shaving .020 off block, .100 off head, trying to get to 9.75:1 CR. Building a new intake for EFI and putting twin 2.0L turbo's on it. Back yard port job on head and bigger valves. I'm hoping for a lot out of the leaning tower of power! Making the trani a race trani with performance parts from Oregon Performance Transmission. Probably going to need a cam but the stocker isnt too bad in this year, I'll have too see. Your bringing me down about the read end. Spent a ton on that surelock. If I have too I'll upgrade to an 8 3/4 that fits my spring pattern.
 
tiffinohio said:
That makes sense. I checked my frictions and they all are the same thickness +- .005. I emailed the guys at Oregon Transmission where I got the rebuild kit to be sure if this is the case. I'm a little concerned about the 7 1/4 rear end too. I'm hoping the surelock will save it. Trying to keep things as stock as possible. The manual is from the Automatic Transmission Service Group. It shows for a 225CID 4 frict + 4 steels front and 4 frictions + 3 steels in the rear.
Click to expand...

They must have sent you all thin discs like the rear clutch uses. If clearance properly that lets you get another disc in the front clutch. I just did that on my 904. The ATSG manuals have a lot of what the factory service manual has but they've added some extra stuff and I've found a few flaws but all in all it's a fair book.

tiffinohio said:
There really wont be anything stock about it. Just the stock components with upgrades. More super stock I guess. Boring 0.060 over, Shaving .020 off block, .100 off head, trying to get to 9.75:1 CR. Building a new intake for EFI and putting twin 2.0L turbo's on it. Back yard port job on head and bigger valves. I'm hoping for a lot out of the leaning tower of power! Making the trani a race trani with performance parts from Oregon Performance Transmission. Probably going to need a cam but the stocker isnt too bad in this year, I'll have too see. Your bringing me down about the read end. Spent a ton on that surelock. If I have too I'll upgrade to an 8 3/4 that fits my spring pattern.
Click to expand...

The suregrip 7-1/4 is a lot tougher than the open differential 7-1/4 but I bet it still won't last. And you don't have to upgrade to a 8-3/4. An 8-1/4 rear is a pretty tough little bugger and they were many used in A-bodies. I ran one in a Dakota I raced that ran low 14's and it held up for numerous trips down the 1/4 mile and 12 yrs of road service. That's what I'd go with if it were me. Since it has a rear cover like your 7-1/4 it'd look somewhat similar and it's a whole bunch cheaper than an 8-3/4
 
To get the 4 on 4 clutch packs the best way is to replace both the foward & front clutch drums with 4 disc ones. Ita a lot easyer that way & they are not hard to get a hold of.
 
Thanks for all the input. Is there a specific rear drum that allows the 4 friction/3 steel setup instead of the 3 friction/2 steel setup or is it all in the piston/pressure plate thicknesses. It seems a bit senseless for Chrysler to mfg different rear drums for different setups. It seems more plausible that it is the parts inside the drum. I can have the piston and pressure plate milled like I did for the front clutch (0.050) but I would need 0.125 total milling and that seems like a lot. I'm a little nervous about milling .050 off the front drum pressure plate considering warpage concerns. Any experience here. I'm not opposed to purchasing a new rear drum, just not sure how to find what I am looking for.
 
Yes the the drum is basically the same except for the grove for the snap ring is cut ferther down so one more clutch & steel can be installed in there. You can have a machine shop cut out this grove the thickness of one more clutch & steel or get a used 4 disc drum.
 
tiffinohio said:
Any idea how much I can mill off the pressure plate of the read drum? 0.125????
Click to expand...

I have never seen anything written in stone but my personal feelings are not to take more than .050" off. That pressure plate needs to be plenty thick to withstand the pressures it endures or it'll bend and fail. As I mentioned earlier in post #3 there are different thicknesses of snap rings that hold the clutch/steel stack in place. That's the first place you look to gain or reduce clutch plate clearance. It's possible your carrier has the thickest selective snap ring in it now which is .085" thick. The thinnest snap ring measures .061" thick so just by using it you can gain .024" of clearance.

I also wouldn't worry one bit about using a 3 clutch rear clutch carrier for a 350 hp application if I were using upgraded frictions like you are. Set the clearance to .030~.040 and you'll be fine

www.wittrans.com is a great place to buy good used parts reasonably. Look up your transmission and click on the drum you need and a page will pop up showing all the available drums for it. I just looked and they show 4 and 5 clutch carriers but let me warn you not to use a 5 clutch carrier unless you also use a rear drum that's made to accept 5 clutches. A 5 clutch carrier will appear to go together but in reality the bottom disc will not engage onto the splines so it won't work

Also keep this in mind. Your parts supplier sent you all thin clutch discs so that means if you get a 4 clutch carrier it's going to take 5 clutch discs and 5 steels to take up the extra space left because of using thin discs in a carrier designed for thicker discs. Even then you may have to have the pressure plate milled down some to get the proper clearance. I just rebuilt a 904 and converted the front carrier to 5 discs/steels by using thin discs and had to have .030 milled off the pressure plate to get the proper clearance

BTW: earlier you said all the clutch discs they sent measure the same. What was that measurement? I'm assuming right about .061" thick but it's good to verify
 
That is great advice. I think I will stay with the 3 disk rear drum. The frictions do all measure 0.061." I did get a 4 disk front drum which fits 5 thin in it like you said. I put all the parts on the rear hub an like you said, the bottom friction was barely in contact with the splines. I think under pressure the friction would slip out. So I bagged that idea and went back to the old three disk front drum that holds 4 frictions with my milled pressure plate. This has been quite a learning process. It is still surprising to me that Chrysler would make so many variation of the same part!

Thanks a lot.
 
The 7 1/4" is tougher then you think .
Just don't do to many neutral starts or you'll break a spider gear. Ask me how I know.

Ok , ok I'll tell, it took over 500 and wouldn't you know it was just out of the 50,000 mile warrantee . This was in our '63 Dart 270 4 door with 225 slant 6 and 904 with 3.21 gears and bias ply 6.50 x 13" B.F. Goodrich Silvertown tires.

Man that was fun , sitting' at at a light in neutral, light changes, push 1 button and stand on it. Screeeeech !! Peg leg stripe :)
 
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