Differences in 904 sun shells

[furrystump]

So I’m building a 904. The core is a 1968 904 with 4 pinion aluminum planetary’s front and back. So the issue I’m having is the low gear set kit I bought came with a sun shell. It is different from the 68 sun shell in that the opening is much larger than than the 68 shell. You can put the shell together like an easter egg.

 

[furrystump]

So this is the 68 shell on the clutch retainer as it should be.

 

[furrystump]

 

[furrystump]

Stuttering!!! Here is the low gear shell!

 

[furrystump]

I can install the low gear set on the 68 sun shell. It doesn’t hit the sun shell, but it opens up the gear set end play. The material the 68 shell is made of is thinner, which is why the gear set end play opens up. I can add an extra thrust washer between the shell and the round wire snap ring. Thoughts?

 

[oldkimmer]

Not sure when the change over was in the 904s, but I think 71 or 72. Then your new parts will fit. Bigger parts means a tougher tranny. Kim

 

[furrystump]

So they changed the front clutch retainer dia or lug size in the early 70’s?

 

[rmchrgr]

The later ('78-up) low gear sun shells are different than earlier models. You have to get everything that goes with it to make it work - the planetary set, the shell, sun gear, snap rings and bearings, etc.

The low gear shell is on the left. See the step at the bottom? The step accommodates a spacer used with the 'low gear' planetary. You have to have that piece or it won't work.

I believe all 904 sun gears are the same except the low gear one uses this machined spacer. I don't know why they went with this setup, maybe has to do with the lockup feature on the later units. The lockup units have different input shafts and I think the overall stacking tolerances might be slightly different. Either way, you can definitely use the low gears in an earlier unit but again, you need all the parts.

Here's a thread I started several years ago regarding 904/999, might help shed some light on your buildup.

How to identify a non lockup A999 transmission with pics!

 

[furrystump]

Thanks, I saw some of those pictures in my searching. The kit I bought had the pinion/sun gear/sun shell and two .045 washers between the round wire clips. The washers are larger in diameter than the washers in the stock shell. Same thickness. You need the larger dia so the actual thrust washer can ride on the washer. I see the step, that is to allow clearance for the larger diameter low gear pinion unit? My low gear steel pinion fits in the “non low gear” shell, the big “low gear washer fits in the “non low gear” shell. The only difference that I think I need to address is the two shells are made of different thicknesses of materials. Low gear one being thicker. Looking to see if anyone has ever run a standard shell with a low gear set. I have thrown one of the original flat washers on top of outer big washer. It seats on the thrust washer for the rear pinion set and brings the gear train end play right into spec.

 

[furrystump]

Thanks for all the replies. Called John Cope, who I got the kit from. He says just swap shells and make sure the gear train play is good. I really just wanted to make sure, only refreshed my 727 once before. New to 904’s.

 

[rmchrgr]

It seems odd that the sun shell would be made from thicker material but I guess it's possible.

This quote is from a Moparts thread started by Scott/dizuster when he made a post about swapping 904 parts.
I know this may not really answer your question exactly but it could help understand what's going on.

"Low gear sets can be switched out of late style 904's but the thrust plate that goes between the snap ring and the sun shell must stay with sun gear. There are at least 2 distinctly different thickness thrust plates in the sun shells."

everything you never wanted to know about 904's

I'd try to anticipate where your end play is before you put the front pump in.

 

[furrystump]

I can see why you need a bigger washer inside the shell with the low gear set. The pinion carrier is larger in diameter and so is its four tab thrust washer. If you don’t use the big washer that comes with the low gear shell, the small washer doesn’t cover the entire surface of the 4 tab thrust washer. It only covers about 1/2 of it. Yeah, I have it assembled temporarily. The gear train end play is .030. Haven’t put the pump in it, but will to check overall end play. I’m going to check it and pull it out and take a bunch of pictures in case someone else runs into it.

 

[rmchrgr]

Cool, glad you figured it out. Guess I did not know that exact little tidbit about the area of the thrust washer but I knew there was something within the stack that had to be retained for everything to fit together properly.

When I built my trans I skipped the low gear set because my motor makes good torque and I didn't think I needed the extra starting line ratio. We'll see how that works out if/when I ever finish my car.

 

[furrystump]

Yeah, it may not work for me, but I wanted to swap out the aluminum front planetary for a steel one anyway. Plus the 10 second small block butterscotch duster in the class runs a low gear set and he was pretty confident with that choice. We’ll see. Here’s the 68 sun shell with the low gear big washer installed and the original 68 washer in front.

 

[furrystump]

and here is the two stacked on the rear of the shell taking up the difference in the shell thickness. The round snap ring grooves are a bit farther apart on the low gear set sun gear.

 

[furrystump]

So, I had read somewhere that the starter was slightly moved on the 904 when compared to the 727. This is a 727 case with a 904 dust shield. So they moved it on the 727 because the converter was bigger? Ok, but all small block auto’s have 130 or 131 tooth ring gear and 10 tooth starter gear. So the share the same tooth per inch of circumference. So they share the same ring gear dia? and if ALL that is true why oh why will my 727 B&M dual pattern flex plate possibly hit the starter when mounted to the 904?? Can’t wait to see what moving the starter up 1/2” does to my original 340 exhaust manifold clearance!

 

[furrystump]

Well, went out to US 131 in Martin Mi for the FAST series opener. The track prep was great. It was HOT 88 degs hot! and damp on Wednesday, but got drier Thursday and Friday. Reading about the possible improvement by changing from a 727 to a 904, it seems 1 to 1 1/2 tenths could be gained from the swap. My change included another 9.5” PTC converter (same diameter as the 727 one) with an increase in flash from 3340 to 3500. The new one actually flashes to 3530. Very happy with the flash. Just basically your standard 904 build. Nothing exotic, but did go low gear set. I also needed a new drive shaft, so I went with PST 3.5” aluminum with 1350 front and 7290 rear joints. It was 7 lbs lighter than the steel 3” of theirs I had been running. The best ever in mineshaft air for the 727.
11.719 at 116.86
The best so far with the 904
11.535 at 117.34
Very pleased with the change, was hoping for a 11.59 in great air and got low 11.53’s at 3200 feet of DA. Now I’m going hunting big blocks!

 

[furrystump]

Shift comes quick! Dotted line is 727. Weird that the 727 never like a higher shift point. My guess is the 904 is at the higher rpm such a shorter time.

 

[PRH]

I was wondering what you changed.

Looks like a nice pick up

 

[Scampin]

Congrats again man! Just got the stroker motor in the AAR, going to be building a low gear 904 and a new converter for it next. Just doing little testing in the driveway and the "3500" stall converter in it turns the tires over at 1900.. We'll be hunting for some barracuda by fall.

 

[rmchrgr]

Reading about the possible improvement by changing from a 727 to a 904, it seems 1 to 1 1/2 tenths could be gained from the swap.

That's the conventional wisdom. Less rotating mass. A "Pro Flite" is a 727 case with 904 guts so that should tell you something.

 

[furrystump]

Congrats again man! Just got the stroker motor in the AAR, going to be building a low gear 904 and a new converter for it next. Just doing little testing in the driveway and the "3500" stall converter in it turns the tires over at 1900.. We'll be hunting for some barracuda by fall.

The AAR scares me…….actually the crafty old driver scares me more! Old age and treachery always overcomes youthful exuberance! Ok, so it’s a stretch saying that I’m youthful! What evvvvv!

 

[Brooks James]

So they changed the front clutch retainer dia or lug size in the early 70’s?

IIIRC 1972