904 race build

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Duane said:
What I did on my pump is, using a punch I drove the sealing ball bearing that is staked in place in the pump right out. I then enlarged the diameter of the drilled passages. Then to re-seal the passage again instead of another ball bearing, I drilled and tapped for a 1/8 pipe plug. The diameter of the fluid passage for high gear is very small in the 904. Be careful not to enlarge the passage too much or your drill will break through the two ring sealing grooves. Mine just barely did this but did not affect its operation once the drum is on. Do at your own risk lol.
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Ok so it looks like the same concept of enlarging an engine oil pump passages.Thanks guys.
 
MHEAD said:
I have been using 904 transmissions for about 25 years and haven't had any major issues till recently.So with the added power that my cars been making I lost the sprag last year.Then we took the guts out of it and reinstalled in another case but now it slips going into third.So I figured it maybe time to put some good parts into it.
So the main questions is how do I modify the main pump support for more flow to high gear?
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Forgot to mention a couple things.
Most rvmb are set around 125 psi line pressure. Most transbrakes are set 170-200. My transbrake is from tranzact as well.
My motor should make similar power to yours and Joe told me that the 904 needs at least 170 psi line pressure to live at that power level. You can buy the higher line pressure spring from A&A trans.
Also you need 5 clutch front drum if you don't already have it.
Also while this is controversial, some cars have no sprague trouble.
I have talked to some other racers who after a stroker motor build claim they cannot keep one in the trans afterwards.
There is a relatively new 30 element sprague available for the 904 that can handle much more torque. It is a caged dogbone design.
You might need that too.
 
TrailBeast said:
What valve body is in it?
Flow volume doesn't help, pressure does.
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Agree with this post but I think improved flow speeds up the shift.
One of the features I liked about the griner and tranzact brakes
Is that they remove all the shift valves and unnecessary fluid passages. The billet bottom half has the fluid passages enlarged
and shortened to speed up the shifts. The use of an accumulator is eliminated completely. But I agree it is the pressure that does the clamping. The reason that I drilled and enlarged the passage to the hig gear clutch is that I noticed on my pump that this passage size becomes the limiting restriction size in the flow bath. This passage is huge in the billet valve body to speed the transbrake release, the common modification between the sealing rings also improves the flow. But the size of the drilled passage in the pump itself remains smaller than both of these on the 904, so I opened it up as well.
Also where the two pump halves bolt together, the flow path has square corners, so I rounded those to assist flow to high gear.
 
Duane said:
Forgot to mention a couple things.
Most rvmb are set around 125 psi line pressure. Most transbrakes are set 170-200. My transbrake is from tranzact as well.
My motor should make similar power to yours and Joe told me that the 904 needs at least 170 psi line pressure to live at that power level. You can buy the higher line pressure spring from A&A trans.
Also you need 5 clutch front drum if you don't already have it.
Also while this is controversial, some cars have no sprague trouble.
I have talked to some other racers who after a stroker motor build claim they cannot keep one in the trans afterwards.
There is a relatively new 30 element sprague available for the 904 that can handle much more torque. It is a caged dogbone design.
You might need that too.
Click to expand...
So I was playing around with a four clutch drum and put in five blue raybestos frictions but only got .040 clearance.The blue frictions are .064 thick.I would think I need another. 020 clearance from what I've been reading. Can the pressure plate be milled down or are there other options?
I assume you are talking about the ati sprag.Been kicking that idea around for sure.
That's good info on the vb.
 
MHEAD said:
So I was playing around with a four clutch drum and put in five blue raybestos frictions but only got .040 clearance.The blue frictions are .064 thick.I would think I need another. 020 clearance from what I've been reading. Can the pressure plate be milled down or are there other options?
I assume you are talking about the ati sprag.Been kicking that idea around for sure.
That's good info on the vb.
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Yes the ati sprag but a&a sells it too. Do you also have the thin steels in that drum? If not that would give you some additional clearance.
 
Duane said:
Yes the ati sprag but a&a sells it too. Do you also have the thin steels in that drum? If not that would give you some additional clearance.
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There the steels that came with the frictions. Steels are .068.Are there thinner steels available?Snap ring is .060
 
I have the pressure plate cut to get the necessary clearance....usually around .020 is needed...
 
Duane said:
Yes those are the thin steels.
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So I got into this trans and in our big hurry to get back racing we must have forgotten the thrust washer in the planet's. Not sure if this affected how it shifted into 3rd but who knows.

15889769942754922532119165309884.jpg
 
MHEAD said:
So I got into this trans and in our big hurry to get back racing we must have forgotten the thrust washer in the planet's. Not sure if this affected how it shifted into 3rd but who knows.

View attachment 1715524349
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So just an update,
I have installed a ati sprag,billit servos, 4.2 lever,multi front drum spring,blue frictions when done,red front band,kevlar rear band,new pump gears.
Dropped off pressure plate at machine shop to have .025 taken off.
Still debating purchasing a low band apply vb but still not sold on it.What brand company would you all recommend?
I have aluminum four pinion planet's from another case but not sure how they hold up?I just put the rear planet gears on a scale steel versus aluminum and they are one pound different. Thanks guys for all your help it's appreciated.
 
MHEAD said:
So just an update,
I have installed a ati sprag,billit servos, 4.2 lever,multi front drum spring,blue frictions when done,red front band,kevlar rear band,new pump gears.
Dropped off pressure plate at machine shop to have .025 taken off.
Still debating purchasing a low band apply vb but still not sold on it.What brand company would you all recommend?
I have aluminum four pinion planet's from another case but not sure how they hold up?I just put the rear planet gears on a scale steel versus aluminum and they are one pound different. Thanks guys for all your help it's appreciated.
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Ok so earlier in your thread, you said you lost the sprag, please explain why you think it failed and why you are not sold on a low band apply valve body. Why do you think the sprag failed the first time.
 
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Duane said:
Did you not check your geartrain endplay? Surely it was out of spec
With a missing thrust washer.
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I didn't check the gear train end play but I'm sure your right it was way out of wack.
 
Duane said:
Ok say earlier in your thread, you said you lost the sprag, please explain why you think it failed and why you are not sold on a low band apply valve body. Why do you think the sprag failed the first time.
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Well I don't do first gear burnouts.Only thing I can think is a few times that leaving the starting line my car spun and then hooked back up.This was maybe two tire rotations.
 
MHEAD said:
Well I don't do first gear burnouts.Only thing I can think is a few times that leaving the starting line my car spun and then hooked back up.This was maybe two tire rotations.
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Agreed and that is textbook sprag killer. That is also why I think some people have sprag failure and some don't. If cars sticks always, no trouble. Now ask yourself why John Cope makes every one of his valve bodies low band apply, even his non transbrake models. If you bought a billet solid reverse servo, you have what you need to use a low band apply. The key to using that servo is do not use the stock band adjustment setting with that servo. Cope says 2 turns only on a double wrap band. Gets the band released quickly so you don't get bind up on the 1-2 shift but the sprag is held by the band right up till you pull second gear. So even if your car hooks and then spins and re hooks, the band is holding and protecting the sprag to avoid failure.
 
Duane said:
Agreed and that is textbook sprag killer. That is also why I think some people have sprag failure and some don't. If cars sticks always, no trouble. Now ask yourself why John Cope makes every one of his valve bodies low band apply, even his non transbrake models. If you bought a billet solid reverse servo, you have what you need to use a low band apply. The key to using that servo is do not use the stock band adjustment setting with that servo. Cope says 2 turns only on a double wrap band. Gets the band released quickly so you don't get bind up on the 1-2 shift but the sprag is held by the band right up till you pull second gear. So even if your car hooks and then spins and re hooks, the band is holding and protecting the sprag to avoid failure.
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Okay that makes sense.
 
MHEAD said:
No the $30 dollar ones.
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Those are just stock replacements.
The good ones are like $250.
According to John Cope, very high compression motors can break the ears off the stocks gears upon firing the motor. He claims there is a very violent back and forth on the gears from the high compression that sometimes break the pump gears. Ymmv.
The good ones are not the same. The drive lugs are much bigger
and there is only a chamfer on one side instead of both sides which is supposed to not break your converter hub because there is more surface area. See the pics for the difference. One pic of each side of the gear, stock is on the left.

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Duane said:
Those are just stock replacements.
The good ones are like $250.
According to John Cope, very high compression motors can break the ears off the stocks gears upon firing the motor. He claims there is a very violent back and forth on the gears from the high compression that sometimes break the pump gears. Ymmv.
The good ones are not the same. The drive lugs are much bigger
and there is only a chamfer on one side instead of both sides which is supposed to not break your converter hub because there is more surface area. See the pics for the difference. One pic of each side of the gear, stock is on the left.

View attachment 1715524791

View attachment 1715524792
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Didn't know these were available or they had a problem breaking.My brother has a 904 converter that he used in the past that has a cracked hub.I was going to try it a some years ago but found the crack before I installed it.
 
MHEAD said:
Didn't know these were available or they had a problem breaking.My brother has a 904 converter that he used in the past that has a cracked hub.I was going to try it a some years ago but found the crack before I installed it.
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Cracked hub on a 904 is very common. Years ago when I bought my turbo action pro series converter, Paul Forte asked e if I wanted the thicker hub on the converter. In order to use it, you have grind down
The raised area that is on the pump. The splined section that the input shaft is inside of. This makes room for the thicker walled
Converter hub. The thicker hub is much stronger. The other mod that helps with the same thing is the better pump gears I showed you because they eliminate one of the chamfers on the gears so that there is more surface area in contact with the hub. Also the lugs are stronger for obvious reasons.
 
MHEAD said:
Didn't know these were available or they had a problem breaking.My brother has a 904 converter that he used in the past that has a cracked hub.I was going to try it a some years ago but found the crack before I installed it.
Click to expand...
Here are some pics of the mods on my 904 pump. Yes some are overkill.

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Well I chickened out on modify the pump but I was thinking about doing exactly what you did.If I ever switch over to a trans brake I may do that.Don't think my caribe bur would fit in between the sealing rings.
I may just use the four pinion aluminum planet's being the steel set are a little chewed up.Got .026 clearance on the gear train. Thanks for the pics.
 
MHEAD said:
Well I chickened out on modify the pump but I was thinking about doing exactly what you did.If I ever switch over to a trans brake I may do that.Don't think my caribe bur would fit in between the sealing rings.
I may just use the four pinion aluminum planet's being the steel set are a little chewed up.Got .026 clearance on the gear train. Thanks for the pics.
Click to expand...
Yes the passages is to speed the brake release. Like I said probably overkill.
 
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