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Old 11-20-2004, 12:59 AM
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T-400 34 Element Intermediate Sprag

I'm rebuilding a '75 T-400 and want to install an aftermost 34 element intermediate sprag. I haven't disassembled it completely yet.

I understand the '75 drum has a check ball relief in the drum, and to use the 34 element sprag I need the early style clutch piston and drum which has the relief in the clutch piston. I have an early hub but no piston. Can I drill a relief in the old style hub, matching the relief I expect to find in the '75 hub, and proceed with the 34 element sprag? I can't find the early clutch piston and GM doesn't carry them.

Can someone give me a quick and dirty on how to make this work?

Thanks, Larry

This "check ball" is supposed to prevent centrifugal apply of the high-gear clutch at high rpm, like to bleed off pressure.

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Last edited by 68NovaSS; 11-20-2004 at 01:14 AM.
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Old 11-20-2004, 11:41 AM
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Cool

You need a relief in the drum or the piston .

If you need to drill a hole , a .050 hole will be fine . No larger though.

If you are dual feeding the direct clutch , make sure you install all the return springs
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Old 11-20-2004, 04:25 PM
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Thanks Crosley.

Any chance you can be more specific? I'm not clear where the hole has to be drilled, (using the older style hub, yes?) and at what angle if any, and unclear about the "double feeding the direct clutch" and the return spring issue. I don't know why I wouldn't reinstal all of the ruturn springs.

I'm assuming (hoping...) the newer style hub in the transmission has the bleed hole and I can duplicate it. If it were easier, I would opt for drilling the piston, if of course, I knew more about that possibility.
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Old 11-20-2004, 09:09 PM
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Cool

Drill the hole you need in the drum with NO hole in the same area as the later roller clutch sprag drum with a bleed / check ball hole you have in front of you.


If I understand you, you have a later drum, you want to use an earlier style that has no bleed hole? Duplicate the hole location and you are set.

If you want to drill the aluminum piston ( it's easier to drill aluminum) , drill the hole in the same location as the steel drum bleed hole is. that location is between the middle seal and the outer seal.

The clutch drum does not know the difference if the hole is in the piston or the drum..... between the middle seal and outer seal , you will be fine

Since you are not familar with dual feeding the clutch pack , forget i mentioned it.
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Last edited by Crosley; 11-20-2004 at 09:12 PM.
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Old 06-24-2005, 07:50 PM
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Crosley - Help!

Crosley, I'm finally getting to that T-400 modification we discussed way back when. For me to use a 34 element sprag I'm told I have to use the earlier intermediate drum, which has no checkball or bleed-off - we discussed that earlier.

My question is now, reading Ron Session's book on modifying the T-400, he states emphatically that when drilled, the early drum must have a checkball. .

This is a one-time shot I want to get it right. Do I in fact need a checkball in the hole I drill into the earlier drum or piston?

If I decide to drill the drum, I'll drill at the same angle as the later drums' bleed hole; If I drill the piston I simply drill a hole, not over .050, right through the face of the piston, in from the outer edge enough to put it between the inner and outer seals, yes? Do drill the piston only or drill through the clutch spring retainer plate as well?

Upgrading to the 34 element sprag is a relatively common modification, I don't understand why there isn't more information available about doing it. Do you know of a web site where I can get more info?

Thanks again, Larry

Last edited by 68NovaSS; 06-24-2005 at 07:53 PM. Reason: Typo
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Old 06-24-2005, 09:49 PM
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drill one hole .050 max where the fluid can escape as I indicated between the middle & outter seals.


I don't care what the book says.

You need a bleed hole to let pressure out when the clutch pack is not applied. a .050 will be fine
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Old 06-25-2005, 12:38 AM
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That's what I'll do once I get it apart. Hopefully I can see where you're talking about.

I assume I won't be able to do any air tests on the pack, with no checkball it wouldn't hold 30# or so pressure?
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Old 06-25-2005, 11:13 PM
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the clutch will air check , may need a bit higher psi




Quote:
Originally Posted by 68NovaSS
That's what I'll do once I get it apart. Hopefully I can see where you're talking about.

I assume I won't be able to do any air tests on the pack, with no checkball it wouldn't hold 30# or so pressure?
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