T-400 Sprag/Direct Clutch Checkball - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-26-2004, 05:57 PM
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T-400 Sprag/Direct Clutch Checkball

I purchased a TCI, 34 element sprag for my '75 TH-400. My rebuild instructions tell me that in order to use this sprag I must use the earlier (pre '71) direct clutch drum, which I have on hand.

I'm using a TransGo 1-2 shift kit and top shelf Hughes rebuild kit.

I'm being seriously warned that I require a 'check ball', which the '75 direct clutch PISTON does not have, nor does the earlier clutch DRUM. The essential message is that I MUST have a check ball in either the drum or piston or both is even acceptable.

Can someone tell me the facts here and where the balls are located in the drums and or piston? Seems I heard the requirement may be related to high RPM usage, I don't expect to go past 6500 at most. Is there a fix or do I need to purchase an older piston with the check ball?

Also, when disassembling the trans I checked the rear output shaft end play and it was definitely zero end play. Is that possible?? I'll provide clearance when I assemble it, but it was odd not to find some play. The front input was at .030, a little loose.

Thanks, Larry

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Old 04-27-2004, 08:28 AM
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The t-400 drums must have a ( 1 )check ball in the drum or the piston. GM built them both ways. Actually applies to all trannys

Look at the inside of the drum in the area that would be under the piston. There may be a check ball in the wall of the drum with the sides of the hole staked in to hold the ball in place.

In the aluminum piston look for the same thing, a check ball in a hole with the sides staked in to hold in from falling out. The check balls should be free / loose

Late model t-400 pistons were steel and NEVER had a check ball in them.

These check balls allow residual fluid to escape when the drum is not applied. Residual fluid can apply the piston / clutch pack at higher rpms & burn the frictions
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Old 04-27-2004, 01:04 PM
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Thanks Crosley, so I'm assuming I need to find an early piston for the early drum needed utilize the new 34 element sprag? Guessing there's no other 'fix'? Am I correct that it replaces the sprag in the direct clutch hub?

Do you know if these early check ball pistons are still available?

Thanks again, Larry
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Old 04-27-2004, 10:10 PM
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You can only buy used aluminum t-400 pistons to my knowledge. " Non chain store " tranny shops should have one

If you wish you can drill a .040 hole in the drum and run a setup with no factory check ball. Drill the hole near the edge of the inner drum inside the area where fluid is behind the piston

This will not alter the application the clutch pack. The hole allows the tranny fluid a path to flow out & avoid high rpm application from residual fluid
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