TH400 Direct Drum/Sprag Question

NickTransmissions said:

Will you be turning north of 6000 RPMs on a regular basis?

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Not likely, fairly stock 350 doing some towing and highway driving. The explanation in that video about venting the extra fluid from the dual feed made sense so wanted to check.. Thanks for the info.

Kilian said:

Not likely, fairly stock 350 doing some towing and highway driving. The explanation in that video about venting the extra fluid from the dual feed made sense so wanted to check.. Thanks for the info.

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Hmmm...That may or may not have been a complete explanation, if that is all the content creator mentiond. The direct drum turns at thousands of RPMs relative to the engine when the trans is driving through it when in first or second gear. So any fluid under the direct piston will push the piston forward causing the direct clutch to drag/partially apply, esp just before the 1-2 shift.

This is not a concern for stock to mild/moderate performance vehicles/applications as the excess fluid never creates enough force to overcome the return springs. High rate return springs can be added to further prevent drag in moderate applications but even those wont be sufficient in seriously high rpm/power situations. The addl .050 hole lends a helping hand to the high rate springs by opening up another exhaust circuit to bleed off that excess fluid, preventing clutch drag or centrifugal apply.

So to answer your question, no - you dont need that extra bleed hole.

NickTransmissions said:

You can also install a Sonnax TH400-LB1 boost valve kit. Replaces a potentially worn factory boost valve/sleeve and gives a modest 10 psi LP increase if you use their spring.

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I was installing the boost valve kit over the weekend and cut one of the O-rings. Called Sonnax and got a part number that I can't seem to find anywhere...do you know the size of the O-ring? I figure the O-ring isn't anything special and local hydraulics shop should have a replacement. I assume they could size it as well, but knowing the size will speed things up. (You warned about cutting them in your video and I still managed to do it). Looks to be 1/16 dia with 3/4 ID and 7/8 OD?

Kilian said:

I was installing the boost valve kit over the weekend and cut one of the O-rings. Called Sonnax and got a part number that I can't seem to find anywhere...do you know the size of the O-ring? I figure the O-ring isn't anything special and local hydraulics shop should have a replacement. I assume they could size it as well, but knowing the size will speed things up. (You warned about cutting them in your video and I still managed to do it). Looks to be 1/16 dia with 3/4 ID and 7/8 OD?

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Yea, I've cut them before as well, I wish Sonnax would include an extra O-ring in each boost valve kit...I think they used to sell extra o-rings in packages of 5 or 10 but can't find an active listing anywhere. I don't know the dimensions off-hand but just bring the o-ring with you to the hydraulics shop, I'm sure you can match it up...Buy a couple of them just in case!