TH400 Direct Drum/Sprag Question

[Kilian]

Hello folks, I'm in the process of rebuilding a TH400 transmission for my 1983 K20 4x4 project. It's a 1987 TH400 which will have the roller-type sprag. It's going behind a nothing-special 350 V8 to be used for hauling and towing, such as a stock trailer and farm wagons, so wondering if I should invest in replacing the drum and sprag with the 34 element?

If so, I understand that the drum from a newer 4L80e can work in the TH400. Can anyone detail what parts exactly I need to find to make it work? Will the inner parts of the original direct drum work in the newer one, etc.

Thanks for the help! I've been reading and watching everything that I can find on rebuilding the TH400 and hoping to get it right the first time. I've got some additional questions that will probably be getting posted soon.

 

[squaredeal91]

TH400 Information Thread

Hi All, Since many of us own at least one GM Squarebody truck with a TH400, I figured I'd start another all-encompassing thread, this time on the Turbo Hydra Matic 400 aka THM400...I have basic facts, links and general information along with videos covering tear down and inspection, step by...

www.gmsquarebody.com

Maybe there is something helpful here.

 

[Bextreme04]

Hello folks, I'm in the process of rebuilding a TH400 transmission for my 1983 K20 4x4 project. It's a 1987 TH400 which will have the roller-type sprag. It's going behind a nothing-special 350 V8 to be used for hauling and towing, such as a stock trailer and farm wagons, so wondering if I should invest in replacing the drum and sprag with the 34 element?

If so, I understand that the drum from a newer 4L80e can work in the TH400. Can anyone detail what parts exactly I need to find to make it work? Will the inner parts of the original direct drum work in the newer one, etc.

Thanks for the help! I've been reading and watching everything that I can find on rebuilding the TH400 and hoping to get it right the first time. I've got some additional questions that will probably be getting posted soon.

You won't need anything different for towing normal loads behind a mild 350.

 

[Matt69olds]

As long as your sure you will never swap out the small block for a rowdy romping stompin big block, the upgraded sprag type drum isn’t worth the expense.

Since you are towing with it, I’d definitely do the dual feed conversion and replace the rear thrust washer with a bearing. Drill the feed holes just a bit oversized from stock for a slightly quicker/firmer clutch apply.

 

[NickTransmissions]

Hello folks, I'm in the process of rebuilding a TH400 transmission for my 1983 K20 4x4 project. It's a 1987 TH400 which will have the roller-type sprag. It's going behind a nothing-special 350 V8 to be used for hauling and towing, such as a stock trailer and farm wagons, so wondering if I should invest in replacing the drum and sprag with the 34 element?

If so, I understand that the drum from a newer 4L80e can work in the TH400. Can anyone detail what parts exactly I need to find to make it work? Will the inner parts of the original direct drum work in the newer one, etc.

Thanks for the help! I've been reading and watching everything that I can find on rebuilding the TH400 and hoping to get it right the first time. I've got some additional questions that will probably be getting posted soon.

Any 4L80E drum will work; the 97+ will have the bonded steel/rubber piston. Whether or not it's worth it will depend how heavy the loads will be and how often you'll be towing (per your desc it's not strictly speaking needed atm). Like Matt said, dual feed the direct clutch, rollerize the rear case and I also recommend installing 4L80E once piece teflon sealing rings onto the pump stator as well as the center support (leave the second ring from the top off to facilitate the direct clutch dual feed. 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. Lastly, replace the intrmediate clutch snap ring with a Chrysler A518 direct clutch drum snap ring (.88 or .106 is what I usually end up using).

Here's another TH400 video rebuild ref for you

 

[Kilian]

As long as your sure you will never swap out the small block for a rowdy romping stompin big block, the upgraded sprag type drum isn’t worth the expense.

Since you are towing with it, I’d definitely do the dual feed conversion and replace the rear thrust washer with a bearing. Drill the feed holes just a bit oversized from stock for a slightly quicker/firmer clutch apply.

Thanks for the info!

For the bearing conversion of the rear thrust washer, is GM P/N 9436851 what I'm looking for? (for the bearing). What PN are the shims for the new bearing?

 

[Kilian]

Chrysler A518 direct clutch drum snap ring (.88 or .106)

Is there a PN to search for to get this particular snap ring?

 

[NickTransmissions]

Is there a PN to search for to get this particular snap ring?

No idea on the part number as I buy them all from my local hard part suppliers but you can get them on eBay as well - click here for an example.

 

[Kilian]

No idea on the part number as I buy them all from my local hard part suppliers but you can get them on eBay as well - click here for an example.

Thanks for the link...I've ordered the one from ebay.

Another question...what tool do you use for installing the rear case bushing? I drove it out with a large socket but don't want to damage the new one. GM PN 8623941

 

[NickTransmissions]

Thanks for the link...I've ordered the one from ebay.

Another question...what tool do you use for installing the rear case bushing? I drove it out with a large socket but don't want to damage the new one. GM PN 8623941

You're welcome. I use a bushing driver for that size bushing from the Kent Moore TH400 bushing R/R set...I wouldn't recommend using sockets for them as you you'll likely ruin the bushing...Check out my TH400 bushing installation video for tips/pointer when it comes to bushing installation...If you haven't yet installed sun gear shaft bushings, they can be an 'adventure'...Also, use an early 4L80E pump body bushing for the rear of the case if you're going to rollerize it by using a TH350 pump thrust bearing and shim set - you'll need the extra width of the pump body bushing to locate the bearing and keep it in place.

 

[Kilian]

Also, use an early 4L80E pump body bushing for the rear of the case if you're going to rollerize it by using a TH350 pump thrust bearing and shim set - you'll need the extra width of the pump body bushing to locate the bearing and keep it in place.

Shoot...I've already got a new TH400 case bushing that I was going to use before it was suggested to rollerize the rear bearing. Is it still possible to use the TH400 bushing and still seat it further into the case to help center the TH350 pump bearing?

I'll have to see about finding the K-M bushing driver, is it J-21465-8?

 

[NickTransmissions]

Shoot...I've already got a new TH400 case bushing that I was going to use before it was suggested to rollerize the rear bearing. Is it still possible to use the TH400 bushing and still seat it further into the case to help center the TH350 pump bearing?

I'll have to see about finding the K-M bushing driver, is it J-21465-8?

No, do not do that. Buy the early 4L80E pump body bushing. Those bushings are cheap, dont worry about any bushings you may already have, just set aside...

I linked the set on ebay above (click on the link in bold) - it's pricey but will allow you to install all the bushings correctly with minimal risk...

 

[Kilian]

In your YouTube video, are you still using the TH400 case bushing? The 4L80E front pump bushings appear to be without any notches or channels.

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Also, when inspecting the friction clutch discs, is there a minimum thickness after which they should be replaced or is it best just to replace them when working on the transmission? The ones in mine don't look burned or warped, but I don't want to put back worn discs.

 

[NickTransmissions]

In your YouTube video, are you still using the TH400 case bushing? The 4L80E front pump bushings appear to be without any notches or channels.

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I cant remember what i used in that video but if you can locate the bearing using the TH400 rear case bushing then roll with it. I may have been thinking of the 4L80E rear case bushing in my initial reply as that bushing is much narrower than the pump body bushing.

FYI - quote my posts or mention me if needing a response as I dont get alerts when someone wants me to respond. I just happened to see this when scanning the forum list and figured I click on it...

 

[Kilian]

I cant remember what i used in that video but if you can locate the bearing using the TH400 rear case bushing then roll with it. I may have been thinking of the 4L80E rear case bushing in my initial reply as that bushing is much narrower than the pump body bushing.

FYI - quote my posts or mention me if needing a response as I dont get alerts when someone wants me to respond. I just happened to see this when scanning the forum list and figured I click on it...

Thanks for the tip on quoting your posts. I had edited my last post to include a question about the friction discs if you don't mind taking a look.