Th400 throwing oil out the rear seal? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**americanmusc1e** · 12-28-2020, 02:37 AM

I rebuilt this TH400 a few thousand miles ago. I'm for sure it's leaking out the rear seal. I can put it 1st gear up on jackstands and watch a bead of transmission oil work it's way out the seal. The entire underside of my truck is coated in trans fluid. Very minimal drips on garage floor.

The first thing I did was pull the extension housing and replace the bushing and seal with a spare that I had. Seemingly no change. I then put a dial indicator on the yoke to make sure it's not out of round and it has ~.020" of wiggle in the bushing/splines but almost zero out of round.

This yoke is suspect. It's an ebay special and it lost the freeze plug out the back a few thousand miles ago so it got the grey silicone and washer freeze plug replacement. Though it's not leaking out the back of the yoke. I've found a really nice sonnax replacement yoke but I'm wondering if there is anything I should be checking before replacing the yoke?

cadillac_al · 12-28-2020, 08:40 AM

New bushing and new seal, gotta be the yoke. I haven't bought any new ones in years. I suppose if they are a couple thousandths too small it will leak. It would be interesting to measure a few yokes to see how close the tolerances are.

Old Money · 12-28-2020, 11:11 AM

do inside micrometer measurement of seal. i had same leak. i bought 2 seals 1 from major brand and another from across town. 1 was exact same size as shaft, the other was .oo1 smaller and does not leak. old money.

**americanmusc1e** · 12-28-2020, 02:36 PM

I measured the current yoke as best as I could with my digital calipers. I'm getting 1.830"

This Sonnax yoke lists its journal diameter as 1.886"

I measured a spare bushing and it's 1.892" so it's very possible that the yoke I have now is undersized. The seal that I have has a listed inner diameter of 1.829" which is probably why it's leaking.

I was going to order that Sonnax yoke, but I found a Dana one for less money that will most likely still be an upgrade. Dana lists the diameter as 1.885"

I ordered a Dana yoke and a new Spicer 1310 joint. We'll see what happens.

12-28-2020, 02:37 AM	#1
americanmusc1e Senior Member Join Date: Jan 2009 Location: Austin, TX Posts: 1,079	Th400 throwing oil out the rear seal? I rebuilt this TH400 a few thousand miles ago. I'm for sure it's leaking out the rear seal. I can put it 1st gear up on jackstands and watch a bead of transmission oil work it's way out the seal. The entire underside of my truck is coated in trans fluid. Very minimal drips on garage floor. The first thing I did was pull the extension housing and replace the bushing and seal with a spare that I had. Seemingly no change. I then put a dial indicator on the yoke to make sure it's not out of round and it has ~.020" of wiggle in the bushing/splines but almost zero out of round. This yoke is suspect. It's an ebay special and it lost the freeze plug out the back a few thousand miles ago so it got the grey silicone and washer freeze plug replacement. Though it's not leaking out the back of the yoke. I've found a really nice sonnax replacement yoke but I'm wondering if there is anything I should be checking before replacing the yoke? __________________ -- Josh Instagram: @americanmusc1e OLD SKOOL-- 1970 C10. 454/Th400/3.07 posi Build Thread FARM TRUCK----1949 Chevrolet 3800 Power Wagon Hauler Build Thread 1999 4wd OBS Tahoe - daily. DM me if you can't see photos i posted

12-28-2020, 08:40 AM	#2
cadillac_al Registered User Join Date: Oct 2013 Location: Maine Posts: 2,332	Re: Th400 throwing oil out the rear seal? New bushing and new seal, gotta be the yoke. I haven't bought any new ones in years. I suppose if they are a couple thousandths too small it will leak. It would be interesting to measure a few yokes to see how close the tolerances are. __________________ 76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10

12-28-2020, 11:11 AM	#3
Old Money Registered User Join Date: Apr 2008 Location: Milton wi. 53563 Posts: 128	Re: Th400 throwing oil out the rear seal? do inside micrometer measurement of seal. i had same leak. i bought 2 seals 1 from major brand and another from across town. 1 was exact same size as shaft, the other was .oo1 smaller and does not leak. old money.

12-28-2020, 02:36 PM	#4
americanmusc1e Senior Member Join Date: Jan 2009 Location: Austin, TX Posts: 1,079	Re: Th400 throwing oil out the rear seal? I measured the current yoke as best as I could with my digital calipers. I'm getting 1.830" This Sonnax yoke lists its journal diameter as 1.886" I measured a spare bushing and it's 1.892" so it's very possible that the yoke I have now is undersized. The seal that I have has a listed inner diameter of 1.829" which is probably why it's leaking. I was going to order that Sonnax yoke, but I found a Dana one for less money that will most likely still be an upgrade. Dana lists the diameter as 1.885" I ordered a Dana yoke and a new Spicer 1310 joint. We'll see what happens. __________________ -- Josh Instagram: @americanmusc1e OLD SKOOL-- 1970 C10. 454/Th400/3.07 posi Build Thread FARM TRUCK----1949 Chevrolet 3800 Power Wagon Hauler Build Thread 1999 4wd OBS Tahoe - daily. DM me if you can't see photos i posted



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