How many seals to replace on a THM 350 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

brown7373 · 12-18-2015, 06:53 PM

I want to pull my THM 350 and replace all the seals. Other than the front, rear and pan, what other seals or O-rings should I change?

RedRoad · 12-18-2015, 10:49 PM

I considered trying to rebuild my own TH350 and looked at many youtube videos on the subject. The performance rebuild kits are cheap and the stock ones are cheaper.

Check out youtube and then ebay or amazon and see if its something you want to tackle.

Way back in high school a buddy rebuilt his TH400 for a couple hundred bucks. Shift kit, better clutches and I forget what else. I do remember it seeming like a bomb went off in a transmission with all the parts scattered on several tables. It was down and dirty done over a home made trash can set up to help drop the guts back in but it worked flawlessly (other than a missing electric kick down switch) for many years of teen age bashing both on and off road.

brown7373 · 12-19-2015, 11:27 AM

Actually, my transmission functions flawlessly. It shifts firmly but not harsh at all engine speeds...but it leaks like a sieve.

Gromit · 12-19-2015, 02:34 PM

The coolant line fitting crush washers (2) (these can be hard to find)
Speedo cable o-ring
Manual shaft seal
Vacuum Modulator O-ring
Dipstick housing O-ring or seal
Cable operated downshift cable housing seal
Drain plug seal (or buy the $6 drain plug kit if your pan doesn't yet have one

This book about the TH350 is excellent and you can buy the electronic version for I think $18 and well worth it

Google lets you look at quite a few pages in their free preview here:

GM Turbo 350 Transmissions: How to Rebuild and Modify
https://books.google.com/books?isbn=1613251890

www.ctpowertrain.com I think sells a re-seal kit for the TH350

This is one of those projects that tends to creep - the trans is out so time to clean the underbody and cross member - the driveshaft is unhooked may as well replace the u-joints - well; got the u-joints off may as well replace the carrier bearing and clean everything above the driveshaft..

Next thing you know the War Department is accusing you of caring more about That Old Truck than you do her..

12-18-2015, 06:53 PM	#1
brown7373 Registered User Join Date: May 2011 Location: Fort Pierce, FL Posts: 430	How many seals to replace on a THM 350 I want to pull my THM 350 and replace all the seals. Other than the front, rear and pan, what other seals or O-rings should I change?

12-18-2015, 10:49 PM	#2
RedRoad Registered User Join Date: Jan 2015 Location: Los Angeles California Posts: 927	Re: How many seals to replace on a THM 350 I considered trying to rebuild my own TH350 and looked at many youtube videos on the subject. The performance rebuild kits are cheap and the stock ones are cheaper. Check out youtube and then ebay or amazon and see if its something you want to tackle. Way back in high school a buddy rebuilt his TH400 for a couple hundred bucks. Shift kit, better clutches and I forget what else. I do remember it seeming like a bomb went off in a transmission with all the parts scattered on several tables. It was down and dirty done over a home made trash can set up to help drop the guts back in but it worked flawlessly (other than a missing electric kick down switch) for many years of teen age bashing both on and off road.

12-19-2015, 11:27 AM	#3
brown7373 Registered User Join Date: May 2011 Location: Fort Pierce, FL Posts: 430	Re: How many seals to replace on a THM 350 Actually, my transmission functions flawlessly. It shifts firmly but not harsh at all engine speeds...but it leaks like a sieve.

12-19-2015, 02:34 PM	#4
Gromit Registered User Join Date: Jul 2015 Location: Central Coast, CA Posts: 498	Re: How many seals to replace on a THM 350 The coolant line fitting crush washers (2) (these can be hard to find) Speedo cable o-ring Manual shaft seal Vacuum Modulator O-ring Dipstick housing O-ring or seal Cable operated downshift cable housing seal Drain plug seal (or buy the $6 drain plug kit if your pan doesn't yet have one This book about the TH350 is excellent and you can buy the electronic version for I think $18 and well worth it Google lets you look at quite a few pages in their free preview here: GM Turbo 350 Transmissions: How to Rebuild and Modify https://books.google.com/books?isbn=1613251890 www.ctpowertrain.com I think sells a re-seal kit for the TH350 This is one of those projects that tends to creep - the trans is out so time to clean the underbody and cross member - the driveshaft is unhooked may as well replace the u-joints - well; got the u-joints off may as well replace the carrier bearing and clean everything above the driveshaft.. Next thing you know the War Department is accusing you of caring more about That Old Truck than you do her..



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