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I just finished replacing the transmission in my 1979 Chevy K20 for the tenth time. WTH is wrong with th-350's? they just won't hold up to my 400 sbc no matter what you do to them they average between 13k to 25k miles and then first gear will go out. I am running an 400sbc bored .40 over with a .760 solid lift cam, an Edlebrock torker intake with a Holly 750cfm double puper carb, long tube hooker headers with 40 series delta flow flowmasters. The engine is only pushing 450+ Horse power. I am running 4.11 gears with a posi rear and limited slip front, a New Process 203 transfer case. I just don't know what to do, I don't want to swap in a 700r4 because I want to keep the same transfer. Any suggestions?
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Who is rebuilding them for you? If they have done all 10, I would find someone else to build them.
How are you driving the truck to break ten? Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project
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The one I just put in I built myself. The truck gets used daily on the farm I work at. Although this past time he truck never even got hooked to anything and the transfer case broke off the back of the transmission case. The transmission before that only had 1200 miles on a rebuild (Rodger's Automatic Transmissions) I have always had good luck with them previouse times but that time I never pulled anything with it or took it off road and I only drive 7 miles to work and first gear went out in it. Everytime it is always first gear after about 25k miles.
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Have you done any of the upgrades? Red Line clutches, Shift kit, NEW ~1600-2200 stall converter, Hardened Intermediate Sprag, extra clutches, etc?
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The first thing to do would be a failure analysis. What failed and why. You can't look at the transmission as a whole. If you do not have the means or knowledge to determine what keeps causing the failure take it to someone who can. The TH350 is a good trans and can be easily midified to take severe punishment with relatively low cost.
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This may seem obvious, but, are your sure you check your transmission fluid correctly? I mean, many don't check it while the tranny is warm. they check it cold. This makes a huge difference in fluid levels. If you check it cold, it may read o.k. but in reality be two quarts low. Running it too low on fluid will burn it up in a heartbeat.
SgtDel
aka "Andy"
www.delbridge.net
1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4
Edited by: SgtDel 
at: 12/4/06 9:18 pm
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Yes i have been checking fluid correctly, it has a 3100 rpm stall converter. It has all the modifacations done to it. This past time, one of the gears sheared in two. I just don't know what I am doing wrong.
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Gear Ratio?
Tire Size?
Do you use the radiator/trans cooler or a seperate trans cooler from the radiator or a combination of the two inline?
Trans cooling lines kinked, smashed, or twisted?
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The cooling lines are new, it has a 4.11 gear ratio with 33x12.50x16.5 tires. It has a seperate cooler because the last time I thought that could be the problem so I bought a trans cooler.
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It's good to run an auxiliary cooler but you should maintain the OEM cooler also. It acts as a temperature stabilizer/regualtor.
What gear shredded in two? There is a planetary gear set in your transmission but there isn't a one specific gear. Gear ratio's are acheived through the interaction of holding one member and driving one member of the planetary gear set. If your planetary gear set failed tell us exactly what part of it broke so you can fix the problem not just the symptom this time.
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It was the one that goes in in front of the output shut that looks kinda like a drum, can't remember what it is called
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Ok, guessing by your description if sounds like you are talking about the pump.
Where on this pic are you specifically talking about?
Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project
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Row D number 97 the end sheared off of it.
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That's your low reverse output ring gear. If this is the same component that is failing I would check to see if there is unecessary stress on the output shaft. Are your driveshaft angles correct? Is the rear pinion angle correct? If the truck is lifted did you lower the transfercase? Is the driveshaft slip yoke adequate? If everything checks ok I would suggest upgrading to the TH400 and doing a thorough overhaul. Have a different trans shop do it this time though
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Everything sits at stock hieght, the problem with installing a th-400 is that I would have to change the transfer case input or the case itself and I don't wanna do that because it's a pita. I appreciate everyone's advice and if this transmission doesn't hold it the truck will be for sale!
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Another thing to throw out here---I don't know if you are the only driver of the truck, but, I wonder if the extra stress is from putting it in reverse while still going forward, or, putting it in drive before coming to a complete stop in reverse. I have been with people who have a bad habit of using the tranny as a break while backing up (putting it in drive before stopping, etc...)
Just a thought. SgtDel
aka "Andy"
www.delbridge.net
1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4
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I've actually been known to do that from time to time when I'm in a hurry so I'll keep an eye on it and see if this could be the problem, thanks for all of your advice we'll see how things work out
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The TH-350 in a truck behind a torquey motor is a bit light, especially so in a truck with big tires & wheels. You have very little give in your drive train, and the intermediate sprag is light, needs to be the performance part, all planetaries need to be in top shape, and the valve body should be done right. You want to run line pressure low enough for some slippage. One problem is the 2-3 shift, and the clutch drums need to have drain holes... Also the trans case needs to be the truck 4 x 4 model, as the others are too light for a transfer case. There is a lot of trick stuff to do far too much to mention, but all of it is necessary. Most go to a TH400 because it is so strong that everything doesn't have to be perfect, and it is a natural for a truck since your driveline dynamics are so heavy anyway. (The TH350 is better in a car since it is like a lighter flywheel, where the TH400 drums and planetaries are so heavy). But the real deal is to beef a 700 R4 so that you not only have a great low gear so that you don't need a stall converter, you also have OD with 4WD and you also gain gas mileage. Also, its yoke and splines are the same as the TH350, and it is only slightly longer. That is what I went to.
Alansres