TH350 Rebuild Questions

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scenic760

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Hey All!

I'm going to take the plunge and rebuild my TH350 that's in the truck...and while I have that out might as well do the transfer case...and while that's out might as well replace all the u-joints, haha..

I've rebuilt a transfer case before but never a transmission and it looks meticulous but doable... I had a question on cleaning everything...

The trans and TC are both caked in mud so I was going to power wash those off before I started anything..when I disassemble everything, what do I clean all the internal pieces with? I have been using paint thinner then acetone to make sure all the solvents were evaporated on just about everything else I have done with pretty good results...or should i just soak everything in clean ATF?
 

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Pressure wash the outside first.

There are numerous how to books available, and tons of YouTube videos. What tools are needed are easily fabricated. Work slowly, don’t force anything, air check each component, the TH350 is probably the easiest transmission to rebuild (the Chrysler 727 or 904 is by far the easiest to both understand how the work, and to build).

Buy a complete set of bushings, thrust washers, and one extra direct clutch friction and steel. Look up dual feeding the direct clutch on a 350 trans, that modification and drilling a couple holes will really wake up the transmission without the need to buy a shift kit.
 

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Much appreciated gents! The rebuild kit is supposed to show up on Monday... I do plan on going the dual feed route, 5 clutches, drill the holes and I ordered a hardened intermediate sprag as well...seemed like those mods would serve the trans well for not a whole lotta $$?!
 

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Crap, I forgot to mention a wide direct drum bushing (assuming your bushing kit doesn’t come with it). The factory bushing is kinda narrow, a wide bushing helps keep the drum spinning on center. Just make sure you don’t bottom out the bushing on the step inside the drum. If you do, you will either need remove it snd install another, or spend about an hour lovingly sanding/polishing the raised portion of the bushing to it fits over the pump.

Hopefully your core transmission has aluminum accumulator pistons, if not definitely get some. You can delete the 2-3 accumulator in the valve body by installing a 3/8 freeze plug in the feed hole under the piston. Delete the spring, reinstall the piston.

Most likely the 1-2 accumulator spring (under the round cover on the side of the transmission) will be broken. Some people will tell you to leave it out, doing so will definitely make it shift pretty firm. A orange spring was originally used in the performance transmission, most likely you will find a green spring. If you drill the holes in the plate, the spring color doesn’t have much influence on shift firmness. Any color spring is preferable to none.
 

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Crap, I forgot to mention a wide direct drum bushing (assuming your bushing kit doesn’t come with it). The factory bushing is kinda narrow, a wide bushing helps keep the drum spinning on center. Just make sure you don’t bottom out the bushing on the step inside the drum. If you do, you will either need remove it snd install another, or spend about an hour lovingly sanding/polishing the raised portion of the bushing to it fits over the pump.

Hopefully your core transmission has aluminum accumulator pistons, if not definitely get some. You can delete the 2-3 accumulator in the valve body by installing a 3/8 freeze plug in the feed hole under the piston. Delete the spring, reinstall the piston.

Most likely the 1-2 accumulator spring (under the round cover on the side of the transmission) will be broken. Some people will tell you to leave it out, doing so will definitely make it shift pretty firm. A orange spring was originally used in the performance transmission, most likely you will find a green spring. If you drill the holes in the plate, the spring color doesn’t have much influence on shift firmness. Any color spring is preferable to none.

Thanks for this!

So I think the kit I'm getting has the wider bushing.. I did see a youtube video of the guy driving in that bushing and he went juuuust a little too far with it but caught himself before having to do gymnastics on it!

I'm not really wanting the chirp shifting so will keep some spring in there...on the accumulator pistons I'm assuming I need 2 of them if they are not in there? Are these thems?:

https://www.summitracing.com/parts/sox-77998-03k
 

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Those are accumulator pistons for the 4L60/4L60E transmission. Depending on the year of the transmission, it might have aluminum piston in it already.

Transmissions with the factory heavy duty calibration or factory performance models didn’t use the spring in the 2-3 accumulator. I prefer my transmissions to shift with a solid “thud” in my daily drivers, just never cared for the greasy slide GM worked so hard to built into these transmissions. Drill the feed holes in the spacer plate to 1/8, leave out the 2-3 spring, it will have a quick shift just cruising, with a firm shift when your on the loud pedal.
 

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This is a plastic piston, this is what you don’t want.
 

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This one is aluminum
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This one is aluminum
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Well I've got it apart, except I need to make the tool to remove the low piston, and it does have the aluminum pistons...it reeks like burnt transmission fluid BTW...

It looks like it had been recently rebuilt as the frictions didn't look too worn but like I found an e clip loose on top of the intermediate accumulator...looked like it was work down a bit too..

Quick question, I ordered a hardened sprag race from Phoenix Transmission Parts and it showed up today...comparing it to my old one (old one on the right) it doesn't look much different?! The hardened sprag I saw in another build thread was dark from the heat treating?

I was going to read the Cliff Ruggles book tonight to see what kind of wear I should be looking for when putting this back together, does anyone have any tips on some common stuff that wears?

Thanks!
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I’m guessing every manufacturer has their own methods of heat treating the sprag, and material of choice in making them. They may not look identical.

Just check the bushings and thrust washers. Replace anything questionable. Good luck.
 

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@scenic760,

Wow, I just had my truck taken to a shop to get my TH350 overhauled. The shop does a lot of high end hot rods and such so they know what they are doing. Unlike ya'll I just have a stock 350 so I'm not getting anything done to it special. When I put the trans back in my truck after rebuilding the truck I bought a TCI "Saturday Night Special" which at that time was a 1,800-2,000 stall converter. They don't make that converter in a 2,000 anymore, I checked. It's a 2,100-2,200 stall now and I don't need that for the kind of driving I'm doing these days. Besides a stock 350 can handle anything I have to give it.

@Matt69olds, Yeah, I was seriously looking at getting a heat treated sprag but decided against it because I won't need it. I was looking at getting Torrington bearings to but came across an article that said I might have to machine something in order to get them to fit, so I blew it off.

@scenic760, You may be in a better position to get the Torrington bearings. I read that they are better than the flat thrust bearings as far as making the 350 more durable. What do you think about Torrington bearings, Matt?
 

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If you care to see, and have a buddy that works at a machine shop, or you just simply have access, you can put the sprauge on a rockwell tester to see the hardness of it.

Like this one at my shop:

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There is a lot of metallurgy in those sprags. It’s not just the heat treat, it’s the case hardening and the metal they are made from that’s the biggest concern. You can have a sprag with the right heat treat, but if it’s not case hardened and “tempered” correctly it will be to brittle. It needs to be hard where the rollers ride, but soft enough in the center to allow the part to flex instead of cracking. I hesitate to even suggest aftermarket replacements because there is some really poor quality parts being made.

I built a 350 for a friend, the trans went into a mild small block Camaro. The bought the sprag and overhaul kit from eBay, it didn’t last 3 weeks before it exploded. Tore up the pump, obviously the direct drum, and lots of shrapnel. The stock drum a and stock sprag race I reinstalled are still going last I heard.

Same thing with the 4L60/4L60E aftermarket 5 pinion carriers. You need to buy that kind of stuff from a reputable place. The money saved buying junk won’t go far having to fix the damage.
 

Vbb199

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There is a lot of metallurgy in those sprags. It’s not just the heat treat, it’s the case hardening and the metal they are made from that’s the biggest concern. You can have a sprag with the right heat treat, but if it’s not case hardened and “tempered” correctly it will be to brittle. It needs to be hard where the rollers ride, but soft enough in the center to allow the part to flex instead of cracking. I hesitate to even suggest aftermarket replacements because there is some really poor quality parts being made.

I built a 350 for a friend, the trans went into a mild small block Camaro. The bought the sprag and overhaul kit from eBay, it didn’t last 3 weeks before it exploded. Tore up the pump, obviously the direct drum, and lots of shrapnel. The stock drum a and stock sprag race I reinstalled are still going last I heard.

Same thing with the 4L60/4L60E aftermarket 5 pinion carriers. You need to buy that kind of stuff from a reputable place. The money saved buying junk won’t go far having to fix the damage.


This is an area i would love to learn more about.
Obviously i am a machinist, as ive said, namely a tool and die maker, not so much just an operator, so i obviously have an understanding of metallurgy.

As you probably know, I'm not a noobie when it comes to transmissions, ive been fiddling with them for maybe 2 years now.
Not an expert, neither do i know rverything.
BUT, what i wanna learn is, the middle ground between the metallurgy and the parts themselves.
As in- what can be hard, what must be case hardened, what needs to be soft, and all thr why's to follow.

I know that **** inside except actual roller bearings and sprauge elements can't just be 58 rockwell hard, but when someone says, "i kept twisting off output shafts, so i installed a hardened one".. i know they dont mean my version of hard, like 60 RC, tool quality. What does that mean exactly?

I understand what billet, cast, etc all means obviously, just not so much in terms of what "hard" is for high performance applications, and their molecular properties.
I assume it to be like, 4140 steel, thats been heat treated, so it still twists slightly, but it doesnt wear out or break off.

Alot if people just like throwing out big words they otherwise dont even understand, like "chromoly" for example. Its more if a bragging right amongst circles to say you have..... "chromoloy axle shafts in muh rear end"

I hate general lore and internet mechanics when it comes to transmissions, or rear ends, or whatever.

its like, when someone says "well those arent worth a damn, because my buddy that knows a guy blew one up one time", that screams to me, i dont understand this, so im going to immediately shoot it down.

It costed me two 700r4s with an improper TV cable adjustment when i was younger to learn why people in general despise the units. 1 guy didnt set his right, so he tells his buddies "all" 700r4s are junk.
Ive taken them and the 4L60e apart enough to know weaknesses, and know why the generic end user hates them, they have flaws in stock form, so they get a bad rep from everyone that doesnt know how to upgrade them because its not "plug and play"


- you get the point..i also understand there is physical limitations to all these various units before you must move to the next step in a unit (i.e. th350 to a th400)


The issue i keep running into, is, when i discuss this with far more experienced builders that possess more than 20 IQ points, they can never give a solid answer on the metallurgy, just the why, obviously because they are not equipt witt that knowledge

The "whys' i already know, or im learning as i go.

Maybe sometime you could have a telephone call with me, and maybe explain some of this?

Its just purely for my personal knowledge.
 
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