Editing TH350 rebuild tech

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This WIKI is not intended to be a replacement for an ATSG or Ron Sessions tech manual. First time builders will likely need a tech manual. I would recommend Ron Sessions book "TH350 Handbook" in the HP Book Series. It has good pictures, alot of good info, but has some outdated info. Outstanding value for the money.

This is not a step-by step pictorial, don't assume I have shown where every part goes, there will be some thrust washers not illustrated that were installed and not taken an individual picture to show it because of bandwidth considerations.


That said I hope this helps some understand the TH350 or maybe it'll help when you're elbow deep and have a question.

First,
A TH350 is known to be rough on bushings.
Bushings are critical in that they allow the rotating parts to ride true on center, and for the lube to make it to the rear of the trans.
Be prepared to replace almost ALL the bushings in a TH350. Might as well order a bushing kit, as well as a thrust washer kit. I would recommend you get at least 2 additional sun gear bushings individually also.

Once the case has been cleaned, all threads checked and or repaired, and the rear case bushing installed you can start assembly.

The first item to go in the case will be the low/reverse apply piston.
Its function is for oil to force it towards the front or bell housing side of the trans, applying the low/reverse clutches. This allows reverse when applied simultaneously with the direct (high gear) clutches, and allows manual low gear when applied with the forward clutches. Manual low gear creates engine braking on deceleration. The low/reverse clutches do not apply in automatic range 1st gear. The low sprag (low roller clutch actually) causes first gear when only the forward clutches are applied.

In the rare application that does not need reverse or low gear engine braking the low piston and low clutches can be deleted, and the feed holes in the case blocked.


Install new seals on the low piston, these are square cut seals, not lip seals. 

[[File:TH350lowpiston.jpg]]

Then push it into the rear of the case evenly by hand, aligning it properly so it will seat (there is a "tang"). Look at the park pawl notch to help index it.


[[file:TH350caselowpiston.jpg]]

Then install the return spring and retainer using the tool pictured.

[[file:TH350lowspringtool.jpg]]

[[file:TH350caselowtool.jpg]]

Next you install the output shaft

[[file:TH350outputassem.jpg]]

Be sure you have the output ring gear to case bearing in the case.
Also be sure on disassembly you inspected and replaced as necessary the input shaft to output bushing shown here

[[file:TH350outputbushing.jpg]]

Next goes in the output planet to output ring gear bearing or thrust washer depending on year.
Then the planet.

[[file:TH350planet.jpg]]


Now you can install the low/reverse clutches, beginning with a steel, then a friction, alternating, ending with a friction.

[[file:TH350lowclutchesinstalled.jpg]]

Note here,
If you have a light duty core with low count low pack, unless it is a transbrake application or will be manually shifted into low for engine braking often, fewer clutches is OK, in some cases, where you are after every lat ounce of ET, using fewer clutches can cause less frictional loss in 2nd and high gear. "Turbulator" steel plates from a 4L60E will also help and are a direct drop-in.
I am using Alto red frictions in this unit, however a stock tan clutch (I prefer Borg Warner) is more than adequate for even very high HP cars. A Hi-Energy Borg from a 4L60E is also a good choice.

Once the low clutches are installed, you can install the anti-clunk spring (or case-saver). Both are pictured, the case-saver is solid and spreads the load over a larger area. Hold in place with assembly gel or vaseline and then install the low support.
Be sure the low sprag inner race is not brinelled or has any unusual wear.

[[file:TH350lowsupportcasesaver.jpg]]

Once it is pushed into place, install the snap ring, with the ends on each side of the spring or case-saver.
Now is the time to air check the low clutches. You do so by applying pressurized air to the passage at the rear pan area of the worm casting in the case. 

Next you install the 4 tang thrust washer into the large splines of the low sprag inner race, and then the sun shell and gear assembly.
Before this step, for HD use, you would have wanted to install additional sun gear bushings, 2 on each end, and re-drilled the lube holes.
Shown here.

[[file:TH350sungearbushings.jpg]]

Drop the sun shell/gear in, spinning in the output carrier until it falls in place.

[[file:TH350inputplanetthrust.jpg]]

Install the input planet next, already pictured.


Now,
To prevent excessive endplay, a pump thrust washer can be installed on top of the input planet, then the regular thrust washer. Then install the small outside snap ring to hold it all in place.
Check the output shaft for proper rotation, there will be some drag, especially in one direction.
Check for excessive output endplay. If endplay is excessive, it can be tightened up by dis-assembling and shimming up the output ring gear bearing from the case.

Now you drop the input ring gear on, spinning into place.

photos needed

Then install the 3 tang thrust.

For racing and heavy towing use, you will want to machine the direct clutch piston down approximately .160" to allow for one extra friction and steel plate for a total of 5 frictions, and 5 steels in the directs. This can be accomplished on a brake lathe. I try to keep a few machined on hand, and machine several at a time when I do so. If you need one email me and if you can supply a core, I MAY be able to sell you a piston.
Stock measurement is ~.835, I cut them to .675". Ideally you would pre-assemble, and check clearance using a 4 clutch pack, meaure the additonal thickness of the extra friction and steel, then cut the exact amount needed to leave you the exact clearance you desire. Rule of thumb on most auto trans friction clearances is ~.010" clearance per friction. So a 5 friction direct setup would work great with .050" clearance. It will work with less, I wouldn't go less than .040", and will work with quite a bit more, but excessive clearance can result in delayed engagement, busted lip seals, and unsatisfactory shift quality.

Stock 4 piston direct piston vs. machined for 5.

[[file:TH350directpistons.jpg]]

Next,
HD applications should also get a hardened intermediate sprag race. It is darker colored than a stock piece.
The intermediate sprag race and intermediate sprag (actually roller clutch) take a brutal amount of force on a 1-2 shift at high rpm. Even more so with harsher shift calibrations. It is the weakest single part of a TH350.
This is not to say a TH350 is not tough, they are under-rated and can reliably handle in the 600HP or lb/ft range based on my experience.

[[file:TH350intspragraces.jpg]]

I also like to always use a NEW intermediate roller clutch for HD builds. My theory on a long lived setup is that the springs that force the rollers against the race must be in good condition to helps the rollers grab quicker and more evenly on apply.

The parts go on the direct drum.

[[file:TH350directdrumnsprag.jpg]]

Assembled direct drum sprag and race with retainer

[[file:TH350intspragassemb.jpg]]

[[Category:Transmission]]

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