Contents. OverviewThe GM Turbo Hydra-Matic 350 transmission (aka 'TH350', 'Turbo 350', THM350, etc.) was used between 1968-'86 in RWD and 4WD GM domestic vehicles. The TH350 was still produced many years beyond 1986 for export sales. It is a relatively rugged, compact transmission that lends itself to a wide variety of applications and modifications. Basic rebuild informationNote:. This wiki is not intended to be a replacement for an ATSG or factory tech manual.
First time builders will likely need a tech manual. Recommended is Ron Sessions' book, in the HP book series. It has good pictures, a lot of good info, but has some outdated info. Outstanding value for the money. This is not a step-by step tutorial. That said, the following should help give a better understanding of the TH350 and possibly help when a question arises.
The following page was inspired by an original Hotrodders forum thread by jakeshoe, titled. Variations of the TH350 familyThere is also a TH350C and TH250C version that has a lockup torque converter (TCC). Look for the end of the input shaft to have a smooth polished end with O-ring on it, similar in appearance to the 200C/2004R and 700R4/pre-LSx 4L60E. The case has a 'C' cast into it, and there will be an electrical plug adjacent to the shifter shaft on the driver side of the tranny. Same also holds true with the oil pump - 'C' is cast into the pump cover and pump half.Don't confuse the TH350C with a TH350 non-lockup tranny with a screwed-in electrical connector on the passenger side of the case. This is a pressure switch that was used 1972-'73 for the Transmission Controlled Spark (TCS) system. This disabled the distributor vacuum advance in all gears except high gear, and is a single wire switch threaded into a pressure port near where the cooler lines are located.
In the case of this switch, you can either leave it in place, unused- or remove it and plug the port.Hughes, TCI and Boss Hog make TH350C torque converters. Rebuilding the TH350 Bushings and thrust washersA 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. It is also recommended to get two additional sun gear bushings individually.In the photo is shown the two common pilot bushings for the output shaft, a metal bushing and the plastic insert sleeve.
The plastic sleeve design is very durable, you will find professional builders have their own preference. TH350 thrust washer kit Case prepOnce the case has been cleaned, all threads checked and/or repaired, and the rear case bushing installed you can start assembly. 1-2 accumulatorWhile it may be true that removing the accumulator spring from the 1-2 servo will firm up the 1-2 shift, leaving the 1-2 accumulator spring out is not recommended. One reason is the accumulator protects the failure-prone intermediate sprag and another is the spring keeps the cover loaded against the snap ring. Photo shows accumulator cover, spring and piston (snap ring not shown). This is located on right side of transmission case.
Check the bore in the case for scratches or other damage if the spring was found broken. TH350 pressure ports circled Transmission reassembly Low-reverse piston installThe 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 bellhousing 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. TH350 low-reverse frictions and steels installed with planetary and output shaftNote: 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 last bit 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.There are Alto red frictions in this unit, however a stock tan clutch ( like Borg Warner brand) 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). Hold in place with assembly gel or vaseline and then install the low-reverse support. Installed sprag (roller clutch), watch orientation of tangs into the center support housingOnce 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 (picture below), and then the sun shell and gear assembly. Front planet with torrington bearing in placeTo prevent excessive end play, 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 end play. If end play 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.Then install the 3-tang thrust washer onto the top ring gear. Front ring gear with 3-tang thrust washer in place installed into case on front planet Direct drum assemblyFor racing and heavy towing use, you will want to machine the direct clutch piston down approximately 0.160' 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.Stock measurement is 0.835', machining the piston to 0.685' to 0.720' will allow the use of 5 frictions using 0.070' steels. Ideally you would pre-assemble, and check clearance using a 4 clutch pack.
Measure the additional thickness of the extra friction and steel, then cut the exact amount needed to leave you the exact clearance you desire. Rule of thumb for most auto transmission clutch pack clearance is 0.010' clearance per friction. So a 5 friction direct setup would work well with 0.050' to 0.070' clearance.
It will work with less, but no less than 0.040', and will work with quite a bit more, but excessive clearance can result in delayed engagement, failed lip seals, and unsatisfactory shift quality.If the piston is too low, the bottom flat steel plate can drop below the grooves in the drum and bind up the piston. You can leave out center cushion seal if dual feed is used.
TH350 direct clutch drum pistons, piston on left cut for 5 friction useHeavy duty applications should also get a heat treated intermediate sprag race. The heat treatment makes the race less brittle. A heat treated intermediate sprag race is darker colored than a stock part. The intermediate sprag race and sprag (actually a 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 single weakest part of a TH350.
This is not to say a TH350 is not tough; they can reliably handle power in the 600 HP or lb/ft range if properly assembled using the right parts.Several companies offer an even heavier duty direct drum/sprag assembly. It uses a 36 element sprag (similar to what is used in the TH400) and a machined drum; it will withstand more torque than a stock unit or one with just a hardened race. These units can cost $300.00 or more. Direct piston with lip seals installed Dual feedingThis is part of the process of 'dual-feeding' the direct clutches. On a stock rebuild you would normally install this lip seal.You can do this on any rebuild but you must also block a passage in the case near the pump to prevent loss of pressure to the reverse circuit. Dual Feed direct drumDual feeding doubles the area of the piston that has pressurized fluid on it in 3rd gear, more than doubling the capacity of the clutch. It is accomplished by most valve body kits without doing so internally but requires a 'transfer' plate and gasket to be added under the support plate in front of the valve body.Internally dual feeding is preferred because it leaves out the additional gasket and eliminates that one extra possibility for a pressure loss resulting in burnt 3rd gear clutches.All transbrake applications will require dual feed of the direct clutch, possibly as well as manual valve bodies.Shown below is the passage that must be blocked.
One method is to tap this hole with a 3/8' tap, and cut off the end of a 3/8' bolt. The head of the bolt can be slotted so a screwdrive can be used.Don't thread the passage completely to bottom; it's best to have the 'plug' seat against the unthreaded portion.This passage may also be plugged by using a roller from a direct drum or sprag to drive into the passage, but this will be a more or less permanent modification due to the difficulty in removing the roller. Plug the hole indicated for dual feed on TH350Note: Don't assume you need to plug this hole. Only plug this hole if you plan on doing the dual feed modification yourself.You can omit the center seal, plug this passage, enlarge the 2nd and 3rd feed holes in the stock separator plate and you will have greatly increased the torque capacity of a stock TH350 as well as gained a firmer shift.Almost all aftermarket kits use a feed plate between the support plate in front of the valve body and the separator plate. This feed plate 'dual feeds' the directs, so plugging this hole is unnecessary. Also most manual valve bodies, trans-brakes, etc.
Will call for omitting the direct drum center seal and the sealing ring on the stator, but plugging this hole is also unnecessary due to the design of the separator plate.Even firmer shifts can be accomplished with check ball removal and modifications to the accumulators. It is recommend to NOT modify the 1-2 accumulator.
Retain the cushion spring and do not block the circuit. The 2-3 accumulator can be blocked by several methods.Removing the 2-3 accumulator spring and using a spacer (nut, washers, cut down section of tubing) between the piston and the E-clip is the 'shade tree' method.
You can also block the feed in the valve body to the 2-3 accumulator.Installing the direct piston will require some patience, a lip seal installed or 0.010' feeler gauge, and a trick to use is a section of plastic cut from a 3 liter soda bottle to use as an outside installer tool. Set it in the drum, and it will force the lip seal into the drum without using a tool around the entire circumference. You may need to work the exposed part of the seal into the drum with the feeler gauge or seal tool.Use care using the lip seal tool. Use a push-in technique more than a work-around-the-drum technique. If you slide around the drum, and it catches the seal, it will cut it and then you must get a new lip seal and start again. Good lighting, patience, and technique are key here, and a be sure to verify using an air check.Once you have the piston seated in the drum you can install the return springs and retainer. You can buy or make a tool similar to the one pictured below, or use several 5' or larger C-clamps.
One method to compress springs to install the snap ring Forward drum inspectionForward drum procedures are almost identical. No center seal to worry about on a TH350, it typically already has a 5 clutch pack, except light duty applications.
Same 0.700' thickness piston for 5 frictions and steels. Removing the wave plate will cause harsh forward engagement. Reduce clearance of the stack to 0.040'.Forward clutch pack clearances can be tighter than usual; however it must have some clearance.
If too tight it can creep in neutral.Pay attention to this polished stub of the input shaft, it goes into the bushing that is in the end of the output shaft. It is a high wear area. Be sure it is smooth. Minor scoring can be smoothed down.
May consider use of teflon bushing, only a lubrication passage, no pressure, no lateral loading. One method to align the pump halves shownThis is an example of to air check the forward and direct drum. After everything is assembled, I place the drums on the pump (above a hole in the bench) with all sealing rings in place, thrust washers or bearings.Use a rubber tipped air nozzle to apply air to the passages around the pump circumference.
As you apply air to the correct passages, you will see and hear the drums apply the clutches. Be sure there are no leaks from the lip seals. There will usually be some minor leakage at the sealing rings. Iron rings air check better than teflon on the bench.
Heavy duty brake band should have weld reinforced struts Intermediate clutch6 cylinder applications use an intermediate piston measuring 1.175'-1.185' thick. The 6 cylinder uses two frictions.The V8 intermediate uses three frictions. The piston is about 0.990' thick.The wavy cushion plate should be used.There are two types of intermediate return spring/retainers.
With the early type you had to manually place the loose springs onto the piston's spring locator bosses,then the retainer went over that. The later retainer has the springs attached to their respective retainers (the bosses are not used). So if the intermediate piston is smooth, use the retainer with the springs attached to it. The late retainers will work in a transmission that originally used the early type retainer, but not vice versa. Intermediate clutch clearanceAfter the pump is set into place to test end play clearance, you can also check the clutch clearance on the intermediate clutch pack. Clearance of 0.050' to 0.070' will work well.
Th350 Anti Clunk Spring Installation Instructions
There are thicker intermediate steel plates available if need to tighten up the clutch pack clearance. Intermediate steels (see page 40) in 0.068', 0.078' and 0.089' thicknesses, some manufacturers may use slightly different thicknesses than these.Also, a scraped low-reverse friction can be used under the intermediate piston if need be. A scraped friction will measure about 0.032'. Notice the open line in the gasket, this gasket installs against valve body, not the case Valve body modificationsRemoving check balls and opening up orifices in the separator plate is a common modification to firm up shifts. Just remember the one check ball that needs to stay is the one just below the modulator.
Separator plateBelow is a picture of the feed holes. These regulate the amount of fluid, or how fast it can apply the clutches. Drilling them bigger allows a faster apply. However, bigger is not always better.Remember that as the load increases on the trans, so does the pressure. The increase in pressure will cause the fluid to move faster and shifts to be firmer. If you drill the holes too big the shifts become too harsh at light throttle openings. This causes undue wear on the internal transmission parts, rear differential and axles, drive line and u-joints.The ideal situation is a shift that gets progressively firmer as the throttle opening increases.
Drilling the 2nd feed hole to 0.125' and the 3rd feed hole to 0.125'-0.140' works well for many applications.Also note above the 2-3 accumulator hole marked in the upper portion. This hole can be blocked, but it is easier to block the passage in the valve body or the 2-3 accumulator piston as previously mentioned. Shift kitsOften shift improver kits are installed at the time of the tranny rebuild. Various manufacturers make shift improver kits, TransGo has a good reputation. Racing valve bodyCoan reverse manual valve body. Notice the machining of passages.
Please note the DIY cannot just mill the valve body as valves have also been changed in the valve body. GovernorImage below of an OEM TH350 governor and cover. Note the direction of the plastic drive gear teeth. If the plastic gear breaks or wears out, there will be no upshifts since the governor is not rotating. Shift point changes Governor adjustmentShift point changes made via the governor weights and springs.
Stronger springs and heavier weights will bring earlier shift points. Lighter springs and weights have the opposite affect.The springs have a greater affect on shift points at light throttle. After about 5/8 throttle the weights have more affect on shift points. Gear ratio and tire size will also play apart of the shift points and changes made. Two types of modulators shown. Low vacuum design on left, ID by the black and red strips.
You will find the TH350 with 6', 9' and 12' tail housings.There is also two 4WD output shaft lengths, the shortest and more common is seen below. A longer 4x4 output shaft was used for around 1.5 years. It is a 2WD shaft of 6 inch length that is cut off about 1.250 inches. Used on 208 transfer case with adapter. Gear ratiosRatios of the TH350 were consistent throughout its production:.
Th350 Anti Clunk Spring Removal
First gear- 2.52:1. Second gear- 1.52:1. Third gear- 1.00:1. Reverse- 1.93:1 Aftermarket parts. An aftermarket low-gear planetary is available that will lower the first gear ratio to 2.75:1. Reference material.
$7.99 Buy It Now 4d 11h, 30-Day Returns,Seller: (64,247) 100%,Location: Sayville, New York,Ships to: US & many other countries,Item: 17 Don't let clunking damage your transmission!This 'small-type', 4 piece, steel case saver kit is used to replace the 'U'-shaped, stock center support anti-clunk-spring and will save your transmission case from irreparable damage.Perfect for High-Performance, Heavy-Duty, Racing and Regular-Duty transmissions.Transmission removal and dis-assembly is required to install this case saver kit.Please verify your transmission before you order. Fits the following transmissions:GM Turbo Hydramatic THM250/THM350/THM350CTH250 (TH-250) 1969-'79 TH250C (TH-250C) LOCK-UP 1980-'84TH350 (TH-350) 1969-1981TH350C (TH-350C) LOCK-UP 1980-'86TH700R4 (TH-700-R4)-4L60 1982-'9-'934L60E (TH-4L60-E/4L6OE ) 1993-UPTH4L65E (TH-4L65E)TH4L70 (TH-4L70)Automatic Transmission Case Save Sold in 4 Pack as Pictured One Pack of 4 will repair one transmission These case savers replace the u-shaped spring at the center support and allow reuse of the case. Will Fit All the Below Transmissions: TH350 350C 700R4 4L60 4L60E Condition: New, Interchange Part Number: TH350,TH700-R4,4L6OE,4L65E,4L70,4r,350C, Surface Finish: Carbon Steel, Manufacturer Part Number: K35354, 44761A, Other Part Number: TH250C,TH350C,4L60,THM,TH700R4, Brand: Tri Component Torque Converter-Transmission Parts See More.