TH350 Automatic Transmission - Rebuild, Buildup

1. Pump, Intermediate clutches, Intermediate overrun brake band, Direct clutch drum, Forward clutch drum, Front ring gear, Front planetary, Sun gear/drive shell assembly, Low overrun clutch support assembly, Low/reverse clutches, Rear planetary/rear ring gear/output shaft, Low/reverse apply piston.

1. Pump, Intermediate clutches, Intermediate overrun brake band, Direct clutch drum, Forwa

In the past few issues of Car Craft, we've been transforming our project Cheap Street Chevelle from junker to jewel. It started out as a non-running heap, but since then we have upgraded various aspects of the powertrain to obtain improved quarter-mile performance as well as better looks. In reality, as we hopped up one aspect of the car, another (weaker) part of the car broke--which is pretty typical of how street machining goes in the real world. For example, we added a higher-lift camshaft, but then fell victim to valve float, because the stock valvesprings were worn out.

Such is the case for the stock TH350 transmission in Cheap Street Chevelle. We rebuilt the engine, added a 2800rpm torque converter and ran sticky rear tires, which generated quicker e.t.'s at the dragstrip but overtaxed the stock transmission. Shortly after our dragstrip fun, the tranny began to slip while driving the Chevelle around town.

Keeping with the theme of Cheap Street Chevelle, we decided to rebuild the stock tranny ourselves with good parts rather than replace it with a high-dollar, full-race unit. We ordered a B&M TransKit (PN 30229) that had all necessary rebuild goodies; plus, the parts are much beefier than stock, which will be perfect for our street/strip Chevelle. In this feature article we don't cover every aspect of the rebuild, but we do give you a majority of the tech info needed to do the job. To further help you in your rebuilding efforts, a complete, detailed, easy-to-follow 24-page instruction manual is included with the B&M TransKit.

2. Contained in the B&M TransKit (PN 30229) is everything needed to rebuild a GM TH350 automatic transmission. The kit consists of items such as: forward, intermediate, direct and low-reverse clutches; forward, intermediate, direct and low-reverse steel clutch plates; an intermediate brake band; a 2-3 shift valve spring; a pressure regulator spring; a modulator plug and plug spring; a middle support and oil transfer plate; a manual low control valve sleeve; separator plates and gaskets; a transmission filter and gasket; an oil pan drain plug kit and an oil pan gasket.
2. Contained in the B&M TransKit (PN 30229) is everything needed to rebuild a GM TH350 aut
3.In order to rebuild the transmission properly, you'll need to remove it from the vehicle. Carefully disassemble the transmission and thoroughly clean it inside and out--using a "hot tank" usually delivers great results. For this rebuild story we don't go through disassembly stages; rather, we show you how to rebuild the tranny once it is gutted and cleaned.
3.In order to rebuild the transmission properly, you'll need to remove it from the vehicle
4. The shift lever/detent (park rod) must be removed to gain access to the shift lever seal. Removing the old seal is easy, as is installing the new seal.
4. The shift lever/detent (park rod) must be removed to gain access to the shift lever sea

5. Install the output shaft assembly in the transmission case. A snap ring holds the rear ring gear in place. Then, slide the rear planetary on the output shaft and down into the ring gear.
5. Install the output shaft assembly in the transmission case. A snap ring holds the rear
6 A&B. Because the B&M TransKit comes with one extra clutch for the direct drum, the surface of the direct piston must be machined down 0.140 inch. The extra clutch increases the torque capacity of the transmission. The machining only takes a few minutes on a lathe, so it should be inexpensive for a machine shop to perform the work.
6 A&B. Because the B&M TransKit comes with one extra clutch for the direct drum, the surfa
6B.

8. Stack and install the low/reverse clutches and steels, as shown, with the small detents together (arrow). Be sure to soak the clutches in clean automatic transmission fluid for 15 minutes before assembly. Install the clutches wet, one at a time--do not dry them off before installation. The steels that go between each clutch have special notches that align with the trans case, while the clutch notches align with the planetary.
8. Stack and install the low/reverse clutches and steels, as shown, with the small detents
9. Install the piston spring retainer in the direct drum using (in unison) the intermediate race, a compressor tool and a pair of snap-ring pliers. Rather than buying an arbor press to compress, we used Vice-Grip pliers as our low-buck compressor tool.
9. Install the piston spring retainer in the direct drum using (in unison) the intermediat
10. Carefully set the low overrun clutch assembly into the transmission case. The assembly only goes in one way--the outside notches align with the trans case and the inside notches align with the planetary.
10. Carefully set the low overrun clutch assembly into the transmission case. The assembly

11. Position one end of the low overrun clutch assembly snap-ring retainer with its end against the anti-clunk spring (arrow), then begin to push the rest of the snap ring down, radiating outward until you get to the other end. If pushed down properly, the ring should snap into place and remain locked in position.
11. Position one end of the low overrun clutch assembly snap-ring retainer with its end ag
12. Set the inner race washer on the low overrun clutch assembly (arrow A), then install the sun shell. As you install the sun shell, slightly rotate it so the teeth (arrow B) align. When the teeth align, the sun shell will drop into place.
12. Set the inner race washer on the low overrun clutch assembly (arrow A), then install t
13. Install the front planetary into the sun shell. To get the teeth to align, slowly turn the sun shell until the planetary drops into place.
13. Install the front planetary into the sun shell. To get the teeth to align, slowly turn

14. Set the front ring gear onto the front planetary. Slowly turning the planetary will align the teeth, allowing it to drop into place.
14. Set the front ring gear onto the front planetary. Slowly turning the planetary will al
15. Be sure to check the pump body for wear. Any large scratches/gouges will cause the pump to make noise, and the pump pressure will be inconsistent. With pump pressure too low, the unit will heat up and eventually fail. If gouged, you must buy a new pump, as the scratches will be difficult to fix with reliable results.
15. Be sure to check the pump body for wear. Any large scratches/gouges will cause the pum
16. Continue to assemble the pump by installing the intermediate spring retainer. The bolts only align one way, so you'll need to rotate the retainer until they are in the correct position. Only install the bolts finger-tight at this point.
16. Continue to assemble the pump by installing the intermediate spring retainer. The bolt

17. Before tightening the intermediate spring retainer, you must align the two pump halves. An easy, low-buck way to do this is to flip the pump over and drop it into the tranny case "backwards." This will align the different sections of the pump assembly. Once aligned, tighten the bolts to 10-18 lb-ft.
17. Before tightening the intermediate spring retainer, you must align the two pump halves
18. Install the forward clutches and steels (five each), along with the cushion spring, into the forward clutch assembly. Insert the cushion first, then a steel, then a clutch, then a steel, and so on. Be sure to soak the clutches in tranny fluid for at least 15 minutes before installing them. If installed correctly, there should be 0.050 inch of clearance between the pressure plate (held in place by a snap ring) and the last clutch.
18. Install the forward clutches and steels (five each), along with the cushion spring, in
19. Install the clutches and steels in the direct drum. Stock TH350 transmissions have four clutches and steels, but the B&M TransKit (and complete B&M transmissions) come with five clutches/steels to increase the TH350's torque capacity. You should have 0.050 inch of clearance between the last clutch and the pressure plate.
19. Install the clutches and steels in the direct drum. Stock TH350 transmissions have fou

20. Install the new thrust bearing (arrow), and slip the direct drum down onto the forward drum. Slightly twisting the direct drum will align the teeth, allowing it to drop into place.
20. Install the new thrust bearing (arrow), and slip the direct drum down onto the forward
21.Carefully insert the forward and direct drum assembly into the case. The assembly will engage the forward clutch hub and the sun shell.
21.Carefully insert the forward and direct drum assembly into the case. The assembly will
22.Install the intermediate overrun brake band and then install the intermediate clutch pressure plate. Afterward, stack the clutches and steels into the tranny case on top of each other, starting with a clutch. Once the clutches/steels are in, set the intermediate clutch cushion on top of the combo.
22.Install the intermediate overrun brake band and then install the intermediate clutch pr

23. Thread in two dowels (arrow) that are used to align the pump. You can make these dowels the low-buck way by sawing the heads off a couple of bolts and then threading them in by hand. Slip the pump assembly down over the input shaft and onto the alignment dowels until the case and pump O-ring contact each other. Then, carefully push the pump assembly down by hand. The pump should go into place without hammering on it. If you have to force it in, either something is misaligned or clearances are incorrect.
23. Thread in two dowels (arrow) that are used to align the pump. You can make these dowel
24.Part of the B&M TransKit is a shift improvement kit. Start the install by carefully setting the No. 1 check ball in position. To hold the check ball in its location (so it doesn't roll around or fall out), lightly coat the ball with a dab of grease.
24.Part of the B&M TransKit is a shift improvement kit. Start the install by carefully set
25. Insert two alignment dowels (two bolts with the heads cut off) into the tranny, then install the separator plate and gasket.
25. Insert two alignment dowels (two bolts with the heads cut off) into the tranny, then i

26. Align the range selector inner lever, then install the middle support plate, followed by the oil transfer plate and, finally, the stock stiffener plate. Thread in the seven bolts finger-tight--the bolts will be torqued to 100 lb-in later on.
26. Align the range selector inner lever, then install the middle support plate, followed
27. Normally, stock TH350s have a spring in the manual low control valve that automatically changes gears at a predetermined rpm. However, the B&M TransKit replaces the spring with a metal sleeve that blocks the valve, allowing the transmission to remain in First gear no matter how high the engine rpm goes. This is beneficial for racers who want to manually shift (at a specific rpm) their cars at the dragstrip for maximum power and lowest e.t.'s. But if you put the trans in Drive, the unit will shift normally for around-town driving.
27. Normally, stock TH350s have a spring in the manual low control valve that automaticall
28. Set the valvebody in place, making sure to hold your fingers over the valve retaining pins so they don't fall out. The arrow points to the pins that are on the underside of the valvebody (you can't see them in the photo).
28. Set the valvebody in place, making sure to hold your fingers over the valve retaining

29.Install the new B&M transmission filter, then bolt on the stock oil pan. The B&M TransKit comes with a removable self-tapping drain bolt. You don't have to install the drain bolt, but it sure makes it easier to drain or change fluid when tranny service is needed.
29.Install the new B&M transmission filter, then bolt on the stock oil pan. The B&M TransK
30. Re-install the stock speedometer gear and retainer. If your old gear is worn and/or chewed up, now is the perfect time to replace it. We found that using an old piece of pipe usually works as a great low-buck install tool. Then, install the rear tailshaft.
30. Re-install the stock speedometer gear and retainer. If your old gear is worn and/or ch
31. Re-install the stock speedometer gear and retainer. If your old gear is worn and/or chewed up, now is the perfect time to replace it. We found that using an old piece of pipe usually works as a great low-buck install tool. Then, install the rear tailshaft.
31. Re-install the stock speedometer gear and retainer. If your old gear is worn and/or ch

Hassle-Free TH350 Performance
Most car crafters like to build up their street machines themselves, but if tranny work isn't your specialty, you may wish to buy a completely rebuilt automatic transmission. B&M offers a wide variety of transmissions that are fully rebuilt and ready for street/strip use. In addition, each B&M tranny goes on a special tranny dyno that checks line pressure, shift points and vacuum needed to trigger a shift, as well as providing an opportunity to inspect the transmission for leaks or problems. When you get a B&M transmission mailed to your doorstep, you're getting a unit that is fully tested and works as it should.
Hassle-Free TH350 Performance Most car crafters like to build up their street machines the