Recalibrates the GM TH400 valve body so shifts apply fast and firm instead of slipping, with Stage 1 and Stage 2 settings in one box.
This is the B&M 20260 Shift Improver Kit for the GM Turbo 400, built for the 1965 through 1987 TH400 (3L80), the TH475 (3L80HD) and the TH375C. Owners usually land here because a truck, motorhome or hot rod with a 400 behind it has started to shift lazily: a long, soft 1-2, a 2-3 that flares when it is pulling a trailer, or a unit that just feels tired compared to how it should. The 20260 recalibrates the valve body hydraulics so each apply happens faster and firmer, and it goes in with the transmission still in the vehicle.
The Turbo 400 shifts entirely on oil pressure. Governor pressure rises with road speed, the vacuum modulator sets line pressure from engine load, and the shift valves move when one beats the other. How long each shift takes is set by the feed holes in the separator plate, the check balls that steer oil to each circuit, and the 2-3 accumulator that cushions the direct clutch. The 20260 replaces the separator plate with a B&M plate that you drill to size, and it changes the check ball layout and accumulator depending on the calibration you choose. B&M's instructions give two levels in the one box: Heavy Duty, for towing, campers, motorhomes and four-wheel drives, which produces firm, noticeable shifts, and Street, for dual-purpose and street/strip cars, which produces very firm, positive shifts. B&M also notes the kit lets you manually downshift into low at any speed and hold low as long as you want.
The TH400 has no computer and stores no trouble codes, so these show up on a road test, not on a scan tool.
B&M is blunt about this: the kit is not a cure-all, and on a transmission that is already slipping or in poor condition it can make things worse. Check the fluid level and condition first, look for ATF in the modulator vacuum line, confirm the vacuum hose is connected, and put a pressure gauge on the line port if the shift is soft everywhere. Our TH400 valve body testing guide walks through separating a valve body problem from a worn clutch pack, and the TH400 common problems guide covers the modulator, governor and pump failures that cause the same complaints.
Per B&M, the 20260 fits 1965-1987 GM TH400/3L80, TH475/3L80HD and TH375C. It does not fit the TH375B, and it is not for the electronic 4L60E or 4L80E. The important catch is the case: GM revised the TH400 case just after the start of the 1988 model year, and B&M says not to use this kit in any transmission built on or after November 23, 1987 (Julian date 327 on the ID tag). The later case has a check ball where this kit expects none, and B&M points those units to its 20261 kit instead. Because so many 400s have been swapped between vehicles, go by the transmission's ID tag, not the vehicle year; if the tag is gone, B&M says the pan and valve body have to come off to see which case you have. Send your year and VIN or a photo of the tag and we will help you check. More parts for this unit are on our TH400 parts page.
Plan on a few hours with hand tools, a drill motor and a vise. Drain the pan, drop the filter, then remove the valve body bolts (three 1/4-20 and eight 5/16-18, ten on some early cases) and lower the valve body while pulling the governor tubes straight down. Keep the six 1/4-inch steel check balls from the old separator plate; you reuse them. For Heavy Duty you drill the marked orifice in the B&M plate with the supplied 5/32-inch bit and run all six check balls. For Street you use the 3/16-inch bit, run only check balls 4, 5 and 6, and remove the 2-3 accumulator spring. Inspect the detent solenoid while it is off: an oval-can unit needs its orifice disc in place and the new metal gasket, while a square-can unit is sealed by its rubber seal and must not use that gasket. A leaking or damaged detent solenoid leaves the unit in passing gear, so a TH400 detent solenoid kit is worth having on hand. Torque the valve body, solenoid and pan bolts to 8-10 ft-lb and the filter shoulder bolt to 10 ft-lb, refill with about 4 quarts of Dexron-type ATF, and check the level hot in neutral. If the pan is coming off anyway, a TH400 pan with drain plug makes the next fluid change cleaner.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
No. B&M says not to use the 20260 in any TH400 built on or after November 23, 1987 (Julian date 327 on the ID tag), because the revised case has a check ball where this kit does not expect one. Those units take B&M's 20261 kit. A 1987-or-earlier unit swapped into a newer vehicle still takes the 20260, so check the tag, not the vehicle year.
Heavy Duty is for towing, campers, motorhomes and four-wheel drives and gives firm, noticeable shifts; you drill the plate with the 5/32-inch bit and keep all six check balls. Street is for street/strip and dual-purpose cars and gives very firm shifts; you use the 3/16-inch bit, run only check balls 4, 5 and 6, and remove the 2-3 accumulator spring.
No. The kit installs from underneath with the pan, filter and valve body off. B&M lists a 3/8-drive ratchet, 1/2 and 7/16 sockets, small snap ring pliers, a small torque wrench, a 1/4-inch drill motor and a vise.
Usually not. B&M warns that on a transmission that is slipping or in poor general condition the kit may make the problem worse. Burnt clutches, a worn pump or a leaking modulator need to be fixed first; the kit is meant for a healthy unit.
Refill with about 4 quarts of Dexron-type ATF and check the level hot in neutral, between Add and Full. Do not overfill; B&M notes that causes foaming and overheating. B&M also recommends a transmission cooler for any towing or performance use.
Inspect the detent solenoid seal or orifice disc, since a leak there leaves the unit stuck in passing gear. B&M also recommends replacing a filter with more than 20,000 miles on it, and the pan gasket comes in the kit.
