TH400 Shift Kit 1965

This is the TransGo SK 400, the original Shift Kit for the GM Turbo Hydra-matic 400. It is a valve body repair kit, not a race kit: it is meant to give a stock-feeling 400 short, smooth, on-time shifts again. If your truck, motorhome or classic car shifts late and soft, bumps or chatters on the 1-2, burns direct clutches, or drops into passing gear with the lightest touch of the pedal, the SK 400 is aimed at exactly those complaints. TransGo lists it for every 400 from 1965 on.

What the SK 400 changes inside a Turbo 400

TransGo's instructions explain the logic well. A 1-2 bump or chatter is not a clutch applying too hard; it happens when the intermediate clutch is still not fully on when the 1-2 accumulator finishes stroking. A soft 2-3 is usually oil leaking away between the valve body and the direct clutch. The kit attacks both with a new separator plate (some holes bigger, some smaller) working with a new pressure regulator spring and modulator valve spring. TransGo says that combination can send up to twice as much oil to the direct clutch, and that it also releases the direct clutch faster on a passing-gear downshift so it does not drag while the 2-3 accumulator unstrokes. Optional governor springs move shifts earlier, and the kit also includes a 2-3 shift TV valve spring and an optional 3-2 control spring.

Symptoms the SK 400 is built to fix

• Late, long upshifts that hang in first or second
• Bump, chatter or slide on the 1-2 shift
• Soft 2-3 shift and repeat direct (third) clutch burn-up
• Over-sensitive passing gear that kicks down on light throttle
• Slow-acting passing gear that drags the direct clutch

Worn frictions, a leaking modulator and pump wear cause similar symptoms on a 400, so do not expect this kit to revive a transmission that already slips in every gear.

How to confirm a shift kit is the right fix

Check fluid level and smell first. Pull the vacuum line at the modulator; ATF inside the line means a ruptured diaphragm, which makes shifts late and harsh on its own. Put a pressure gauge on the line tap and compare to the spec in the service manual. If pressure and fluid are good and the shifts are simply soft and late, the valve body calibration is the target. Our TH400 harsh shift guide covers the modulator checks, and the delayed engagement guide helps if the problem is slow engagement into Drive or Reverse instead.

Fitment

TransGo lists the SK 400 for all GM 400 transmissions from 1965 on, which includes the Buick, Cadillac, Chevrolet, GMC, Oldsmobile, Pontiac and Jeep applications, plus Jaguar, Rolls-Royce and Bentley cars that used the 400. Our listing says 1965 because that is where coverage starts, not where it ends. A 1964 TH400 uses a different valve body and takes TransGo's SK 425, so this kit is not for that year. It is also not for the electronic 4L80E. TransGo instructions call for six check balls on all models, even 1987-up cases that came with seven. If you are not sure what you have, send us the year and VIN. Browse the rest of the range on our TH400 parts page.

Install notes

• The work is all at the valve body and can be done with the transmission in the vehicle. Read every page of the instructions before you start.
• Remove and discard any horseshoe shims at the pressure regulator and install the yellow spring supplied. With the transmission in the vehicle the spring goes in through the window in the case.
• Install the pink modulator valve spring. If you want later, shorter part-throttle shifts after the road test, TransGo says to remove it.
• Start the alignment bolts first so the valve body, plate, gaskets and case line up.
• TransGo warns that plastic accumulator pistons break and recommends aluminum.
• Use a new filter and pan gasket and refill with Dexron-type ATF.
• Replace the modulator if it is original, and repair the detent circuit with our detent solenoid kit if kickdown is dead.
• If the clutches are already worn, combine the kit with a TH400 overhaul kit.

Want firm, high-performance shifts and full gear control instead? TransGo states the SK 400 is not for that; their reprogramming kits, such as the TransGo 400-1&2, are.

Will this fit my 1972 TH400?

Yes. TransGo lists the SK 400 for every 400 from 1965 on. The 1965 in our title is where coverage starts. Only the 1964 TH400 needs a different kit (SK 425).

Will the shifts be harsh?

No. TransGo designs the SK 400 for short, smooth shifts, not hard performance shifts. If you want firm shifts and gear control, a TransGo reprogramming kit is the better choice.

My case has seven check balls. How many do I use?

TransGo's instructions say to install six 1/4 inch check balls on all models, including 1987-up cases that originally used seven. Follow the placement diagram in the kit.

Can I make the shifts happen earlier?

Yes. The kit includes optional green governor springs for earlier shifts. If the part-throttle shifts end up too late after the road test, TransGo says to remove the pink modulator valve spring.

Do I need to remove the transmission?

No. The valve body work is done through the pan, and the pressure regulator spring can go in through the window in the case.

Does it fit a 4L80E?

No. The 4L80E is electronically controlled with a different valve body. The SK 400 is for the hydraulic TH400 only.

• TH400 Harsh Shift: Causes and How to Fix It
• TH400 Delayed Engagement: Causes and Fix
• TH400 Valve Body Problems: Symptoms, How to Test, and Replacement
• TH400 Common Problems and Fixes