TH400 Transmission Build - Turbo 400 Revival - 4 Wheel & Off-Road Magazine

Automatic transmissions are steadily gaining popularity in off-road vehicles. There are the few hard-core old guys (including our trusty leader) that won't do it with anything other than a stick shift, but many feel that an auto tranny is far and away the way to go when off road.

Before we started the rebuild we dropped the transmission off with David Dibs of Orange County Transmissions to get gutted and cleaned. Our TH400 received the entire B&M package. A rebuild kit, a deep-sump aluminum pan, a B&M torque converter, a flexplate for a Chevy big-block, and Trick Shift tranny fluid to make this tranny hold up for not only big horsepower, but also endurance.

Before we started the rebuild we dropped the transmission off with David Dibs of Orange Co

Auto transmissions absorb more vibration and transmit less shock load onto other drivetrain parts than a manual transmission does. The fluid coupling of the torque converter behind the engine provides quite a cushion against shocks that could break axleshafts, output shafts, or U-joints when things get caught or bound up.

Off road, an automatic transmission frees up a hand that would otherwise be dedicated to a shifter, plus it frees up any thought put into shifting, allowing you to fully concentrate on the obstacle ahead. And when you are climbing a hill, an auto tranny deletes any pedal dance that you'd have to do between a clutch pedal and brake pedal as you ascend.

But auto trannies aren't perfect-in fact, far from it. Many are considered weak by off-road standards and unable to stand up to the rigors. The more popular heavy-duty trannies are old three-speeds, since the four- and five-speed overdrive transmissions of today can have frustrating computer controls. A few of the most bombproof: Ford's C6, Chevy's TH400, and Chrysler's 727 are all considered the all-time end-all auto trannies for swapability and strength.

We recently had Orange County Transmissions buff up a Turbo-Hydramatic 400 transmission with a B&M rebuild kit, and deep-sump pan for our Ultimate Adventure K5 Blazer. Dave Dibs of O.C. Tranny used all the integral B&M components, plus added a couple tricks of his own.

And the rest of the Blazer? Check out the first part of the frantic build that Fabworx accomplished in less than a month and a half starting in this issue.

The restrictor orifice is completely drilled out to allow maximum oil flow to the rear servo. We were more concerned about getting enough oil to the servo, not the harshness of the shift. The rear servo will go where the red grease is on the housing, and should be bolted on and checked to see if it will be depressed too much with the rear band. Since we had a brand-new band, it was pressing the rear servo pin and applying the band when the servo was bolted in. The pin was slightly ground down on a grinder.
The restrictor orifice is completely drilled out to allow maximum oil flow to the rear ser
One other thing we needed to get before the build was the correct output shaft for our TH400. There are different output shafts depending on what transfer case the tranny was originally equipped with. We had a 32-spline NP205 output shaft on our tranny, but since we were running an Off Road Design NP203/NP205 doubler kit, we had to purchase the shorter 32-spline NP203 output shaft.
One other thing we needed to get before the build was the correct output shaft for our TH4
B&M supplies a new heavy-duty Kevlar band that will take more heat buildup without failing, but at the same time will also wear more quickly than a conventional band. Don't worry, you'll still get 50K-100K miles out of a B&M Kevlar band.
B&M supplies a new heavy-duty Kevlar band that will take more heat buildup without failing

One upgrade Dibs made was to replace the old roller clutch with a late-model sprag and a drum from a 4L80E tranny. He did so because of the considerable gain in strength.
One upgrade Dibs made was to replace the old roller clutch with a late-model sprag and a d
Look for yourself: The late-model sprag has 34 rollers or "dogs" that make contact, while the early model roller clutch only has eight. Besides that, the late-model intermediate sprag has a much wider race than the old roller clutch design.
Look for yourself: The late-model sprag has 34 rollers or "dogs" that make contact, while
Once the rear gearset is set up with new radial Torrington bearings and dropped into the tranny housing, Dibs used his own custom tool to slide the gearset into place and check for play after the endplates were installed.
Once the rear gearset is set up with new radial Torrington bearings and dropped into the t

After the intermediate clutches are dropped in, a heavy-duty snap ring (right) is used instead of the stock one (left) to ensure the little lugs won't break off the clutches.
After the intermediate clutches are dropped in, a heavy-duty snap ring (right) is used ins
A very important modification is to put a spiral snap ring in place of the C snap ring on the intermediate sprag on the direct drum.
A very important modification is to put a spiral snap ring in place of the C snap ring on
Dibs also recommended adding a new reprogramming spring in the direct drum before reinstalling it. This will help the tranny not bang into reverse as hard as some conversions.
Dibs also recommended adding a new reprogramming spring in the direct drum before reinstal

The input drum, direct drum, and intermediate clutches were installed with the pump left to be placed. The pump cover was machined flat so there would be no leaking problems. After assembling the pump, Dibs dropped the pump into the case and checked the tolerances.
The input drum, direct drum, and intermediate clutches were installed with the pump left t
After the center support was torqued to spec, the valvebody and fluid filter were added. The Third gear checkball was left out of the valvebody to let more oil get to the Third gear clutches. Since we had a factory deep pan, we had a longer pickup tube than a regular pan. The B&M pan requires an extension that goes onto the factory short pickup tube for a standard pan. It's nothing that $10 couldn't fix.
After the center support was torqued to spec, the valvebody and fluid filter were added. T
Once it was done, we thanked Dibs, threw the newly rebuilt B&M/O.C. Tranny TH400 in the back of a truck, and raced for Santa Rosa, California, where the K5 and its engine were being assembled. At Fabworx, they bolted on the new B&M flexplate to mate the tranny to our 462ci big-block, checked that the B&M deep-sump pan was on tight, and added the B&M Trick Shift fluid.
Once it was done, we thanked Dibs, threw the newly rebuilt B&M/O.C. Tranny TH400 in the ba

SOURCES
B&M Racing 9142 Independence Ave. Chatsworth CA 91311 8-18/-882-6422 www.bmracing.com	Orange County Transmissions www.octrans.net
Fabworx www.fabworxinc.com