The beauty of working with Chevys is the amazing level of parts interchangeability. Over the years, Chevy engineers have really made it easy for Bow Tie hot rodders to swap parts designed decades apart. That was until the dawn of the T56 six-speed. This transmission, first designed and built by Borg-Warner and now constructed by the Tremec Corporation, is simply the best manual transmission ever built for a Chevrolet. We shouldn't burst your bubble, but the reality is that the T56 was originally designed for the Viper and then later used in performance RWD V-8 Chevrolet cars.
Until now, the T56 swap into '60s and '70s musclecars required major effort to adapt the factory hydraulic-clutch system. Because of this hassle, everyone looked for a better way to mate the T56 with early stock mechanical clutch linkages. That is exactly what D&D Performance has done.
D&D purchased dozens of brand-new Viper T56 transmissions and converted them into direct-installation gearboxes for virtually any small- or big-block GM musclecar. D&D's owner Don Walsh modified this transmission to bolt right up to either a stock GM aluminum or an aftermarket steel bellhousing. All you have to do is relocate the rear crossmember and have a driveshaft built, and you're ready to go! Let's see how this works.
D&D performs several modifications to the stock Viper trans that creates this bolt-in application. The Viper uses a longer input shaft that D&D modifies by machining down the pilot pin and then welding it back up and grinding it to the 0.590-inch diameter that can be used with either a pilot bushing or bearing. The input shaft is a standard 11/8-inch x 26-spline input that is common for Super T10 and late-model Muncie four-speeds. Next, D&D uses a coated steel plate as an adapter between the trans and a standard small- or big-block Chevy bellhousing. The plate is carefully machined and accurately located with 1/4-inch dowel pins to the transmission.
Normally, adding this plate would create all kinds of difficulties aligning the input shaft of the trans to the bellhousing. Walsh eliminates this problem with the dowel pins and by bolting the transmission's bearing-retainer collar (which is what the throwout bearing rides on) to the front of the adapter plate. Since the plate is pinned to the trans and the bearing retainer collar is bolted to the plate, transmission alignment issues are eliminated. Of course, this still means you really should align the bellhousing to the centerline of the crank to ensure ideal trans operation, but this is not required to bolt the T56 in place. The plate is securely fastened to the T56 with eight metric fasteners.
To mount the transmission, we'll assume you have a 11/8-inch x 26-spline clutch disc in the bellhousing. If not, you'll need to swap your coarse-spline clutch for a fine-spline piece. Bolt the adapter plate to the bellhousing using D&D's four Allen-headed, countersunk 7/16-inch bolts. With these four bolts tight, you can now install the T56 trans to the bellhousing. The transmission weighs around 140 pounds dry, so unless you're Mr. Buff, you should seriously consider using a trans jack to help bolt the trans in place. We were installing this T56 in a '65 Chevelle with help from Tim Moore at Moore Automotive. The effort also required some floorpan hammer clearance, and we had to trim two nonessential ears off the front of the trans case with a Sawzall. Now D&D trims these ears off the transmission before they ship it to you to save you the trouble.
The D&D T56 Viper trans mounts the shifter about 2 inches farther forward compared to an LT1-style Camaro T56 box, but the large shifter housing still required some sheetmetal surgery to clear even the large hole we'd cut for the Super T10 shifter location in our Chevelle. We had to trim about 2 inches farther back to clear the floorpan. You may have to trim more depending upon the condition of your floorpan. The shifter comes up through the floor in a very convenient place, although this trans won't work with a bench seat--you'll have to have buckets.
Back underneath the car, we moved the stock crossmember back about 7 inches and drilled new mounting holes in the frame. The D&D T56 employs a custom extension housing that allows you to use a stock Chevy trans mount as well as a mechanical speedometer. If you are bolting a T56 in an early Camaro, this may require cutting and reshaping the crossmember to bolt to the subframe and to the trans because the subframe does not extend far enough to the rear of the car. Another alternative might be a custom-built tube crossmember. Universal kits are available from Art Morrison Enterprises.
The D&D T56 trans is a full 5 inches longer than an early Muncie four-speed, which means you'll need to either shorten your existing driveshaft or build a new one. Since we're constantly swapping transmissions in this Chevelle, we decided to have Denny's Driveshafts build a new driveshaft. We carefully measured the distance between the end of the transmission and the front part of the yoke on our 12-bolt, and the distance that the output shaft extends from the end of the extension housing. With these two measurements, Denny was able to construct a 3-inch-diameter, Nitrous-Ready steel driveshaft outfitted with 1350-style U-joints that are much larger and more durable than the stock 1310-style joints that were in the car.
Changing to the 1350 U-joints also required replacing the stock 1310-style yoke on the rearend with one of Denny's steel 1350-series yokes and a new pinion seal. We performed this swap at Moore Automotive being careful to torque the pinion nut so that the pinion bearing preload did not change. Finally, we filled the new T56 trans with Royal Purple synthetic ATF-style gear lube and connected our mechanical speedometer. The D&D T56 comes with a seven-tooth gear on the output shaft, and the custom extension housing will accept the standard four-speed-style speedometer gear adapter, which bolts right into the case. With the installation complete, we fired the Chevelle up and let the clutch out on the 2.66:1 First gear. Not only is this a world-class transmission with excellent shift feel and movement, but D&D blueprinted this trans so it shifts almost as if you were one with the transmission. After a couple of First through Fourth gear blasts, we took the Chevelle on the freeway to try out both the Fifth gear 0.74 and Sixth gear 0.50 overdrive ratios. Coupled with our 3.55 rear gear, Fifth gear ends up the equivalent of 2.62:1 while Sixth is a Bonneville-bound 1.77:1. In Sixth gear, that brought our cruise rpm from 3,200 at 70 mph to a much more sedate 1,600 at the same speed, while in Fifth gear the rpm is 2,400. This turned our rpm-spinning '65 Chevelle into a much more sedate highway cruiser. With our 500hp 420ci small-block's mondo torque and excellent throttle response, this is the perfect transmission to complement the rest of the drivetrain.
The new D&D T56 transmission runs $2,499 ready to bolt in with the adapter plate and a stock Mustang-style shifter. While this sounds like a lot of money, consider that converting a used LT1 trans--including a hydraulic clutch and a mechanical speedometer drive--would run around $2,000 to $2,300. So for only a couple hundred more, you can have a brand-new, stronger, ready-to-bolt-in T56. Unless you like spending more money and working harder, the right choice should be obvious.