Classic Chevy 5 Speed Transmission Insight - CHP Insider

Jeff Mortenson Of Classic Motorsports Group Tells All There Is To Know About Swapping An Overdrive Stick Into Your Classic Chevy

Stephen Kim Aug 1, 2009 0 Comment(s)
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"Developing a new five- or six-speed swap kit isn't particularly complicated, but it's a time-consuming process that requires lots of patience. First, we test-fit parts that we know will fit in all models of a vehicle platform, such as the bellhousing, clutch, and flywheel. The next step is mounting the trans at the correct height to achieve a proper driveline angle. If our desired driveline angle creates interference between the trans and the floor, we modify the floor for clearance. After the trans is bolted in, we can determine if a stock or an aftermarket Hurst Blackjack shifter is necessary to position the shifter in the stock location. After that, a prototype crossmember is fabricated with the goal of duplicating the correct driveline angle we achieved earlier in the R&D process while still providing enough room for the exhaust.

"As for the hydraulics, most of it is a bolt-in affair. The most difficult part is figuring out where to mount the master cylinder. We make our own custom brackets for Chevelles and Camaros in order to position it properly on the firewall where it's out of the way of other components.

"With the exception of our Corvette kits, no mods are performed to the exterior of any of the transmissions that we sell. That way, if you want to pull your trans out and sell it later on down the road, it will fit in any car because it hasn't been modified."

In the Works
Due to the tall 0.50:1 Sixth gear ratio and limited shifter locations of the standard T56, CMG only offers it in '67-74 Camaros and '68-74 Novas. However, CMG will soon be offering six-speed swaps for every Chevy musclecar where a TKO swap was once the only option. "The main drawback of the standard T56 is that it took a lot of work to relocate the shifter to the locations necessary in most Chevy musclecars. Plus, most of them had a lower torque capacity than the TKO," Mortenson explains. "With the new T56 Magnum, all that's changed since it accommodates several shifter locations, and it is rated at 700 lb-ft. In conjunction with the Hurst Blackjack shifter, we'll now be able to offset the shifter to place it in the stock location. We are working hard to have our T56 Magnum kits readily available within the next few months."

Sticky Pedals
Owners of many late-model GMs with factory six-speeds often complain of hydraulic issues, such as sticky clutch pedals and improper clutch disengagement. Since this is an undesirable characteristic in any application, CMG made sure to eradicate the problem in its six-speed kits. "The problem isn't necessarily a product of the T56 itself, but actually the result of GM's hydraulic system," says Mortenson. "In order to smooth out clutch engagement, GM inserted a restrictor valve into the fluid supply line. Many racers have found that drilling out this valve reduces the tendency for the clutch pedal to stick on the floor. Our solution is to use a Tilton master cylinder, which prevents this issue from surfacing in the first place."

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Clutch Actuation
"Although Tremec transmissions work with a stock mechanical linkage, CMG has a complete bolt-in hydraulic clutch setup that will convert your car to a modern hydraulic system. In the past, mechanical linkages were preferred in high-horsepower applications due to the fact that they could handle greater pressure plate clamping loads. However, this has changed with modern advances in clutch technology. For example, CMG offers an optional LUK 11.5-inch clutch rated at 600 hp that's compatible with our hydraulic system. Our hydraulic setup includes a throwout bearing, a master cylinder, a remote mount reservoir, and all fluid lines and hardware.

"The most important part of the conversion is the master cylinder firewall mount bracket. The stock firewall is too thin to properly mount a clutch master cylinder, and our bracket is designed to provide a solid foundation for it. Additionally, the bracket positions the master cylinder pushrod at the correct geometric angle for proper pedal feel."


Classic Motorsports Group


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