Transmission Drivetrain Breakage

Written by on January 1, 2009 Don't Let Your Sprag Sag
Transmission Drivetrain Breakage View All 10 Photos
Mopp 0901 01 Z+transmission Drivetrain Breakage+front View 1/10 Chrysler TorqueFlite transmissions are some of the toughest automatics ever built. This month we'll show you how to make yours even tougher by installing a bolt-in overrunning clutch, or sprag.

While the Chrysler TorqueFlite transmission is one of the strongest automatics your car could be equipped with, able to survive hard-core racing, neutral drops, and most of the other abuse we can dish at them, the TorqueFlite still has a weak link. Like most automatic transmissions, the TorqueFlite, whether 904 or 727, has an overrunning clutch assembly that's designed to prevent extreme transmission damage if the transmission is allowed to free-wheel due to a broken U-joint, axle, or driveshaft. The overrunning clutch assembly is commonly referred to as a "sprag," and the clutch housing is pressed into the rear section of the transmission case. This pressed-fit assembly of the sprag can create a problem that will destroy the transmission case in a high-powered or abusive application. Fortunately, there is a cheap fix for this weak link, and it comes from TCI transmissions.

There are many ways to cause an overrunning clutch failure, but drivetrain breakage is the most common. When a drive line component behind the transmission fails under power, the engine revs freely and so does the transmission. The overrunning clutch is designed with spring loaded rollers which help slow and stabilize the transmission's rotating assembly. Since the energy of the transmission and engine are absorbed in the overrunning clutch, and then transferred to the housing, or sprag, which is pressed into the aluminum case, the sprag will actually try to spin itself out of the transmission case, losing its pressed fit and causing irreparable damage.

Prior to the availability of a bolt-in sprag from the aftermarket, many racers, this author included, would drill and tap the factory sprag, then insert longer governor housing bolts to hold it in place. Since the sprag is made from tool steel, this process involved drilling with carbide bits, and usually more than one broken tap. Thankfully, the folks at TCI have made this modification much easier by providing an overrunning clutch kit that includes a pre-drilled and tapped clutch housing, new springs, and new rollers. For the cost, this is one of the best modifications you can do to your TorqueFlite, but since it does involve tearing the transmission completely apart, we recommend performing the upgrade during rebuild like we did. If you're having a shop rebuild your transmission for you, it shouldn't cost too much extra to have them install the bolt-in sprag for you. While we hope you never need this upgrade, if you ever break a U-joint, axle, or driveshaft, you'll be happy you installed a bolt-in sprag.


TCI Transmissions
Ashland, MS 38603
Inline Performance Specialist
Webster, FL

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