Why our Mustang now has an aluminum driveshaft

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Update by Tim Suddard to the Ford Mustang Fastback project car
Mar 24, 2023 | Ford, Vintage Race Cars, Mustang, driveshaft, Mark Williams Enterprises

After installing our engine and putting the car on the ground, we found some bad news: Our driveshaft was about two inches too short.

How could a Mustang driveshaft not fit into a Mustang? we asked.

As with most issues surrounding our Mustang’s drivetrain, the answer revolved around the Roltec transmission. This Chevy-based transmission is some 1.75 inches shorter than a Top Loader or Ford-based T-10.

After getting on the phone with Mark Williams Enterprises, they walked us through the process for measuring out a new driveshaft. (One tip: bury the yoke into the tail shaft and then backing it out 5/8 inch.)

Final answer: We would need a 55 3/8-inch driveshaft.

The next question: steel, aluminum or carbon fiber?

The steel driveshaft would weigh 19.8 pounds, aluminum would check in at 11.8, and the much more expensive carbon fiber shaves a couple more pounds.

Shaving eight pounds doesn’t sound like much, but it is nearly 50%. The static weight isn’t the real issue. Like a lightened flywheel, a lighter driveshaft helps the engine accelerate. Based on the horsepower rating of our engine and its intended use, the shop calculated that a 3.5-inch-diameter driveshaft would be best for us.

The price difference between aluminum and steel is about as much as the weight difference–imagine that. For a bit over $1000, within a week we had our new driveshaft.

Mark Williams Enterprises shipped us a beautifully made driveshaft that was perfectly balanced and built to withstand at least 1000 pounds of torque in a violent drag racing scenario. We feel competent it will serve our needs quite nicely.

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View comments on the GRM forums
Noddaz GRM+ Memberand PowerDork
3/24/23 10:47 a.m.

"For a bit over $1000, within a week we had our new driveshaft."

It never gets cheaper to race, does it.

GameboyRMH GRM+ Memberand MegaDork
3/24/23 11:12 a.m.

One thing made of carbon fiber that can be had fairly cheaply is tubes, makes you think about chopping up a junkyard shaft and affixing a carbon tube cut to length where the steel used to be...

Edit: Looks like Robbie had similar thoughts:


Tom1200 UberDork
3/24/23 6:52 p.m.

I made a similar faux pas with the Datsun. In my case I'd found a driveshaft via Dave at Futofab, sadly I'd measured wrong and was off by about the same 1.75"

Fortunately I simply moved the motor and tranny back in the chassis.

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) GRM+ Memberand MegaDork
3/24/23 6:59 p.m.

"Pounds of torque"?  Why not the simpler "torques"?

MyMiatas HalfDork
3/24/23 7:57 p.m.

Is that second picture just a demo of what the old one looked like when it was too short?? 

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