I don’t have exact answers to your questions. I‘ve had a tapered driveshaft on my 56 Fairlane since 1980. A driveshaft shop made this for my T85C R11 OD transmission conversion. Green as I was then, I hadn’t realized it may have come off a later Ford till your post and that the shop reversed ends (small end is at the transmission) opposite Ford’s intent.

Your questions prompted some searching. Ford played up the tapered driveshaft design in both 1957 Ford and Mercury literature. Ford’s 49-59 Illustration Catalog shows the tapered driveshaft used from 57 through 59. I don’t know about later years. Ford had a very aggressive racing program underway in 1957 until the American Automotive Association banded OEM’s from using race wins in advertising, at which point Ford pulled out of racing along with other OEM’s. Perhaps Ford foresaw a racing need for the higher critical speed that prompted the extra tooling investment and manufacturing steps to make the more costly tapered driveshaft. Cost is likely why Ford discontinued them.

Tubular driveshafts have a critical speed (CS) above which they can fail from harmonic vibration. Dana gives the following formula in RPM for steel:

CS = 4769000 x (1/L²) x √(OD² + ID²)

where L is the unsupported shaft length, usually U-joint centerline to U-joint centerline. Tapered driveshaft patents I examined start with a single heavy wall tube that is expand in a pre shaped casing to the desired larger diameter length. My driveshaft likely began as a measured 2” OD tube (likely 0.375” seamless) with its longer end expanded like an inner tube in a tire to a measured 3 7/32” OD tube giving it an estimated 3.017” ID if I worked the math right. The stock 57 Ford tapered driveshaft length for OD transmissions (P/N B7A-4602-E) is listed as 47 31/64” in the Ford parts manual.

Had Ford just used a non tapered 3 7/32” OD tube the same ID as estimated above, the resultant driveshaft critical speed would have been less than half of the lowest critical speed of the tapered driveshaft. I do not have exact ID’s of either tapered end, so I won’t say more without measured thickness data. If anyone knows, please share. Unless one plans on revving their driveshaft well beyond 7000 RPM, any well-built conventional non-tapered driveshaft should work fine. If one already has the tapered driveshaft, consider yourself lucky to own a piece of racing history and use it.

U-joints are likely the weak link in any undamaged driveshaft. As new Y-block mods continue unleashing more Hp, the need for a driveshaft safety loop increases.

Jerome