Front driveshaft diameter

[Short 3/4]

My current truck was assembled from several parts trucks from 64-66. There were two front driveshafts in the pile, one 2 1/2” and the other 3 1/2. I picked the smaller one because it was in better shape. I’ve now managed to wear that one out, so am looking to build a new one. Since I’ll be starting from scratch figured I’d make it the correct size…so what would that actually be?

Thanks for any help
Tom

[Richard]

Originally Posted by Short 3/4

My current truck was assembled from several parts trucks from 64-66. There were two front driveshafts in the pile, one 2 1/2” and the other 3 1/2. I picked the smaller one because it was in better shape. I’ve now managed to wear that one out, so am looking to build a new one. Since I’ll be starting from scratch figured I’d make it the correct size…so what would that actually be?

Thanks for any help
Tom

Why not just replace the u-joints? Did you break the shaft?

[Short 3/4]

When I assembled the truck way back when, the thicker of the two shafts was dented, so it got scrapped and I used the thin one. The splines are now completely worn out. Since it sees a fair amount of off-road miles I want to build a good reliable new one I won’t ever have to worry about. The 3/4 ton 64 I got the running gear from did not have a front driveshaft when I got it and I have never seen another 60-66 K20 in person to check out. The few 60-66 K trucks in my area I’ve found are all 1/2 tons, and all so far have the smaller diameter shaft. Can’t remember where I originally got the larger diameter shaft, as I was doing a lot of 60-66 K truck parts scrounging for many years before finally assembling my truck.
Thanks for any help,
Tom

[cadillac_al]

FWIW I'm pretty sure my 1/2 ton and 3/4 ton square bodies have the same size driveshafts.

[cbeerman]

The GMC 65-66 master parts books shows the front drive shaft as a 2-1/2 tube

[Short 3/4]

Originally Posted by cadillac_al

FWIW I'm pretty sure my 1/2 ton and 3/4 ton square bodies have the same size driveshafts.

Thanks, that would make sense because shafts are quite short in a married transfer application, so not as critical as the 4 ft long front shaft on a divorced case.
Appreciate your reply
Tom

[Short 3/4]

Originally Posted by cbeerman

The GMC 65-66 master parts books shows the front drive shaft as a 2-1/2 tube

That’s very interesting. I’m assuming that is for both 1/2 and 3/4 tons, and with or without locking hubs option? The reason I’m curious is if you do the math on a 4ft long 2-1/2” shaft on a truck with 4.56 gears at 60 mph, the critical speed of that shaft would be too high. Of course with locking hubs that would not be a problem, but otherwise you’d have no choice. I sure wish my 64 K20 had still had its original front shaft installed when I bought it, as it did not originally come with the locking hub option. It was originally a utility service truck, so would have been driven daily both on and off the highway. Of course maybe that’s why the aftermarket hubs pictured in my signature were installed when I got it.

Anyway, thanks for the info. I like that this is a safe space for people like me with such obsessions. I’m amazed at all the vintage tech literature you manage to dig up and share.

Tom