The Stovebolt Site Forums The Shop Area Driveline Converting SM420 to open shaft

Have a '51 3100 w/SM420 torque tube drive line, would like to keep stock trans but convert over to open shaft. What needs to be done and where would I get a lead on parts?
Appreciate any answers. Have a '58 pumpkin lined up for the open shaft rear.

You need the open driveshaft rear cover fom any open driveshaft SM420 transmission. You should be able to easily sell your enclosed driveshaft rear cover and the u-joint cover/bell.

Post a Want Ad in the Swap Meet.

likely the only way to find the parts to change the tail of your trans from the closed connection to an open slip yoke will be to find a 54+ SM420 ... and the driveshaft from a later year - might try a post in the parts wanted

Bill

What's wrong with your closed driveline??

PO tried to torch it off. I've removed the pumpkin and separated the trans joint at the hub collar, just mulling over some options,don't want to go the T5 route just yet.

Actually any late '47 1 1/2-2 ton truck would also do although there is a difference between the speedometer drive gear.

Jeff

1954 SM420s fom 3/4 ton upwards would also be set up for an open driveshaft.

Wouldn't there aways be a speedometer gear mis-match, if the original transmission is set up for a 4.11 rear end with standard 16" wheels?

A third member out of a 55 and later will bolt in and it will be a 3.90. A 4 speed out of a 55 and later half ton will be open drive shaft and it will have a speedo gear for 3.90. 3/4 ton used a closed drive shaft to the center bearing and open from there and it will have speedo gears for 4.56 on 7.00 x 15 tires. A 4 speed from a larger truck will have a parking brake on the rear of the trans and will probably have the wrong speedo gears. I never worry about speedos. I just drive them as fast as they will go. I think there is a good chance that your speedo gear you have now would work. There is not that much difference in the rear of a SM420 with or without open drive shaft.

I am using a 1949 SM420 in my '51 burb. The speedo was way off when I put it in, but I took the truck to a local old time speedo shop, and they fixed me up with a very clever little gear box which attaches to the tranny and puts out the right ratio. They did this by putting the truck on a roller/treadmill machine and ran it at some speed like 50 or 60 miles and hour, did a little math and then assembled the little gear box using gears from a big chest.

They have since gone the way of the dinosaurs, but maybe you can find somebody to do that for you.

Kurt

The Speedometer Shop makes custom adapters. Mine is good to
less than 1% error at 60 mph.

http://www.speedometershop.com/rep-pag.htm