The Stovebolt Site Forums The Shop Area Engine Shop using a '49 1/2 t trans in a '47 6100 truck

I just bought a rebuilt 1955 235 engine mated to a 4 speed trans from a '49 1/2 ton to put into my '47 6100 series grain truck. It currently has a '57 235 and the original 4 speed crashbox. I was happily driving home, thinking what a great deal I had just made, when it hit me that the 6100 series truck has an open driveline, unlike the pickup. Can I swap the rear covers and bolt on the open driveshaft flange, or would that be way too easy? Barring that, are the innards interchangeable; in other words, could I use the input shaft & cluster from the '49 and the output from the '47 trans? Outside, the gearboxes look the same, down to the lugs to bolt on my emergency brake mechanism, so I'm hoping that I can use this trans.

Thanks in Advance

Is it an early or late 47. If it is an early with the reverse lockout trigger, almost nothing will interchange. If it is a late 47 with the spring loaded reverse, it should work.

It is an early '47 and does have the reverse lockout trigger, whereas the later transmission doesn't. What are my options? If the '49 trans won't work, what will go in and still accomodate my PTO?

Thanks again.

As crenwelge says basically nothing swaps between the early and late transmissions. Including the bellhousings.

If you have a '55 bellhousing with that replacement motor, you will need to modify the mounts to use that bellhousing. If you have a '49 bellhousing, it should bolt up where the '47 came out. I think.

When you change to the later bellhousing you will need to modify the clutch linkage and I can't help you there, I know people have done it, but I'm not one of them. Send a PM to rustednuts and ask him to read this thread. He converted an early GMC COE to use the SM420 and might be able to expand or correct what I've said.

The '49 4-speed is an SM420. They became the standard 4-speed in '48 and were used into the 60's. The transmission was used in both open and closed driveline configurations and you should be able to simply swap the pieces to convert from one to the other.

These transmissions aren't that hard to come by, you can probably find an open driveline type SM420 as easily as the parts to convert the one you have. If you can locate MotherTrucker he is in Arizona and probably has the parts you need. Check OldGMCGTrucks.com to find him.