The Stovebolt Site Forums The Parking Lot Big Bolts 1949 6400 chevy transmission issue

My big bolt 4 speed transmission frequently grinds when going in 2,3 or 4th gear. Seems less so when I have a load on the dump.

Ideas for problem, can you get parts if you want to rebuild. Truck only has 13,000 miles....former fire truck.

thanks,

Pierre

I think the 1949 GM truck transmission was an SM420.

If that is correct, parts are available.

That transmission should last for much more than 13,000 miles.

Is there water in it (empty into a catch-pan by removing drain plug in the bottom).

Maybe someone else will post helpful diagnostic advice?

In addition to the above, it could be out of oil or there could be a clutch problem. Is the clutch releasing all the way? (Does it shift from neutral into 1st and reverse easily without grinding, engine running, truck stationary?)

Thanks....transmission oil was changed with no difference. Clutch seems fine. It does go into first or reverse easily when stationary.

What type trans oil did you use? The modern multi grade and synthetics oils can be to thin/slippery and don't allow the gears to slow down between shifts.

Mike B

Mike,

Wow you have some great vehicles. I am not sure i guess. I had it serviced by mechanic when I first bought it. What do you recommend. Certainly worth it to change again just to make sure. Doesn't seem right that at 13,000 miles there would be a problem. I understand these are great transmissions.

Thanks for the compliment.

You need to use SAE 90w Mineral Oil.

Mike B

pierrem sounds like clutch is draggy my 51 sm420 has 113000 not easy miles on it shifts fine...when you pull up on a little slope and clutch it will it offer to roll back ?? if not look at clutch.