The Stovebolt Site Forums The Shop Area Driveline SM420 - Compound Low - Muncie 4 speed

Is there an option for a replacement gear for the compound low gear in a SM420 4 speed Muncie gear box? I would like to get a 'taller' gear to replace the compound low 'granny' gear. Any options for this compound low gear?

No options, 465’s 1st. Gear is a little higher.

Ed

tinbarn,

Since this is your 1st post, Welcome to the Stovebolt!

Unfortunately, Ed's reply above is the definitive "short" answer on your question.

Depending on what you are trying to accomplish (sounds like you just want a better starting off 1st gear) you may want to consider changing the transmission to another unit that will achieve that result.

There are many options, from using a period specific 3 speed, a 3 speed with overdrive, a conversion to a T-5 overdrive 5 speed and others.

All depends on what truck, existing drive train and desired end result you are seeking.

Feel free to ask more questions, including ID of your Chevy/GMC truck you are working on, and folks will certainly jump in and try to provide more help as they can.

Dan

Thank you for the great advice. I appreciate that very much. I liked the General Motors Muncie SM420 in my old 1949 Chevrolet pickup and so I thought the SM420 would be a good fit for an old car project with an 'X' member that would not allow extra room for an overdrive. The Muncie 4 speed SM420 is a very short and compact and fits where I'm putting it but the only down side on the build is that compound low gear. I wonder if there's a shop that could turn a one off gear? That would be $$$$$ but that's okey as the SM420 really fits the build. Thanks again -- Gary

Gary,

Glad to try and help out a fellow 'Bolter.

Since we are a site dedicated to our Chevy and GM trucks, we can talk to till the cows come home about your '49 Chevy pickup in all of the forums.

Your "project" would best be discussed in the The Greasy Spoon if you want to continue on it there.

I would be interested to hear more about your '49 pickup, plus in case you hadn't noticed it in other conversations, we love to see pics of what ya got.

Care to share??? If you do, you might consider posting in our Welcome Centre

Catch ya later.

Dan

You would also need to redesign the whole transmission, not just one gear. Making a one-off cluster gear that meshes with 1st. would cost several thousand dollars. Since 1st. isn't synchronized like the rest of the gears, you'd still need to come to a full stop to engage it. Your checkbook will need CPR after that project. Most of us use a SM 420 as a 3 speed with a synchronized first gear, something a regular 3 speed can't do. "Granny gear" is not used often, other than getting a heavy load started moving on a steep uphill grade.
Jerry

Thank you all! This is what I need to know. My questions are answered. Happy Motoring !!! Gary

It's also great for teaching the kids how to use a clutch! I'm sure there's plenty of us on this forum that learned that way!

With careful pr@ctice, it is possible to learn to double clutch and shift into first gear while moving, especial useful when coming to bottom of a hill with a load.

Ed

My granny gear serves one purpose. It's my parking brake.

I call granny gear, parade gear.

Back in the dark ages when hay was square baled, granny gear, a hand throttle, and a couple of bungee cords on the steering wheel got a lot of flatbed trucks loaded. Point the rig in the right direction, lock the steering wheel straight ahead, and idle the engine up a little, then join the crew bucking bales until the truck got to the end of the hayfield. Turn it around, "rinse and repeat"! A thousand bales a day or so, baled, picked up, hauled, and stacked in the barn was about average, working from "can see to can't see"!
Jerry

Granny is also a clutch saver. You don't have to slip the clutch when maneuvering in tight places.

Not only maneuvering, reverse is also the granny ratio, which is ideal in every situation.
I hated my F᠁ that had a tall reverse, bet it would do 25mph backwards, wife smoked that clutch backing it out of the garage on our 17 degree hill.
Changed out that g box to get a granny gear᠁.never got another truck without a granny᠁
Chuck

When our massive front yard oak tree fell, I used my granny gear to help pull most of the massive roots out.

Especially useful when backing a trailer up-hill!

Ed