SM420 question

I searched this topic but couldn't find any post that answered my question. The time has come to pick a tranny setup. I have two options...Option 1 In my garage sits an SM420 with a flywheel and bellhousing from a '55 Gm Pickup. Option 2 try and find a good TH400. My engine is a mild 350 may be pushing 350hp. My rearend is a ford nine inch with (what I believe to be)3.73's. Will the sm420 work with this setup? Any thoughts. Thanks, Jess

 

sure will. try www.stovebolt.com google sm420 and that should pop up some info. 400 turbo will work fine also. guess it depends on if you want stick or auto. trans. www.oldgmc.com has info also.

 

I was just under the impression the old 4-speed wouldn't work because it was geared too low. I want to do some racing also will it hold up?

 

That "farmer four" is a fairly strong trans, but you won't like it for racing, even once in a while. The ratios are for getting a load moving slowly, so the first gear is like a tractor. Second and third are low, too, with quite a jump to high. They don't slam shift very well, either. Go with the 400.
Then, too, the early bell uses a three-bolt starter that is getting hard to find.

 

It's a great Trans for a 4 wheeler but not a 2 wheeldrive hotrod. You'll end up not using first. I'm pretty sure 1st is straight cut too. Toss in a transfercase and Dana 44 front axle,extra dive shaft and you set! 4X4!

 

Had one in my '62 Suburban. You don't use 1st at all...it's a hefty 7.05 ratio. By comparison, the 1st gear in the New Process 4spd OD manual trans I just put in is 3.09.
Second on the Granny 4Speed is 3.58, 3rd is 1.71, 4th is 1:1. So it's really a 3 speed.
If you want to pull down your neighbor's house, that's the trans to use. Even if you want to use it around town, it's not horrible. I put over 20,000 miles on my Suburban in a year and three months, plus plan on a 5,000 mile round trip to Bonneville with it, so I need overdrive.

If you wanted to use the truck as an around-town truck, hitting the Home Depot on weekends, that'd be a fine trans. That you've even mentioned the word "race" means you'll hate the Granny Gear.
Like one guy told me: "Granny ain't good for much...'cept Grandpa."
-Brad

 

Thats a big 10-4. Kind of what I figured.