Sm420

Posted on  by admin
Muncie sm420 transmission for sale

Sm420 Shift Pattern

I'm in the process of figuring out what tranny I want to use.
I've done a search in this forum on the SM420 and didn't come up with what I was looking for.
I'll try to write as much info as I can in this post to let you know what's on my mind right now. I may be right or wrong with what I'm thinking and am completely open to the possibility that I'm wrong. I'd rather figure that out now before I start spending money.
I've got an '81 Scramber that's completely stock.
It's sooner than I planned to start this, but I blew the old 4 cyl. Iron Duke. Since I have to replace the engine, I thought I'd better figure out what trans I'm going to use now and just do this at the same time.
I'm planning on going with a newer 4.0L 6 cyl. with maybe a mild cam and later plan to add some bolt on stuff for more horsepower like an aluminum head. I figure I'll eventually wind up with about 225 hp.
The tires and suspension are now stock, but I'm planning a lift and 35' tires down the road and have a few different ideas on what to do with the suspension. That part of the plan is in flux.
This is not a daily driver, but I will use it to drive to the beach with my scuba equipment in addition to taking it off road. It's definitely a toy and an extra vehicle that I'll drive if the others are in the shop. I want it to be a crawler, but will still drive it to where I'm wheeling and not trailer it. Hard trails like the Rubicon, the Dusy Ershim or the Hammers are more the goal than just seeing how crazy of a rock pile I can get over.
I've read all of the info at Novak and am very interested in the Muncie SM420 for that low 7.05 granny gear. Another choice would be the New Process NP435 with a 6.68 granny gear.
My idea is that with 4:11 gears I could still have decent RPMs on the highway, but give me a respectable crawl ratio in 1st. I'm not really decided on the gear ratio yet.
For now, I'm planning on using the stock Dana 300 transfer case and will probably just re-seal it because it's leaking a bit.
I have not driven the SM420 and have heard that it drives like a heavy truck and that I may not like that. I don't know anyone that has this tranny to try it out, so I'm asking you all what your experience with this tranny is. I'm still looking for one to test drive around the block, but I'm not sure I'll have that luxury. 'Driving like a heavy truck' does not necessarily bother me, but I want some more info about what that might mean.
I know someone with the NP435 in his Scrambler behind a 6 cyl. and while I have not driven it, I know he's happy with it. He runs 35' tires and 4.88 gears and comments that he may have been a bit happier with a slightly higher gear ratio on the highway. He says he rarely needs the granny gear, but that it's nice to have when it's needed.
Do any of you run the SM420 and if so what do you think about how it shifts?
I'm open to any comments and suggestions and hope to learn from the collective wisdom of the board.
Thanks in advance.
Christian

Sm420 Rebuild Kit

IDENTIFICATION - SM420 4spd, RWD or 4WD Top loaded, tower shift 'granny-low' transmission found in light and heavy duty applications. Cast iron case with cast iron top cover. Non-synchronized 1st, but has only brass type synchro rings for 3rd/4th. All gears on the countershaft are removeable except for 1st. SM420 TRANSMISSION SMALL PARTS KIT FITS '48-'67 GM 4-SPEED WITH 10.5' CASE 1 PTO (SP250-50B).