Muncie SM420 or New Process NP435

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

both are old school

cast iron granny boxes. NP435 is a bit more modern, SM420 has been around since the late '40's. IMHO, the NP shifts a bit better and at one point, had the adapter, the transmission, and had an input bearing turned to run behind an AMC engine. Having said that, I have an SM420 in my cj-5 in front of a Novak adapted D300 (sbc engine) with 4.10's usually 31-33" tires; rpm is pushing 2800 at 60ish mph. I just liked the idea of the 7.0 first and the rig was built for smileage.

You will not win drag races with either of these boxes, they shift like a (old) truck transmission... because they are (can't remember what the NP435 is rated at, the SM420 was used in up to 2 1/2 ton trucks). You will rarely use 1st gear on the street. Your engine had better be up to gear ratios that widely spaced. To test drive, look for GM trucks between 1947 and oh maybe 1967; NP will be found in 70's to maybe mid '80's Fords (there may be a very few Mopars).

Compared to modern gearboxes, both are short; the SM trans is shorter than the NP. They are heavy. Almost indestructible. 1st gear is not synchro, but again, unless you are trying to start a heavy load from a dead stop, you won't use it on the street. You can doubleclutch these from 2nd down, but the drop is so steep that it's not worth doing unless trying to impress friends with old school tricks.

Either one is a good choice; your choice may be determined by what is available in your area.

Oh, almost forgot, it wasn't until the very last year of production that the SM420 had a reverse light switch installed; earlier have a boss that can be drilled out and one installed. There may be a rib that prevents clocking of the SM420 beyond a certain point or may need to be ground to make up to the D300.

Hey 40below,

Thanks for the info and pic of where to grind.

I'm expecting to use 2nd on either of these from a dead stop on pavement.

In reading your post, I remembered that a very long time ago I used to drive an old '72 Ford 14' bobtail truck to deliver furniture. It's actually the vehicle I learned to drive when I was 14. I have a feeling that it may have had the NP435 in it. Even with a normal load it was rare that I'd drop it into first. It had to be a heavy load to use 1st.

From what you describe it sounds like the SM420 would work well with 35" tires and similar gearing to what you have. That should keep my RPMs down at around 60 - 65 mph. The crawl ratio should still be low enough for anything I imagine I'll do.

I'll have to try to find someone who's running a 4.0L 6 cyl. with the SM420. I'm thinking that it should be enough power even if it's only running at a stock 180 hp as long as I'm not expecting to race it, but it's a good thing to check first.

Thanks again.

Christian

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72 Fords IIRC were T18 or 19, but 'eh, you have the idea; old school granny gear boxes are similar in the way they drive. NP435 will have an aluminum top cover; Novak has the pretty pictures to ID.

As for the six cylinders, yes, you will have enough power (had a 69 Chevy 4x4 with a 250 and SM465, the 65 I got the SM420 from had a 292) you just have to plan ahead a bit on the highways or if towing; the transmissions in question have ratio gaps a mile wide compared to modern boxes. On a trail I don't think it would be a problem.

re: casting ribs on SM420's, some have them, some don't and it seems they weren't always in the same place. Not a surprise for a basic design in production for so many years. More of a FYI in case you were considering clocking the 420 when attaching to a D300. I have no idea about possible interference if you are moving up to an Atlas, sidestepping to a NP205 or chain drives.

attached pic shows why rib had to be ground; interference with T-case shift rails. Also, you can get an idea on the clocking problem. Can it be done? oh, probably, but setup is in a 5 so length is critical.

40below,

Thanks for the follow up post. It was helpful in gaining a better understanding of why people might not like shifting this tranny.

With the ratio gaps that wide, I'm guessing people are just not used to letting the RPMs go a bit higher before they shift. I think that it would be a minor issue for me if I'd even consider it an issue at all.

Off road, I'd imagine that I'd be in agreement with you about it not being a problem.

It's good to hear that I'll have enough power, too.

I still want to try to drive one before I make my final decision. This Scrambler is not going to be a daily driver and this tranny sounds like it might actually be more fun for me to play with off-road.

I appreciate your input and would love to hear from others about their experience with this tranny.

Christian

Was wondering if there was any update on your decision and if a SM465 was ever in the mix as far gearbox choices go?

Sfisher said:

Was wondering if there was any update on your decision and if a SM465 was ever in the mix as far gearbox choices go?

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I have not made a deciision yet, but am leaning heavily toward the SM420 on theory.

I have simply not found one to test drive and I'm not in a hurry to do this yet. I'm working on some other things like going with fuel injection on my 258, doing a 1" body lift and putting some body armor on it to keep the carnage down when I lift it. I also have narrow track axels (around 50") and want to go wider (and beefier) before putting the 4.5" Rubicon Express lift on it. I don't want it rolling around like an animal in a circus act everytime I turn around.

I considered the SM465 as a viable choice, but decided that if I was going to go that route, I'd be better off with the SM420 for the following reasons.

The SM465 has a 6.55 1st gear where the SM420 has a 7.05. I may as well get the lowest gear if I'm going in this direction, which should also give me the widest spread of gears for best performance on both off road and highway if my transfer case and diffs are properly geared.

The length and height on the SM465 is 12" and 18" respectively where the SM420 is 10.4" and 17". It's a biit shorter and not as tall for slightly better angles on the rear drive shaft and maybe easier to get in the same space as the SR4. I may be wrong here, but that was my perception.

The SM465 weighs 175 pounds and the SM420 only weighs 135 pounds. By all accounts, I understand that the SM420 is bulletproof, so why not save 40 pounds?

The price difference between these transmissions for a fully adapted turnkey transmission from Novak is very slight. The current price on the SM465 is $1,590 and on the SM420 is $1,740.

So...that's my current stance, but new information could still sway me.

Christian

If you are keeping an AMC engine (258, 304, 4.0) hard to beat a T18 or T19. I used a ford version T18 in my cj-7. It bolts to the stock bellhousing, uses stock pressure plate and flywheel, ford disc, and an advance adapter plate on the rear for the t case to bolt to. The adapter aint too much (cant remember now) and you can find these trannies in the junk yard easy enough. Just a suggestion When I did the swap in mine, I bought a rebuilt tranny with the adapter plate already installed for $1,200. Tough to beat price wise.

spankrjs said:

If you are keeping an AMC engine (258, 304, 4.0) hard to beat a T18 or T19. I used a ford version T18 in my cj-7. It bolts to the stock bellhousing, uses stock pressure plate and flywheel, ford disc, and an advance adapter plate on the rear for the t case to bolt to. The adapter aint too much (cant remember now) and you can find these trannies in the junk yard easy enough. Just a suggestion When I did the swap in mine, I bought a rebuilt tranny with the adapter plate already installed for $1,200. Tough to beat price wise.

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This is something to still consider given the price. I've compared the specs to the SM420 and it's really pretty close.

I see that the T18 has a pretty low 1st at 6.32:1 and noticed that reverse is actually super low at 7.44:1.

The T18 weighs 145 pounds, which puts it very close to the SM420 at 135. It is 11.87 inches long and 17.5 inches high, which makes in only a tiny bit longer and taller than the SM4.20.

Thanks for the input!

Christian

As I was deciding between the 465 and the 420 I decided to buy the 420. In part because of the info from this thread. Found it in a '65 GMC truck for $120 so not a bad deal. Since I have the T-5 still it looks like it shouldn't be too difficult to get it mated up with the bellhousing. Now just need the rebuild kit, adapter for D300 and front seal.

Sfisher said:

As I was deciding between the 465 and the 420 I decided to buy the 420. In part because of the info from this thread. Found it in a '65 GMC truck for $120 so not a bad deal. Since I have the T-5 still it looks like it shouldn't be too difficult to get it mated up with the bellhousing. Now just need the rebuild kit, adapter for D300 and front seal.

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Let us know how you like it once you have it installed.

I'm expecting it to be a great success, but I'm running on theory.

Christian