Sm420
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
Item 2 GM MUNCIE SM420 4 SPEED TRANSMISSION SIDE COVER C10 C20 C30 C40 K10 K20 K30 K40 2 - GM MUNCIE SM420 4 SPEED TRANSMISSION SIDE COVER C10 C20 C30 C40 K10 K20 K30 K40. Item 3 SM420 GM Chevy Transmission Rebuild Kit 54-67 4 Speed Rock Crawler 3 - SM420 GM Chevy Transmission Rebuild Kit 54-67 4 Speed Rock Crawler. Get the best deals for sm420 transmission at eBay.com. We have a great online selection at the lowest prices with Fast & Free shipping on many items! I have some Muncie SM420 transmissions for sale. I am asking 250.00 each. These are the unbreakable transmissions that were used from around 1955 thru 1966 or 67 in all of the 34 and 1 ton GM trucks. These were even used in some larger trucks. These are in excellent condition and we can remove the shift tower to inspect them before you purchase. Great for any heavy duty application. SM420 TRANSMISSION REBUILD KIT FITS '48-'67 GM 4-SPEED WITH 10.5' CASE & 1 PTO (BK108) SM420 TRANSMISSION REBUILD KIT FITS '48-'67 GM 4-SPEED WITH 10.5' CASE & 1 PTO (BK108). The SM420 is the old Chevy heavy duty granny four speed. Of all the heavy duty granny transmissions, it has the lowest first gear and a particularly short overall length (the Jeep T-18 is the shortest when you add transmission to transfer case adapter.).
Reasoning:The SM420 has a first gear ratio of 7.05:1. When used with a 3spd transfer case and a 4.11 ring and pinion ratio, your final crawl ratio will be 67.5:1.
Availablility:
The SM420 was available from 1947-1967 in 1/2 to 2.5 ton chevy trucks. The only change made was in the last year when a reverse switch was added to illuminate reverse lights. Some of the parts for the SM420 have been discontinued by GM, so finding some of the parts, such as gears, is difficult.
Required Parts:
Sm420 Overdrive
To adapt the SM420 to the 2F (or F) and 3spd t-case you need:
3spd bellhousing (will bolt up to 2F)
3spd flywheel and pressure plate (I'm not sure if the 4spd will work)
Chevy 11' clutch disc (from something like a 75 4wd 1/2 ton chevy pu)
Special pilot bearing. It is the same inner (I think) and outer diameterfor the 3spd tranny, but the inner shaft is about 3/8'
longer, so it sticks out further. This isnecessary because the input shaft of the SM420 is a bit shorter than the3spd
Cruiser tranny.
Sleeve around the input shaft of SM420, where the throwout bearingrides.
Muncie Sm420
Ring around the front bearing retainer of the 420. It isabout half an inch thick and has a wall thickness of about 1/2'. Itfits around the front bearing retainer and assuresthat the 420 sits snugly in the 3spd bellhousing.
On the back side all you need is the 3/4' adapter plate that you boltto the back of the tranny, then bolt the t-case to that.
The gear that rides on the output shaft of the 3spd tranny will slideright onto the output shaft of the SM420.Discussion:
One thing about the 3/4' plate style adapter is that it rotates thet-case down so that the front output shaft flange is able to clear thetranny (it is still a close fit though and some people have to do a bitof grinding on the tranny). This means that your t-case will sitabout 1.5' lower than stock, which I find very helpful with the driveshaftangles on my spring-over.
Another thing that will have to be delt with is the linkage for the transfer-case. I have seen a few people build their own shifters, one type with two sticks, and the other like the Toyota single shifters. If you have an early model truck (like I have) you can use the stock vacuum shift for the 4wd, and just lengthen the rod going to the low range lever.
If you have a Cruiser that originally came with a 4spd tranny, you willhave to address the problem of having different driveshaft flanges betweenthe 3spd t-case and the stock pinion flanges.
There are 3 basic ways that I know of to make the change.
1. Split the driveshafts at the slip-yoke and use the 3spd driveshaftsection at the t-case end and the 4spd driveshaft section at the differentialend. Make sure to keep the U-joints in phase and to have the driveshaftbalanced before you install it.
2. Change the pinion flanges from the 4spd to the 3spd, they will boltright in. Then use the 3spd driveshafts
3. Change the output shaft flanges on the transfer case. Youwill only be able to do this on the rear. If you were to do thison the front I believe you would run into problems with interference between
the flange and your SM420.
I think that is basically all of it. I got my adapters fromClassic Cruisers in Salida, CO. They were the most helpful and seemedto be reasonably priced.
This conversion is at the owners risk and I assume no part in any ill fate that might occur because of my directions. Use at your own risk.
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