SM420 or T18?

I am in the decision process on a 4 speed to put behind my 225 V6 - kind of down to an SM420 or a T18. I will probably want to put this drive train in a '69-'71 CJ6 which I have yet to buy.

I was close to committing to the T18, but I have an pre-'54 SM420 sitting in the shop that I thought I would scrape some crud off of and take a look at to see if it would do as well as a T18.

I have a couple reasons to go with the SM420

- I already have it
- It's a bit 'old school' - when I was a kid my dad had a '50 3/4 ton Chevy truck that I drove a lot. The 7:1 granny low was a lot of fun in that truck.
- The total package behind my V6 would be maybe 3-1/2" longer than the T18 package, but that should not matter in a longer vehicle.
- I can use a standard Buick bell housing for the SM420

Of course, there are lots of reasons to go with the T18 as well.

I noticed that there are a couple bosses on the PS of the SM420 that I might use to anchor a bracket to extend the D18 twin stick shifters… Anybody done this?

Any reason I shouldn't use this transmission?



The gears and shifter are in fairly good condition - would need a rebuild and a new front retainer…

 

I would say that, 1) since it's going in a -6 and, therefore drive shaft length will not be an issue and 2) since you already have it, I would just go with the 420. Main reasons for opting for the T18 is that you can end up with the shortest possible conversion and, apparently, parts availability. I have heard that 420 parts can be scarce but, also that rebuild kits are out there. Do your homework on that. May also have to grind a bit on the transmission case and/or the front bearing retainer (twin stick shifter housing) on the Dana 18. Tons of info here on both options.

 

I have the sm 420 as you know. I paid 75 bucks for it and its from a 1951 chevy pick up. I did nothing to it other than change the oil and remove the output and install the novak adapter.

The transmission is a 4 speed, but low really is pointless for street. The transmission shifts slow, but is fairly bulletproof. The bell housing for a buick ,depending on which one you have and provided its not clocked in some strange way, will work. I had to machine a recess into the bell housing for the shift rails to clear and allow mating from the bell housing to the transmission. You will have to drill out the bottom two holes and insert the mounting bolts into transmission from inside the bell housing. You will have to cut the shifter and reweld it to clear the dash and make it so that it clears your seat as well. The shifter housing is much larger than a t-90 or whatever came in a cj6 and will require a bit of fabrication to make a new cover.

I do not think it is possible or worth your energy to use the threaded boss in the transmission for the shifters with the dana 18 t/c.

I had an issue with getting the correct T/O bearing to work with the diaphragm style clutch. I also and a real issue with the transmission input shaft splines running out of groove and pushing into the clutch hub. This caused the clutch to never release as it was smashed into the flywheel. I only needed less than 3/16th of an inch to make it work, so i machined the clutch hub back to clear the shallow end of the splines and solved my issue. I am not unique in that problem, but documentation is difficult to find on the internet. I also had trouble making sure the pilot bushing was correct.

I actually really like my sm420. I never had any issue with my first t-90 either. I have no real experience with a T-18, but am sure i have driven trucks with this transmission in it. SM420 parts are available, but they haven't made it since 1969, so its really old. That being said, since you have it, for me it would be a no brainer. The Novak adapter is like 400 bucks and is a perfect bolt on part. Its worth it, in my opinion.

 

Pro's of the SM-420: you have it and it appears to be in decent shape, lowest first gear of all the light truck transmissions at 7.02-1, fairly compact, very strong. Cons: They haven't commonly been produced since 1967 so parts can be hard to find, 3 versions (that I'm aware of) so makes it even harder to find parts for, requires a longer adapter, front driveshaft/yoke clearance can be an issue, tend to be noisier than other "truck" 4 speeds due to the angle of the helix on the gears.

Pros of the T-18, parts are readily available, decent low first gear at 6.32-1 (if you get the wide ratio version), one of the shortest combinations in a Jeep application so best for rear driveline clearance, no front driveline/yoke issues if set up correctly, quieter than an SM-420 (doesn't mean it is "quiet" though), probably the easiest of the truck style transmissions to work on.
Cons, 2 different first gear ratios possible, not as low as a first gear as the SM-420, there are different versions that require different blocking rings etc. so need to be aware of that, you don't have one in your posession.

Just as an aside, from my research 1967 was the only year the SM-420 had a backup light switch so that can be an issue if you require/want one.

 

Chuck - I paid $75 for this SM420. It works, but needs a front retainer and probably a bit of internal work. I am concerned about the bulge that extends past the front of the case at the top. Will this interfere with the bell housing?

 

Nick -- I am going to rig a back up switch on my T90 as in the photo below. I might be able to do the same with the SM420.

 

Yes. you will need to machine that part of the bell housing to allow it to bolt up. If you look at the AA bell housing it looks to have that space open to accept the shift rail.

 

SO Chris, you just cut/ground it off flush with the face of the transmission?

 

T18, I love mine, I also have a sm465 as well as a sm420. For working on fit and parts as well as weight I like the t-18 the best.

 

Actually, i put it on my mill and cut a pocket with an end mill. I am sure it could be done with a grinder as effectively. Its really not difficult. A small die grinder is all you really need.