SM420 vs T18

Which would you choose? 63CJ5 283 Chevy, D18, Warm OD, Dana 27/44, 33s. It currently has, I believe, an M21 that the 283 was matched with when it came out of a 67 passenger car. Other future upgrade will be D30.

Have done the search and seen some of the pros and cons and actually spoke to Herm a few years ago, but am very interested in your very experienced opinions. Been away from the project for a few years due to $$ and family and am looking to get a fresh start.

Thanks!

 

Personally I'd go with the T-18. Mostly due to parts availability and front driveshaft/yoke clearance. The front will need adapted (can be done with no adapters a couple ways) but the rear can be done with factory parts. The SM-420 is noisier and parts can be hard to find. The front won't need to be adapted but the rear will and there can be clearance issues with the front driveshaft. The biggest upside to the 420 is it has a 7.02-1 first gear. Lowest of all the light truck transmissions.


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T-18.

 

Thanks Guys!

That 7.02 would be nice, but considering the T18's 6.40 1st I think I'll be in good shape.

Can you elaborate a bit on having to adapt the front?

 

I vote hands down T-18.
The gear ranging is much more uniform.
The bellhousing adapter depends on what engine and the CJ's wheel base.

 

Had options for both transmissions here for about the same price, $100 each on CL.

looked at the available parts options, went T-18.

if there are adapter-less ways to set up the t-18 I'd like to know more. Otherwise I'm thinking the Novak stuff was looking pretty good.

thanks!

 

Another consideration being that I already have a chevy bell housing the sm420 should bolt right up, right? With the T18 I'd need a $400 adaptor. I see on Herm's site you can get T18 with GM or Ford pattern. Does that mean the GM pattern bolts up to a GM bell housing?

 

If you start with an IHC or Pre-1980 Jeep T-18, you can use use the main shaft and adapter to connect to a Dana 20 or Dana 18. You have to mix and match junkyard parts, and it requires some minor machining, but it can be done.

Search old posts - it's been discussed a lot.

 

Yes, SM420 bolts to a GM bell.

Ford and GM transmissions can mostly be used with GM and Ford bells and engines. There's an article about adapting Ford transmissions to GM engines on the Novak site. No adapter needed.

 

Excellent article, thank you Tim!

 

I have an adapterless T18 to BOP bellhousing.
For starters it's a wide ratio Jeep T18.
That puts the PTO outlet on the left side where one can use it if desired.
It has the short 6 spline mainshaft and the standard Jeep D18/20 rear adapter.
The front of the case was welded and drilled.
That modification provides the GM saginaw bolt pattern.
It's now essentially a GM Borg Warner.
The maindrive is the Jeep 1-1/16" x 10" OAL.
This T18 will bolt direct to the BOP.
It's 11.875 OAL plus .9375" thick rear adapter plate.

I don't use it because I'm picky.
In my mind it has a defect.
I want to use a 1-1/8" driven disk hub not a 1-1/16".
So I'm planning a similar T98 with 1-1/8" modified maindrive.
Besdes that I think the T98 was the better design of the two.

 

I would also recommend to stay with the B&W T-18.............but stay with the Borg Warner Ford version and not the Jeep version...............parts on the Ford version are plentiful and reasonable since these Transmissions are still in use in a lot of Commercial Drilling pumps and oil rigs & Fork lifts. If you looked at the Novak Website your looking for a case that looks like the one pictured with the two extra bosses between the standard 4 bolt pattern...........in this picture this case has had the two extra holes drilled and the top holes have also been elongated to fit a Buick V6 Bellhousing..........On the Chevy Bell housing there may be an issue with the OD of the candlestick / throwout bearing housing hole but in most cases turning down the Ford housing will allow it to work.............if you have a Chevy truck bellhousing with a large hole that can also be fixed................You can count the rotations between the input shaft to the tailshaft to verify the 6:32 ratio or pull the top cover off which I would suggest anyway and count the front input shaft gear which should be 17 teeth for the 6:32 ratio..............also take a look at the large sliding gear in the rear and the counter shaft at the rear along with the reverse Idler............these three gears are non syncro and take the bulk of the damage as people try to get it into gear prior to a full stop..............Check for chiped teeth.......a little rounding is OK.....second gear through 4th have syncro's. Good Luck!

 

I don't read anywhere about SM-420's or T-18s hand grenading.........
Why all this worry about parts for SM-420?
Used trans are cheap, and I haven't had any trouble locating what I needed to do my 420 swap.
I don't see 420 parts as an issue on this.
If you get a good used transmission, working condition, you would be okay.
Just replace the bearings and move along.
Disclaimer: I'm no expert, and I haven't driven mine hardly at all since the 420 went in.

 

The sm420 is found easily in the junk yards around here, I bought 7 of them a few years ago for $75 a piece and 1 for spare parts for $10.

 

I have both transmissions. I originally had an SR4 and a T5 to replace that but the T5 went south and I had to choose. I got an SM420 for free and then nickmill built me a T-18. A decided to run the T-18 as it would be an easier set up and my SM420 is now in the corner of my garage awaiting an eventual rebuild anytime between now and 10 years from now.

I think T-18's would be the way to go and I only have the SM420 because it was free and it is described as the "creme de la creme" of offroad transmissions.

 

Ha ha!! I have all four sitting in my shop.... wait. What? What about the NP435 and SM465?? I think that with all of today's adapter technology, you're okay with anything you can make work for you. When I first started working on my M38a1 last year I planned to put an SM465 in. Because that is what I had. And I've used them in a lot of different full size trucks and love them. Then in a case of clever swapping, I became the owner of a SM420. My wife had to have a clutch in her 73 f@rd bronco, so we have the NP435 too. This last weekend I got a T18 with a Dana 20 factory adapted to it. So in theory, I could get my small block mounted up to the T18 without any adapters... I love that idea!!!

 

There are 3 versions of the SM-420 I'm aware of. In the land if rain here in the PNW any transmission that sits outside, even in a vehicle, is subject to rust from filling with water or condensation. Factor in the 420 hasn't been widely used since 1967 and hard parts can be an issue. The last one I built we had to get parts from across country and was not cheap. If you can get multiple transmissions for cheap, or know that the parts inside are good, then Great. If the transmission is an unknown I'd pass unless it has absolutely no moisture in it, you can look inside, and know what to look for. If not to any of the 3 above, I'd suggest using something you know you can get parts for.


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I love my SM420. It allows me to crawl really nicely.

It has served me well with only one issue. The snap ring that holds the 2nd gear dog into 1st gear broke as I pulled into the driveway after three days on the road and trail. I think it was probably improperly installed since the trans looked like it may have been rebuilt at some point. Availability for the ring was almost non-existent, with Novaks having only used copies for $50. I chose to waterjet one out of some spring steel sheet. Since that repair, I have had not one issue with it.

 

I can only speak theoretically at this point, since my T-18 is sitting on the floor of my shed yet to be installed, but it doesn't have any aftermarket adaptors. I started with a Jeep T-18 from a J-20, with the wide ratio gearing. R&P sent me a GM case, and the shorter input shaft, as well as a larger synchro assembly. My dauntless bellhousing bolts right up without a front adaptor and the trans/transfer case section has a factory adaptor. Looks to me like the perfect setup, but we'll see!

 

Great discussion guys.

All I can add is that the SM420 transformed my rig. Well worth the effort.

I know repair parts are hard to find but in such a light vehicle there's not much stress on the parts. I don't ever expect to wear the thing out.