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Muncie SM420 Transmission Installation in an Early CJ-5
Author: Jon M. Heikka, A.K.A. 69CJ
Editor: Adam Sparks, A.K.A. Sparky
Why would you want to anyway?
Well, in a side-by-side comparison, the SM420 provides you with a stronger transmission and a wicked low first gear.
Side-by-Side Specs
Transmission |
T14A (Warner) |
SM-420
|
|
Height |
? |
17" |
|
Length |
8.75 " |
10.4 " |
|
Weight |
? |
135 Lbs |
|
First |
3.10:1 |
7.05:1 |
|
Second |
2.61:1 |
3.58:1 |
|
Third |
1.00:1 |
1.71:1 |
|
Fourth |
None |
1:1 |
|
Reverse |
3.10:1 |
7.05:1 |
|
Crawl Ratio* |
37.21:1 |
84.63:1 |
*With 4.88 diff gears, Dana 18 (2.46 low), trans in first gear.
T14A on top, SM420 on bottom.
You can typically find SM420 transmissions in pre-1967 GM trucks. They can also be found in some school buses and dump trucks (I don't think you'll ever find a T14 in a school bus or dump truck). Only 1967 SM420's had a reverse light switch built into the case. I suppose one could be made, but I went with the toggle switch on the dash method rather than drilling and tapping the top of the trans. For additional SM420 information, check here.
The Down and Dirty Procedures
Disassembly: Remove the floor panel to expose the transmission. Remove drive shafts. Disconnect and remove all linkage assemblies. Drain oil from both the transfer case and transmission. Remove the transfer case. Remove the transmission. Remove bell housing. (If you're changing the clutch too, then remove the clutch, flywheel and pilot bushing.
Reassembly: Install adapter to transmission. Install bell to transmission. (If you're replacing the clutch, install pilot bushing, flywheel and clutch assembly). Install transmission to engine. Remount cross member in new location. Install tcase. Hook up all necessary linkage. Install modified drive shafts. Fill tcase and trans with appropriate fluid. Adjust clutch. Replace floor. As you should be out by now, go buy more beer and think about how you're going to make a boot for the shifters.
Modifications and Installation Details
Bell housing
The SM420 bolt pattern and the input-bearing cap will match up perfectly to the original bell. The mod here is to the two lower bell to transmission bolt holes. The shape of the SM420 doesn't allow enough room to run the two lower bolts in from outside the bell. The fix is to drill out the two lower bell holes and then thread them into the tranny from inside the bell. I've heard some others say they have to grind a little bit off the top/center of the bell to provide clearance for the center shift boss which sticks out just enough to keep the bell from mounting flush with the trans. In my case,they mounted flush with no problem - so keep this in mind as a "maybe." The bell and transmission will have to be lifted into place as a unit when mounting to the engine.
Clutch
Since I was going all the way in on this job, I decided to stick a new clutch in there as well. I used a LUK goldline replacement clutch. For $195, you get a new disk, pressure plate, throw out bearing, pilot bushing, and some spline grease. To get the old pilot bushing out, I kept pushing wet toilet paper into the bushing cavity until it was packed solid. Then, I tapped a bolt into the pilot-bushing hole and the bushing popped right out. After cleaning everything out, I tapped the new bushing in with a piece of wood and a hammer. The wood is softer than the bushing, so it'll distort before the bushing will. Obviously, the new LUK clutch plate is nice new silver one.
Before installing a new clutch, take the time to get the flywheel machined. It cost me $28 at the local Napa machine shop. Keep in mind the odd-fire flywheels are balanced and can only mount up one way to the crankshaft. The bolt pattern is drilled so you won't make a mistake AND there's corresponding punch marks on the end of the crankshaft and the flywheel. Align these punch marks when you lift the flywheel back into place and the bolt pattern will match right up. Clean the flywheel and pressure plate surface very good with brake cleaner before clutch installation. In addition, keep greasy fingers off all friction surfaces. Use a clutch alignment tool to line everything up while tightening the pressure plate & tighten the plate down equally otherwise you may damage it during the install. After torquing everything down, you're ready to install the SM420.
The first pic above is the flywheel before machining. The flywheel on the bottom is after machining (before cleaning), with the new pilot bushing is in place.
The Adapter
(Novak Adapter kit number 422-4-10 is available here). If you have an overdrive, be sure to tell them, as you'll get a different spud shaft.
The SM420 must be readied to receive. Use an impact wrench (or other suitable tool) to remove the rear drive shaft yoke. Once the bolt was off, no puller was necessary. As you can see, this SM420 came from a 2WD donor.
Then remove the rear cover. Take care NOT to allow these two shafts to travel rearward with the cover removed. If they move to far, you could end up dropping a bunch of roller bearings into the case. While you won't have to go through a complete disassembly to fix this, it's still a pain (yes, I got to experience this). See how the back of the adapter looks? The kit comes with a gasket for this. Note the absence of any oil passage holes. If oil it going to flow between the trans and the transfer case, it's going to have to be well above the fill line and travel through the adaptor via the spud shaft opening.
Here it is bolted up. You'll use the same bolts from the cover to attach the adaptor. I sealed them with a little RTV to help deter oil leaks. You'll also notice the absence of any oil passage holes in the adaptor.
Note the ribs on the side of the SM420. If you are running the original small front yoke on the D18, the extra length of the Novak adaptor (as compared to the AA adaptor) will provide enough clearance for the yoke,u-joint and a small diameter drive shaft. If you've upgraded the front D18 yoke to a larger unit, you'll find that it still clears, but it's close. As I run a rear drive shaft up front (larger diameter), I needed a little more room and ended up grinding some of these ribs away with a 4" grinder.
Installing the transmission
Make SURE you're clutch is aligned like it's supposed to be. I used a floor jack to lift the transmission into place. I also looped a ratchet strap over my cage spreaders and under the trans to help support it while I lined it up. I did this by myself, but another set of hands would have been helpful. Once the lineup was close, I threaded two long bolts through the two top holes in the bell and started them into the block. This really helped to line everything up even better and it was but a few seconds later that I heard the shaft clunk into its proper spot. From there, I put the correct bell to block bolts in and torqued them to spec.
Crossmember
As everything is a bit longer now, the cross member needs to be moved rearward and bolted in. On the original setup, the cross member mount was at the rear of the T14. You can retain this mount and adapt it to the tcase adaptor, or you can buy a new GM mount from Novak for about $17 (USD). The Novak mount is smaller, and bolts right up to the tcase adapter without modification. As the SM420 has no place to attach the cross member, the adaptor and the (side stabilizer) on the transfer case will be your only mounting points on the cross member.
You can see the GM style mount in the photo above.
In order to line everything up, I installed the driveline and the cross member, then lifted it into place. After checking to ensure nothing was binding, I drilled the new holes in each frame rail. I used steel spacers on each side to lower the cross member about inch, then bolted everything up with Grade 8 hardware.
Another thing, you'll need to modify the cross member for is the tcase stabilizer. On the passenger side of the tcase, just behind the nose cone, there's a mount that helps to stabilize the tcase when the driveline twists. After reinstalling the driveline and cross member you'll see that this stabilizer is now centered above the cross member. Drill the cross member so the drilled holes lines up with the transfer case side mount. I then inserted a rubber puck between these two pieces and threaded a bolt with a large washer from top to bottom. I slipped another rubber puck on the bottom of the cross member, then bolted it together.
Linkage Issues
Single Stick Dana 18
If you want to retain the single stick, you will have to fabricate a mount to stabilize the linkage. On my T14, there was a spacer between the bell and the trans; this is where the single stick linkage bolts up. The SM420 has two available bolt holes on the passenger side; you can use these to mount the linkage. It's not a direct bolt up, but if you have a cutoff wheel/grinder, drill, welder and minimal fabrication skills you'll really have no problems. I took a small piece of 3/16" plate and drilled it to match the two SM420 bolt holes. Once this was in place, I simply maneuvered the original single stick (and mount) into a position that would work and tacked it in place. When I moved the shifter back and forth through it's positions, I found there was a little binding where the linkage was rubbing up against the side of the SM420. I little bit of grinding on the case was all that was necessary to provide sufficient clearance for this part of the linkage. Once I knew everything was going to work, I finished the welds, cleaned it up and hit it with some hammerite. If you want to avoid fabricating a custom linkage, convert your D18 to a twin-stick setup.
After everything was in place, I cycled the linkage and found that the side of the transmission case needed a little grinding to keep the linkage from binding when shifting into 4L. After I had enough room for the linkage to work smoothly, I cleaned everything up and touched up the paint (new Ford gray).
Overdrive
I used the original O/D linkage mounting bracket as a donor to fabricate a new one, which bolts up similar to the original configuration on the T14. First, cut off the tab that has the O/D shift stick pivot. Then flip the bracket over and drill it to match the two bolt holes shown in the pic. In the pic above, you can see a bit of the roughed in O/D linkage. It's bolted to the top cover. You can grind or cut a substantial amount of metal from the bracket to give it a cleaner look. Once it's in place, tack on the tab with the O/D shifter pivot and test fit. If all is well, finish weld, clean it up and paint it.
Clutch
I used Advanced Adaptors chain setup ($132 w/tax) to replace the original cable unit. This piece was extremely easy to install. After removing the original linkage hard line and cable, I bolted up the one end to the clutch tab and the other to the fork. I'm still running the stamped steel Jeep fork and the cable fit perfect. From there, I played with the positioning of the bracket until I had the best fit with the chain snug. I clamped it with a C-Clamp, then drilled the two mounting holes in the lower part of the frame rail. The bearing on the clutch tab was rubbing against the M/C plunger where it attaches tot he brake pedal. This was occurring because over time, the hole in the clutch tab had become elongated from the clutch rod.
The fix was to disassemble the entire unit and plug weld the clutch tab. I then drilled a new 5/16" hole slightly lower on the tab, and then trimmed the top of the tab for that little extra clearance. I reassembled everything and made sure it was aligned correctly, no more interference. The pedal will feel different with this new linkage and it'll take a few shifts to get used to it. Nevertheless, the AA chain linkage provides for smooth clutch activation.
Drive Shaft Modifications
The rear drive shaft had to be cut down to measure 16.5" at the center of its stroke. Measurements were from the ebrake mounting plate to the center of the rear diff u-joint. The front drive shaft had to be lengthened to 28" at the center of it's stroke, with measurements taken from center to center of the u-joints. Cutting down the rear shaft and retubing the front shaft was same day service and cost $138 (USD) at a small local drive shaft shop. The front drive shaft shown isn't yet hooked up to the front axle, so don't judge the clearance by this photo. It was tighter than it appears.
Once everything was in and adjusted, I trimmed the floor covers to fit the taller transmission and relocated shifters. It took a couple of beers to figure out how to cover the now larger hole in the floor. Several things were considered, and I finally settled on asking my wife to sew up a custom boot out of the same black vinyl she used to make the safari top. As this hasn't been made yet and needs to be fit into her schedule, I'll wrap it up here.
Special thanks to Adam Sparks for building and hosting such a great board for early CJ's!
Thanks for reading & see you on the trails!
Regards, Jon
(If you want to shoot me an email, you can find me here)