When it came to a transmission for the project, I considered many different options and finally selected the Muncie SM420. This heavy-duty truck transmission was built by GM and installed in 1/2, 3/4, 1 and 2-ton trucks, buses, and heavy equipment from 1947 to 1967. The SM420 is fully synchronized in second through fourth gears, and all gears are helically cut except first and reverse, which are spur gears. Ratios are as follows: 1st gear = 7.05:1, 2nd gear = 3.6:1, 3rd gear = 1.7:1, 4th gear = 1:1. First is a super-low granny gear that will provide an outstanding crawl ratio for slow speed rocky trails. (My crawl ratio will be 65:1, calculated as 1st gear 7.05 x low-range 2.46 x axle gears 3.73.) This will allow me to run 3.73 axle gears for fuel economy and highway cruising without giving up my crawling ability. The SM420 will marry directly to the Buick 225 V6 bellhousing, simplifying installation. This is a very compact transmission at only 10.7" in length, perfect for a short wheelbase flattie, although it's no lightweight with its cast iron construction - it weighs in at 135 lbs! Novak Conversions offers an adapter kit to mate the SM420 to the Model 18 transfer-case. After considering all these factors, my transmission choice became pretty clear. I even found a transmission online in very good condition for $200 (gotta' love eBay!).
The SM420 is a top loaded, top-shifting transmission with the shift mechanism located in the "tower" and top plate. This tranny has a 1 1/8" ten spline input shaft and a ten spline output mainshaft. A common wear item on this transmission is the two pins in the top of the shift tower which hold the shifter in place. These were very worn, so Bill (from ATS) removed them and welded in replacements to keep things tight while shifting.
The front bolt pattern is the consistent GM Muncie/Saginaw four-bolt pattern as found on GM bellhousings and transmissions from the 1940's through the 1990's. The bellhousing from the Dauntless 225 V6 will bolt directly to the transmission.
To the right is the tranny in its original condition. The outside of the case had the expected dirt caked to it, but cleaned up easily. Note the PTO access plate on the near side and the rear yoke that indicates this unit was originally installed in a 2-wheel drive truck. The rear yoke will be removed before installing the transfer-case adapter for the Model 18.
I pulled the top plate to gain access to the internals, then removed the drain plug. With a bucket under the drain hole, I thoroughly cleaned the interior of the case with brake cleaner until it drained clear fluid. I was pleasantly surprised to find the main shaft and gears to be in good condition. The synchros and bearings showed normal wear, so I ordered a rebuild kit to freshen up the small parts and seals while the tranny is out of the vehicle and had ATS Transmission perform the rebuild. After cleaning the outside of the case, I sprayed it with several coats of engine enamel.
The SM420 didn't come with a check/fill hole for the gear oil, meaning it was necessary to remove the PTO access plate entirely. To make ongoing maintenance easier, I removed the plate, drilled a hole at the necessary height, and welded a nut to the inside of the plate. Now it has an easy access hole through which to check the oil level and refill if necessary.
(Sidebar: The 1970 Jeep originally came from the factory with a T-14 3-speed transmission behind the Dauntless V6. It's interesting to note that this setup included an OEM adapter between the transmission and the bellhousing. Because the SM420 bolts directly to the Buick bellhousing, the adapter becomes obsolete. This saves some crucial inches that are needed for the rear driveshaft in such a short wheelbase vehicle.)
When bolting the bellhousing to the transmission, I found that the very front of the top plate didn't quite fit flush against the bellhousing. I used a grinder to remove about 1/8" of material from the bellhousing, which resulted in a tight, flush fit.
When planning the build, I decided to forego the original through-the-floor pedal design and to instead retrofit a late CJ firewall-mounted pedal assembly. There were several reasons for this, including the desire to replace the original single reservoir brake system (with the master cylinder located below the floor) with a late model dual reservoir system, and also to replace the original mechanical clutch linkage with a modern hydraulic clutch and slave cylinder system. The original clutch linkage was prone to binding when off-road due to the flexible frame. This clutch system change required a longer clutch arm and throw-out bearing.
After installing the entire bellhousing/transmission/transfer-case unit into the chassis, I test fit the slave cylinder mounting bracket (also from Novak conversions) and found that the new clutch arm didn't have enough throw to accommodate the slave rod - it hit the back of the bellhousing window (see the pictures below). Luckily, Novak offers an adjustable clutch arm pivot that threads into the OEM location, resolving the fitment issue. For the clutch master cylinder, I went with a universal unit from Speedway Motors. It's high quality and mounted easily to the firewall in alignment with the clutch pedal.
In the pictures that show the internal bellhousing, you can also see that I reinforced the areas surrounding the two lower bellhousing-to-transmission bolt mounting holes with an epoxy putty. I saw what appeared to be the start of hairline cracks in these areas, although it could have just been decades old casting marks. Regardless, it couldn't hurt to reinforce the areas for additional strength.
The original flywheel (and engagement ring teeth for the starter) was in good shape, only requiring a fresh resurfacing on the metal lathe. This is one heavy-weight flywheel, which assists in smoothing out the odd-fire V6 and also contributes to the engine's torque.
I installed an OEM replacement clutch and pressure plate assembly to ensure strong clutch engagement for the Jeep. One item I was missing was the inspection plate that mounts to the lower bellhousing to cover the clutch and flywheel. I searched extensively on the web and ordered a couple of inspection plates that were purported to fit before I finally found The Jeep Guy. He makes correct inspection plates for the Dauntless V6 bellhousing from thick gauge steel, and all of the mounting holes lined up perfectly. It required minor trimming due to bellhousing differences, but otherwise was an exact fit.
Another item that needed to be addressed was mounting the transmission to the frame crossmember. I decided on a heavy-duty urethane mount from Energy Suspension, but because the drivetrain isn't stock I had to measure and fabricate a mounting plate of the correct dimensions to correctly position the weight over the crossmember. I used 1/4" plate for the primary bracket plate to ensure adequate strength.
Overall I'm very pleased with the way the drivetrain fit within the chassis. Everything should have adequate clearance, and should be stout enough to last for years to come.
The original shift lever that came with the transmission had been cut and welded in the middle, and when I tried to fit it with the body tub installed it became clear that a new one would be needed. Due to the transmission's installed position in the frame, the shift tower actually protruded through the tub underneath the dash. Again, I looked to Novak conversions for their 24" one-piece replacement shift lever for the SM420/SM465 transmissions. This stout piece of steel comes completely straight, ready to be custom bent for your application.
To bend it into position, I used my 12-ton hydraulic press and a propane torch to heat a small section and create a bend, test fit the bend to the body tub, and repeat. Multiple times. This was absolutely tedious work, and took several hours to complete but was also surprisingly satisfying. I created a significant bend just above the shift tower to clear the dash, and another further up to clear the center console with cupholder installed. I topped it off with an early-style wooden shift knob positioned at the right height for comfortable shifts. I finished off the lever with several coats of satin black Rustoleum paint.
When it came to the cover for the transmission tunnel, I had to design and fabricate something custom. I had purchased an original CJ-2A transmission shift cover early on in the project, but with the transmission swap and the larger opening in the fiberglass tub, it would have been more effort than it was worth to modify it. So I measured and cut a piece of sheet steel for the top and sides of the transmission tunnel. After cutting the holes for the transmission tower and twin transfer-case shifters, I spot welded the three pieces of sheet steel together, then shaped and smoothed the edges to conform to the tunnel. I measured and drilled the cover for the shifter boots. Once I was satisfied with the fitment, I gave it several coats of black paint.
The finishing touch for the transmission was to install an extended breather hose with a check valve, and route it high up on the firewall. As with the axles, this will provide an extra measure of safety against ingesting water during deep water crossings.