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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.


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.

The last 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.