SM420 Install-Shade Tree Mechanic’s Version

There are a number of sites that describe the actual procedure involved in installing an SM420 into an FJ40 with either the F or 2F engine, so I thought I’d try to highlight real world issues with this project, and discuss some of the solutions to these issues. The install took place during Memorial Day weekend 2002 on my relatively stock ‘71FJ40 with my good friend and fellow shade tree mechanic Billy Atkinson.

Before installing or even thinking about an SM420, ask yourself this question:

Do I want to give up a smooth Toyota Transmission for one that belongs in a Strohman’s bread truck?

I’ll start with the acquisition of the SM420 itself. Everyone knows that the SM420 was used in GM trucks from ½ ton -2.5 ton trucks from 1947-1967. Junkyards that specialize in classic autos are more apt to have these lying around, in fact I literally tripped over one at the yard I purchased mine at. Get to know the guys that run the yard, sometimes they know complete histories of some of the autos they have in stock. The SM420 I picked up was out of a 1972 Chevy 2wd pickup, in which the 420 replaced the automatic transmission that was in the truck from the factory. The truck was owned by the yard owner for most of its life, and he knew everything about that transmission and suggested it should make a good candidate. Junkyard scrounging can oftentimes be fun, and truly a pain other times. I suggest bringing power tools, especially reciprocating saws and grinders. I brought along an abbreviated assortment of hand tools, a propane torch for heat, PB Blaster for cutting rust, Sawzall, highlift Jack, grinder, pry bar, and a small rented generator to run the power tools. I also brought along a friend, because with the SM420 weighing in at 135 lbs, I wouldn’t be able to carry the thing by myself out of the yard. Once the transmission is out, take off the inspection plate on the side and inspect the gears and shafts. Check the quality of the gear oil itself, noting any foreign substances in the oil, such as water and/or particulates such as metal chips and chunks. Check the magnet in the base of the transmission for metal filings, chips, gear teeth, etc. Spin the gears and check the meshing and gear engagement. Check gears for any pitting, galling, missing teeth, anything out of the ordinary. Other than shafts and gears, the rest can be rebuilt, which is what I did.

I purchased a rebuild kit from Novak, a company out west that specializes in Jeep conversions. The kit consisted of all the bearings, seals, gaskets, and synchro rings to restore the transmission to perfect operating condition, price of the kit-$200. Before ordering the rebuild kit, make sure you know which variation of the three SM420’s that were offered are in your possession. I have heard of horror stories involving putting in the wrong synchros and experiencing harsh shifting only because the wrong year kit was purchased. The tranny I purchased was of the later variations because it had two distinct horizontal ribs on the fill plug side of the case. I will discuss these ribs later in the discussion. Another thing I learned, try not to decipher the date code, because sometimes GM or Muncie for that matter, will leave off the designator for the decade. What I thought was a June 10, 194? turned out to be a June 10, 1964. This may be why some people order the wrong rebuild parts. SM420’s are rather easy to rebuild, even for the novice shadetree mechanic such as myself, but I opted for the professional rebuild due to time constraints. I found a tranny shop locally in Newfield called Sure Shift, and dropped off both my SM420 and my Toyota early Tcase for rebuilding. I purchased the small parts rebuild kit for the Tcase from Specter Off-Road for $165. Rebuild charge for the Trans and Tcase together was $200, not bad for the piece of mind knowing it is put together correctly.

Now comes the other parts for the installation of the SM420 behind a stock F engine. An adapter plate was purchased by Mike Smythe or SpaceGhost on the LCML, which came with an assortment of hardware, a bearing, gaskets and seals for $200 or so. Not bad considering the machining time involved with the plate itself. Mike was a delight to work with, offered some tips during the process, and answered a few questions I had. That kit takes care of the tranny/tcase junction, now you need the front kit, and all the clutch parts. I bought an 11” GM 10 spline clutch disk for a ’75 Chevy application from Napa for $45 or so, found out they don’t sell the cheap plastic alignment tools, but ended up finding one at work. I purchased a “Heavy Duty” pressure plate from Spector, although I’m not sure what’s so Heavy Duty about it, it looks like my original one to me($117). I picked up the front kit which consists of a sleeve for the tranny nose, the pilot bearing spacer, and the pilot bearing from Warden’s Auto for $65. Another point to mention, DO NOT expect a quick delivery or short lead time from an auto shop in Placerville at the height of the off-road season. Danny Warden is great to work with as well, he was just severely swamped at the moment, so be prepared for wait time and order parts early. It’s also a good idea to replace the entire flywheel and pressure plate hardware, especially if you haven’t torn into the clutch in some time. Check the rear main seal, throwout bearing, fork, fork boot, and all the other clutch components for wear and replace if necessary. I picked up a new Speedo cable while I was at it because I haven’t had a working Speedo in years, thought it would be nice.

Now you have all the parts in front of you, hopefully on a work bench in your shop or garage, and not the tail gate of your pickup at the apartment. Let’s just say, the back of my truck is completely covered in gear oilJ. Ahh, this brings me to some good points about apartment dwellings, and their inherent issues with auto repairs. Luckily I had a car port to work under which provided shade on sunny days, and shelter from the rain. There were also rafters in the car port that can be quite advantageous when one can’t find a friend with a cherry picker or shop crane. Lifting the transmission and Tcase combo in an out of the truck through the floor with tranny cover removed is the way to go. I couldn’t take the hardtop off, but that would make it much simpler. The setup involved running a tow strap from the rafters, through the door openings, across the top of the cab. From there I attached a come-a-long, or cable hand winch from the straps down to the chain wrapped around the Tranny/Tcase combo. This put some stress on the hard top, but worked like a charm otherwise. I had everything available, except the come-a-long which I purchased at the hardware store for $35.

Adapter plate install went according to the procedure indicated on other sites, such as ExtremeBends.com and others. Pretty basic, attach the plate, torque the bolts, put in the gaskets, make the “special” bolt with the washer, spacer, and spring pin, yada yada. I should put emphasis on making sure ALL GASKETS ARE IN PLACE before bolting the thing together. I woke up two days later after the install from a really bad dream that I forgot the Tcase gasket, and wouldn’t you know, we really did forget that gasket. We had to bolt the Tcase on and off so many times to check clearances between the front flanges and tranny case, that we simply forgot the gasket on the last time, oh well. That brings me to another point, tranny grinding! Don’t be afraid to grind a little off the side, with the thickness of the casing, there is a lot there that can be taken off. I did grind the corners of the flanges, but put emphasis on the tranny. Remember those two horizontal ribs that helped clearly identify the age of my transmission? One rib has to be completely grinded off to provide clearance for the flanges. Grind until there is sufficient clearance, .050” or so. Once everything is bolted together, we’re ready to install. Again, remember all the gaskets.

The clutch is very accessible from inside the truck as one locates oneself in the hole in the floor boards, looking straight into the bell housing. I should clarify, a somewhat thinner person such as myself may fit in the hole in the floor and leave adequate room to maneuver. Warden’s pilot kit goes in first, pounding in the spacer and bearing until it’s fully seated. Mine took a lot more force than should have to seat, I’m guessing Danny was a little on the high side of tolerances for the spacer, because I pounded like no tomorrow on that metal ring. Clutch assembly provided yet another real life issue that required true shade tree mechanic skills to remedy. The clutch disk would not sit flush on the flywheel, rather interfered somewhat on the springs along the inner circumference of the disk. I had Napa resurface the flywheel, but never did check to see if it was still in spec. I’m guessing that over the course of the flywheel’s life, a little too much material is missing, which put the disk closer to the flywheel bolts. This is nothing a grinder can’t fix. I proceeded to grind down the heads of the flywheel bolts until the disk springs had clearance. There was still a slight catch, but it is so slight that once I start the rig, I’m sure they will wear down. I did make sure that I grinded the bolt heads flat, and left enough material to catch with a socket or wrench. Clutch assembly went rather smooth after that.

Now the tranny/tcase combo is ready to go in. It’s a heavy unit, so if you are many beers into the day, wait until tomorrow to do this step. There are a few things one should consider in this step, and it all revolves around the tubular cross member in the FJ40’s and the parking break drum on the Tcase. I suggest putting together the brake before bolting the unit in place, because once bolted in, it is difficult to pull the drum off due to binding on the cross member. We did not put the brake cable in when we raised the unit into place, but was able to get the drum off by slightly tipping the unit down and towards the rear. I suppose one could also grind a little off the cross member to provide clearance for the drum. I didn’t want to service my brake for quite some time, so I threw in some new shoes. There is another technique, which I found out later due to my dream about the missing gasket. To replace the gasket, I supported the Tcase from above with the tow straps and chain come-a-long, and unbolted the Tcase. Once the tcase was free, I lowered slightly to separate the tcase from the tranny and put the gasket in place, and bolted everything back up. This led me to believe that it is easier to install the transmission first, and then bolt the Tcase on with the transmission in the truck. This would also help with the cross member, and will also have to be the technique to service the E-brake. I did not, and don’t think I would ever cut out the cross member, but rather work around it. We finished the basic install during the wee hours of the night, the details would have to wait until morning.

Thing of Beauty

Many people complain about vacuum actuated transfer cases, but that day I loved it. I did not have to make a new linkage, but simply bought longer vacuum hoses from Napa. I could not find the stock thick walled hose, but I don’t think that the new hoses can be collapsed. I will need to verify when I debug the project. I did need to lengthen the shifter rod that actuates the high/neutral/low switch on the Tcase. A couple extra inches in the rod is all you need, as there is some adjustability in the rod end. I ended up cutting the rod, and welding in 1.75” of steel to increase the length of the rod. With the adjustable end, the linkage fit perfectly.

Now you are ready to test drive, right? Well, most likely your driveshafts are not right, as in the case with mine. The front was too short, and the rear was too long. I measured for new shaft length by measuring the distance between the U-joint centers, and dropped off the driveshafts at Fleet Pride (Universal Joint Sales) in Watertown NY.

The driveshafts were modified at the tune of $90 total, and fit like a charm in the cruiser. Once the driveshafts were in place, it was time for the official first test drive. I fired the cruiser up, and the first thing I noticed was some strange noises coming from the clutch area, which I chalked up to the springs on the clutch disk making slight contact with the flywheel bolt heads. No big deal, I figure they’ll wear in eventually. I took the cruiser for a test drive around the block, and I must say, the SM420 sure is not the quietest transmission in the world. It’s not really that smooth either, but it shifted with no problem between all gears. After a few trips around the block, it was time to test 4wheel granny low, I mean, that’s the whole reason for making this swap. I popped it in first, jumped out of the vehicle as it slowly made its way down the driveway, looked underneath the rig to make sure everything was spinning, then jumped back in. 67:1 is very low, and it idles somewhere around 1mph. Incredible.

I was somewhat in a time crunch, in that I was still finishing details a couple of weeks before the Coal Mine Cruiser Classic 2002 down in PA. Finishing details included putting the transmission cover back on, as well as the skid plate. I’ll point out now, because I never heard anyone else complain of this, but neither the cover nor the skid plate fit real well. In fact, I had to cut sections out of the skid plate, because the Tcase was lowered and moved back a few inches. The skid plate now looks like Swiss cheese, so I might be making another one soon. The tranny cover was cut by a PO when they switched the column shift to floor shift, and just as I figured, the hole wasn’t really in the correct spot. So I butchered a hole in the cover with a Sawzall. It’s not the cleanest, but it worked, and the hole is covered by vinyl matting.

So the initial test drives seemed to work out alright, with just a few weeks to go before the Coal Mine Classic in PA. Off-road testing was not able to be done before going to PA like I had wished, so the real test was at the coal mine. Once down there, the transmission performed way beyond my expectations. Rock trails that I had some difficulty with last year, were much easier this year with that granny low. I should also point out that my soon to be wife likes to drive, but not in rocks. Once she figured out that granny low does all the work in the rocks, she had a blast driving it. Soon after I installed the SM420, I switched from 31” tires to 33” tires, and I think for now I’ve found the perfect gear ratio for me. I would definitely recommend this swap for anyone out there searching for low gears, because it’s relatively easy to pull off for a true shadetree mechanic.

Next cruiser project reported from a true shadetree mechanic point of view may be power steering for the old boy.