Disclaimer:  I have finished installing an sm420 in my Land Cruiser and the following is a chronological listing of a few of the things that I have learned in the process.  It is not necessarily accurate or complete and your experience may be different.

1.  Find a good SM420
Sm420s were used in GM truck from 1947 through 1967.  They can be identified by the large round bulge for the reverse gear on the passenger side and the PTO cover on the drivers side.  They can be found in anything from the 10 series 1/2 ton pickup to the 60 series 2 ton trucks, buses and vans.  When you are looking for an sm420, don't overlook these large trucks, vans and buses, as they may have less mileage than a pickup of the same vintage.  In selecting one, first drain the tranny and pull the PTO side cover and check the magnet on the bottom of the case for large chunks of metal.  Any big chunks may be a sign of catastrophic failure and you might want to find another one.  Tiny flakes and fine dust is normal.

After you get your tranny home, it is a good idea to disassemble it and check for excess wear on critical parts.  The sm420 is relatively easy to disassemble and reassemble with a mallet, drift punch and a bearing puller.  Get a copy of the manual for the sm420 that explains the disassembly, inspection and reassembly procedures.  You can download a zipped copy of the scanned pages of this manual from Don Kinzer's web site.  Thanks Don!

 Don Kinzer's sm420 manual

4.  Remove the rear bearing from the output shaft using a bearing puller.
5.  Lift the output shaft out of the case with first, second and third gear still attached. If the needle bearings fall out of the imput gear pilot hole, don't worry, you can fish out all 18 of them after you get the counter shaft out.
6.  Disassemble the main shaft as described in the manual.
7.  Tap the input gear out of the case using a brass drift and a hammer.  Be sure to turn the cut out on the input gear to clear the counter shaft gear.
8.  Tap counter shaft rear bearing out of case.  Use a large drift punch and a hammer to "walk" the bearing out of the case.  I tapped on the rear most gear (first gear) bearing on two teeth at a time.  When the snap ring was about 1/4 inch out, I used the jaws of my gear puller to pry the bearing the rest of the way out of the case.

9.  Remove the rear bearing from the counter shaft with a bearing or gear puller.
10. Remove the counter shaft from the case by rotating it up and pulling it out.  I needed to push the reverse lever all the way forward and pull the front gear of the counter shaft behind the reverse lever to get it out.  I did not remove the reverse idler shaft or disassemble the counter shaft any further.
11.  Clean all parts for inspection.

6.  Inspect the clutch hub for wear.  Because of the design of the clutch gear, the hub wears in the center.  I was concerned by the amount of wear on this part shown in the photo, but several people who are more familiar with this transmission have told me that it is normal.  The wear shown is 0.010 deep on both sides of the teeth.  I believe they were right, because the transmission does not pop out of gear.

Rebuild kits are available, but they only contain the bearings, gaskets, synchro rings and small parts.  If any of the gears or shafts on your transmission is bad, you will need to get another transmission.

My tranny came from a bread delivery van that read 50,000 miles on the odometer and I decided that everything in my tranny was fine and just reassembled it with new gaskets and seals.  It turned out that this was a bad assumption.  Even though the clearances passed the wear limit tests listed in the manual and there was no significant wear evident on the synchro rings, the transmission ground when shifting into 3rd gear.  I took it out and put a rebuild kit in it.  The kit contains new 3rd and 4th gear synchros, all new bearings, new snap rings and gaskets.  I got the kit for $130 from:

Stick Only Transmission Parts
9900 Glenoaks Blvd. Suite 2
Sun Valley, CA 91352
(818) 768-4747
(800) 225-8824
They were very good to deal with and helpful over the phone.
 
 

Reassemble the transmission as outlined in the sm420 manual.  Where the manual calls for a special service tool to reinstall bearings, you can  "walk" the bearing on the shaft using a drift and tapping it with a hammer instead.  Tap alternately on the opposite sides of the bearing.  You may only move it about .002 inch per tap, but it you tap it enough, you will drive it home.  When you install the output shaft rear bearing, you can use the Toyota transfer case input gear or PTO gear to pull the bearing on the shaft.  Install the gear on the splines then install the sm420 output shaft washer and bolt.  When you tighten the bolt, it will pull the gear onto the shaft.

After you reassemble the transmission, it is a good idea to double check the parts diagram and sort through the parts you have replaced to make sure that you haven't forgotten anything.  It is easy to forget about the oil slinger for the countershaft and the main shaft pilot bearing spacer.
 
 

You can determine the date of manufacture of your sm420 by decoding the casting mark on the top of the drivers side.  The code starts with a letter that stands for the month (A-L), followed by two digits for the date and two digits for the year.  In the example below, H3061 means August 30, 1961.

1.  Parts
Get all your parts together.  I got a kit from Mike Smythe (msmythe@iei.net).

 Mike Smythe's adaptor kit.

It is fairly complete for the 2F and contains everything except the front seal for the sm420, the pilot bearing adaptor and throw out bearing adaptor.  The last two items are available from Classic Cruisers of Salida, Co.

http://www.classiccruisers.com/

You will also need a F engine 3 speed bell housing, a female drive shaft slip yoke from a 3 speed and a GM clutch disk.  I found that the chevy 11 inch clutch disk worked perfectly with my 2F flywheel and Centerforce pressure plate.  In fact, I could not measure any difference between the GM clutch disk and the Centerforce Toyota clutch disk.  I went ahead and used the GM disk anyway.  The diameter of the disk and the dimension of the splines were the same as far as I could tell with my micrometer.

2.  First bolt the output flange and front U-joint together and grind down the points on the flange to the diameter of the lock washers on the nuts.

 

3.  Fit the transfer case to the sm420 and grind the reverse hump of the tranny to clear the 3speed output flange.  Temporarily bolt the adaptor plate to the back of the sm420 aqnd then bolt the transfer case to the adaptor plate.  Then install the flange/ U-joint assembly on the front output shaft.  It should be obvious where the flange hits the bump.

 
 
 

4.  Mark the area where the flange hits the hump with a pencil.  Slide the output flange off.  Cover the front output shaft of the transfer case and grind away the transmission hump in the marked area with an angle grinder.

 
 

5.  Slide the output flange assembly back on and check the clearance.  You want about .050 inch clearange between the tranny and the output flange.  I needed to grind about 1/8 inch of the tranny hump to obtain the correct clearance.  It is a good idea to take out the drain plug to get an idea of how thick the casting is at this point.  They seem to be about 3/8 inch thick.

 
 

6.  Make the special lock washer to retain the input gear of the Toyota 3speed transfer case gear onto the output shaft of the sm420.  Center punch one of the raised teeth on the original washer for the sm420 output shaft as shown in the picuture below.  You will need to drill a 3/16 inch hole in the tooth and make sure that the hole is centered so it is contained completely within the tooth as shown in the second picture below.  Drill a matching 3/16 hole through a "fender washer" installed on the sm420 washer.  Drive a 3/16 x 3/4 inch roll pin into the two holes as shown in the third picture below.


 

When it is installed on the sm420, the roll pin will fit into the input gear on the transfer case as shown in the picture below.

 

7.  Trim the lower stud that screws into the transfer case at about the 7 o'clock position so that it is flush with the front side of the plate and does not protrude into the cut out for the sm420 lower bearing.

 
 

8.  Install the two studs at 7 and 9 o'clock with red Locktight and tighten to about 25 ft pounds.  You can double nut the ends of the studs to tighten them.  Install the gasket for the rear bearing retainer for the sm420 and then install the plate.  Install the Toyota washer #90201-35070 onto the sm420 output shaft and then slide the N307LOE bearing on to the shaft and gently tap it into the recess on the plate.  Install the 8 bolts and lock washers using blue Locktight as a thread sealer and torque the bolts to 25 ft-lbs.

 
 

9.  Install the transfer case onto the adaptor plate.  Make sure the seal is installed.  (The spring side of the seal faces toward the transmission).  Install the Toyota gasket between the transfer case and plate.  Install the transfer case and the 3 bolts and 2 self locking nuts on to the studs.  Torque the bolts to 45 ft-lbs and the nuts to 30 ft-lbs.  Insert the input gear through the PTO port on the transfer case and slide it on to the sm420 putput shaft.  I bought a new 1/2 inch x 20 grade 8 bolt for the end of the sm420 main shaft that is 1/2 inch longer than the original one.  Install the special locking washer and the bolt and torque it to 100 ft-lbs with blue Locktite.  Then bend the edge of the fender washer over the head of the bolt to lock it in place.

So how do you keep the gear from turning while you are trying to torque that bolt to 100 ft-lbs?  It is easy, you just remove the top cover of the tranny and engage two gears simultaneously, like 1st and 4th.  I stuck my tape measure in to keep 1st gear from sliding out.

10. Next step is to fabricate the spacer that goes over the front bearing retainer of the sm420 so that the Toyota throw out bearing will fit properly.  The OD needs to be 38 mm, which is very close to 1.5 inch, while the OD of the bearing retainer is close to 1 3/8 inch.  1.5 inch 16 ga tube is a little too big OD, so I cut a slot lengthwise in the tube.  16 ga is a little too thick of a wall so I filed it out slightly with a half round file until the Toyota bearing slides easily on the spacer.  18 ga tube may work without filing.  If you buy one, you won't have to fool with this.

After fitting the spacer, I soldered it to the sm420 bearing using acid flux, plumber's solder and a propane torch.  JB weld would probably work just as well.  I installed the new seal in the bearing retainer after soldering the spacer.  I then installed the index ring around the bearing retainer by heating it slightly and tapping it on.  Test the fit of the bearing retainer in the 3speed bellhousing before you install it.  I retained the index ring on the bearing retainer simply by using flat washers under the 4 bolts that attach it to the transmission.  Now it is time to button up the transmission and transfer case assembly and get ready to install it.

11.  You will need to fabricate linkage for the transfer case.  After looking at what other people have done and staring at my OEM 4 speed linkage parts and the pieces of steel in my junk pile, I decided to fabricate a new 2wd/4wd shift lever and use my original 4speed shift lever and linkage to work the hi/lo output selector arm.   bolted an adaptor plate to the side of the sm420 using the threaded holes on the side of the transmission and washers to space the plate out about 9/16 inch from the tranny.  I welded a 5/8 x 6 inch bolt to the adaptor plate to use as a pivot shaft for the lever.  Then I fabricated a bell crank with a lower arm that operates the 2wd/4wd fork on the transfer case and an upper arm that the Toyota shift lever bolts to.  The Toyota shift lever operates the OEM 4 speed hi/lo arm on top of the transfer case through the modified original 4sp link rod. I heated and bent the original 4sp Hi/Lo linkage arm to fit.  I The final linkage looks like this:

The blueprints for the linkage are shown here half size.  If you want to use it as a template, be sure to enlage it until there is 4 inches between the calibration marks at the top of the diagram.  If you would like to buy this linkage adaptor, email me at: cglabe@uci.edu

 

12.  Remove the old transmission and transfer case assembly, clutch, flywheel and bell housing.  Remove the original Toyota 2F pilot bearing.  Drive the pilot bushing into the hole in the crankshaft.  If the cam freeze plug or the rear main seal show evidence of leaking oil, now is the time to replace them.
 

12.  Next install the bellhousing, flywheel and clutch.  Make sure you use a clutch pilot so that the disk splines and pilot bearing are centered.  Lube the splines of the clutch disk with high temp antisieze.  Also put some antisieze grease on the inside of the hub of the throw out bearing.  Install the clutch fork, slave sylinder, linkage and throwout bearing.  Next will be to install the sm420/transfer case assembly.  I removed the transmission hump and seats so that I would have more room to work, so I lifted the transmission assembly up through the cab using a shop crane or cherry picker.

Make sure that the front face of the transmission is parallel with the face of the bellhousing before you try sliding the input shaft in.  You can use a tape measure to make sure that each mounting tab is the same distance from the bell housing.  It won't go in if it is crooked.  A small jack on the back of the transfer case can help level it out.  Gently rock the transmission while turning the output shaft until the splines engage and the nose of the transmission engages the pilot bushing.  Once it is in, install the bolts and tighten them up.  I noticed that the Toyota M12 x 1.25 bolts for the 4 speed do not tighten up properly in the top holes of the 3sp bellhousing.  Instead, I found that 1/2 inch x 20 SAE bolts (the same thread as on the output shaft of the sm420) work fine.  Install the transfer case linkage.  Reinstall the transmission hump, drive shafts, seats and all the othe little things.  Fill up the transmission and transfer case with gear lube.  You will need to either replace the sm420 shifter with a Toyota 3speed shift lever or bend the sm420 shift lever to clear the heater and dash.  I cut the sm420 lever to the same length as the Toyota 4sp shift lever.  The Toyota knob fits on to the sm420 lever.

Happy crawling with your granny tranny!

Special thanks to Mike Smythe and Jim Russell for the help and support!