I was just blessed with an SM420, which was found at a local swap meet this morning. I couldn't pass it up for $40. Complete set-up from a 3/4 ton, minus the 4-speed floor board section. Now I need to tear into it to see if it is good to use as-is or if it needs anything replaced. Does anyone wise know what specifically to look for? I've been through the 3 speed before, but never a 4 speed. I found it hard to pass up the PowerGlide car rear end with the 3.55:1 gears. I have read that you can swap that into a 1/2 ton truck fairly easy. Or the car 3-speed with the Hurst Mystery Shifter still attached... Thanks everyone!
Diagnosis No need to tear it down , just remove the drain plug & drain it into a clean drain pan , lay it over on a slab of clean cardboard and let it drip dry . There should be brownish gear oil that stinks badly , some bronze colored fine silt and that's about it ~ any steel (magnetic) slivers or chunks means teardown time . other wise , button it up and slap it in , fill it slowly and fully with EP85W-90 gear oil and go drive happy .
Unfortunately, the oil was drained out and the top was off when I purchased it. Methinks I might use some sort of solvent/cleaner to rinse it out and make sure there isn't any organic matter inside, as well as any metal shavings. From what I can see so far, the gear teeth look fine. None are chipped or broken. I'm not seeing any strange wear patterns, either. At this point, I'm just wondering why the top was off...
Cleaning O.K. then ; Get a gallon of gasoline and pour it in , swish it 'round a LOT , then tip it out onto an old white towel.... If *nothing* is in the now dirty gasoline , be suspicious ! , time to tear it apart .
Results... Well, I ran the motor fuel through it, like you suggested Nate. It cleaned a lot of old oil out and some old bug parts, but nothing else. No steel, no brass, nothing. I don't know what to think. With the exception of the top being off, I can be certain that it hasn't been apart. There are too many years of grime on it still. The front bearing seems to be a little loose, but that may be because it has no oil in it. The gear teeth that I can see are all intact. It seems to shift OK, but that is inside the trans by hand. I just don't know what to make of it. I certainly don't want to get it all nicely installed only to find that it is inoperable. I also found a nice crack in the top of the u-joint ball housing, running from the gasket surface forward almost through the whole housing. It is centered at the 12 o'clock position so it shouldn't leak oil, but... Speaking of the u-joint...I am thoroughly confused here. Obviously I can't use the 3/4 ton u-joint that came in it. Do I use the u-joint out of my 3 speed? And, what about the speedo drive gear? Maybe I should have held out for a trans out of a 1/2 ton instead of this 3/4 ton model.
Well ; Don't you have another old SM420 to yank the rear housing off of ? . that crack is a serious thing , you cannot have it brazed , I'd not even want it properly welded . These are REALLY sturdy trannies , I say fix the back end and slap it in but then , I'm not the one humping the 350 # of cast iron.... Don't forget the guide pins ! . The input shaft wiggling is normal , this is why it's so important to ALWAYS remove the clutch and replace the pilot bearing in the end of the crank shaft . easy to do and cheap too , it'll improve the shifting greatly by holding the input shaft in proper alignment . Plus , you'll be able to check the clutch disc , clean it of oil or replace as necessary , clean up the flywheel , I know it's greasy and dusty . The U-Joint can be swapped ~ there are two types , light duty and heavy duty , use what you have if the splines match . It's been a while since I had one apart but , IIRC , the speedo drive gear slips over the splines & you re use it , be sure that front yoke bolt is tight ! . As you're there alone , you have to make the call but , looking for properly fitting parts ,matching hardware and all gaskets etc. as you re assemble it and you should be fine ~ I learned this by doing old farm trucks in the mud , using parts culled from junkers sitting way out in the lower 40 fields... I'm no genius so I believe anyone can do it if they but pay attention & work carefully and slowly , if you see a boo-boo , stop and go back , take it apart as necessary to correct any little thing like leaving the ball housing retainer off (oops) . Remember : if you have a seriously dead part , like the cracked rear housing , Joe has plenty as this is a non wear part and there's no shortage of SM420 Parts lying around ~ ALL old junkyards have several that were taken apart , the Hot Rodders also have them cluttering up their garages , don't be shy to offer $35 for the perfectly good takeout tranny , NO ONE else will ever come calling for it . You can do this ! .
Nope...no spare SM420's here. A spare 3 speed I do have, but unfortunately that won't work. So...I'm going back on the hunt. I don't know if it is just the area, or people around here don't check the internet, but I've had no luck on Craig's List or the local classified board. I found 2 SM420's locally at junk yards, but both were for bigger trucks, so no torque tube. At least the rear housing is small enough to have shipped if need be. As far as the clutch and pilot bushing, they have less than 2000 miles. I changed them out when I installed the rebuilt 235 back in 2005. So...I'm confident that they are good to go. Thanks again for the excellent info as well as the encouragement, Nate!!!
Count us in as one of the ones that have a pile of them cluttering our shop. $35 for an incomplete & locked up beater trans. that is being parted out is about right but.... thats only if you show up in person & haul it away.... You want a 4 speed shipped to your doorstep & things get expensive real quick {along with being a PITA to mess with}. We do have the rear u joint housings for open & closed driveshafts.
DZT-Unless you are in central MO, I'll only be needing a u-joint housing for the enclosed driveline shipped to my door. While you're at it, a 1/2 ton u-joint and the driven speedo gear would be great, too... Let me know what kind $$$ we are talking.
See Now I tolja so ! . Trust Joe , hell do you right and treat you fairly . Maybe send him pictures of what you need , include pix of the casting #'s .
As far as why the top was off... Could have been the previous owner couldn't get the shifter out when tearing down their truck. I know that was the case when I did mine. Didn't inspect the rest real close for cracks cause I was planning on (Sorry Nate) swapping out the powertrain... if your tranny was missing the shifter I may know where it came from ... Course that was 10 years ago Bob
I tried to put the car rear end with the 3.55:1 gears in it from a old Chevy torque tube vehicle in my truck. Unfortunately, the rear is not the right width for the truck and cannot be made to work. Maybe yours will fit, but I think you are going to be disappointed. That's why Patrick's sells those extremely expensive gear set conversions for our trucks. After tbuying the gears and all other parts needed to make the conversion, you're looking at well over $1000.00 especially if you have to have a mechanic install. Or, you could be lucky like Nate and get a converted rear end for peanuts Andy
Andy- I didn't pick up the rear end. A few years ago, maybe, but now it is too big of a project for me. I have a family that demands most of my time. I still find the idea of it intriguing, but in reality a top speed of 65 is fine with me.
