The Stovebolt Site Forums The Shop Area Engine Shop More SM420 questions

Are all SM420s interchangeable? Specifically, can I take the main and clutch shafts (with all their gears and synchros in place) from a '49 unit and swap it into a '52 case (keeping the idler and reverse idler gears and shafts in place)?

And what's the difference between the 50-60 models and the 10-30 models? How can you tell which one you have?

Thanks,
John

John, I'm no expert on them, but they did slowly evolve through the years, I think there wasn't much that changed until around 1960. As far as I can tell, the same SM420 was used for all series.

Maybe bringing this back to the top will get someone who knows for sure to respond.

Thanks for the reply, Fred.

I sure have been getting an education on the SM420 this past week. And Tony P has been coaching me through this.

I ordered my gasket set and a new rear seal from Stick Only Transmission Parts waaaaay out there in California. The guy there said there are some differences in the gearsets between the earlier ones and the later ones. So if you go changing out gear sets (as I am doing) the main thing you have to check is the measurements on the 3rd and 4th speedclutch sleeve and the 1st/reverse gear where the shifter forks ride in them -- the "Dog Width." You can match casting numbers on the forks OR (I like this one better) just swap out the entire top cover, shifter assembly and all, along with the associated gearset.

Other than that, he said there wasn't much difference. The gut really impressed me, though. Either he had some sort of reference open in front of him right when I called, or he has the 420 memorized as he just rattled off shifter fork casting numbers to me (not just case casting numbers, but numbers inside the tranny). I was impressed.

Of course, I'm easily impressed... He could have been making the stuff up for all I know about it.

I will be writing up a Tech Tip about this. You experienced guys probably think nothing of tearing into a 420, but it was quite an "adventure" for me and probably is just as intimidating to others like me.

Regards,
John

John,

I had what you would consider the worlds worse looking SM-420. I dissembled, cleaned, glass bead blasted and reassembled it. All new bearings and gaskets I will forward the pictures to you when I get then scanned into a jpeg format this week end. Wait to you see this.

You got the mounted pictures?

Jake

Since I'm a relative newbie to this area, I'll ask some newbie questions.

Is the SM-420 the designation for the "granny 4-speed?"

Is that the usual suspect for what I'd have in my 64 C10 with 292 I6?

And is there supposed to be a dash in there or is it SM420...or does anybody really care.

Thanks,

Here is what I know about the General Motors SM-420 4-speed , and yes this is that so called "Granny" trans, see my webpage for ratios.

Thanks, Jolly. That's a whole lotta info there. Real good stuff. And now I know what to call my transmission. How bout dat?

Thanks again,

Jake, We got the pictures! We're still catching up on our correspondence after Christmas ...

Okay fellers -- I successfully got the new bearings on the main shaft and got it reinstalled in the case (piece of cake). That was about 3 or 4 pm. Since then, we've (Peggy was in the shop with me ) been struggling with the clutch shaft. I can get the clutch gear to clear the countershaft without dumping all 18 mainshaft pilot roller bearings. The trouble seems to be with the clutch gear synchronizer. In reinstalling the clutch shaft, I seem to get it in 3rd and 4th gear at the same time locking everything up. I think the trouble is either in how I'm reinstalling the clutch synchro or the three clutch keys inside the 3rd and 4th gear clutch hub. What's the trick?

About 30 minutes ago I got frustrated, put all the tools/weapons down, shut off the lights and came in. Oh yeah, the dogs and I went for a walk in the falling snow.

Please oh please wisemen of 420's -- what's the trick I'm missing in getting that clutch shaft back in????

Thanks,
John

In this life there are two kinds of men -- mechanics and parts changers -- I'm a parts changer.

I don't know if this will help or not, but their should be a retainer for those 18 roller bearings. It will be at the very foward most spot in the clutch shaft I found it while thoroughly cleaning the area where the rollers are. It is meant to hold the rollers in place as you install the mainshaft and then slides foward and off the rollers after assembly. Then it just sits there. If any of the rollers are out of place the whole mainshaft will not be in the right spot. This may or may not be your trouble but worth a look.

John check this link, it might help
http://members.cox.net/cglabe1/SM420/sm420.html

I had a problem to get the clutch shaft in when I did it. Lots of heavy grease to hold the needl bearing in and also I think I remember that I had the gear assembly the wrong way around.

My memory some times fails me, but after dropping the trany on my thumb , my memory cam back, also it gave me a nice blue finger nail.

Jolly has a lot of good info on his transmiision site. However, the SM465 (later called the CH465) was the successor to the SM420. I didn't know GM light trucks were available with the New Process transmissions. My 71 GMC, 72 Chevy, 76 Chevy, 80 Chevy, and 86 Chevy (OK, so I like GM) all had the 465 4 speed as standard equipment. It had the same ratios as the 420 except for 1st gear, which was 6.55 instead of the 7.06 of the 420. I still think the 420 had a crisper shift, although I can't explain why. My 96 has a 4 speed, too, although its one of those....what do you call them....'automatics'. Its...uhh...a Chevy.

Lookie what's on ebay

Synchromesh Overhaul book

Too bad the slides aren't with it!

My neighbor drilled the hole I need in the case and tapped it so I could put the newer rear bearing retainer (that has the mount for the driveshaft PBrake). So today I'm going to try reassembling the original transmission gears and shafts in their original case and see if it works then.

Worst case scenario -- what happens if I decide to run the gear with the three pitted teeth? Is it just noisy or do worse things happen?

Thanks,
John

Pitted gear teeth are questionable, but not the end of the world in most cases. As long as you aren't trying to power-shift or go rock crawling you'll probably be okay. They can cause a stress riser under heavy load with constant use, but as long as the pits aren't real deep you should be okay with the service most of these trucks see.