SM420 folks - need your help

[HunterMcC1]

I have been running a 1958 SM420 mated to the F motor and a 3sp t-case. I just got done putting a new t-case in the truck and added a brace to the back of the t-case also. Tires all move freely through all gears with no resistance. Put the truck on the lift to check it again with motor running and now the tires will not spin with out some serious rpms on the motor. took it out of 4WD and the front spins freely but the rear tires are stuck like they are between a gear tooth, they will move about an inch forward or back and then stop. Like I said, they will move but the motor has to do some serious work. I took the top cover of the tranny off because it seemed to be the source of the problem, and hooray the tires were free again. Here is my question: has this problem occurred to anyone else? what was the fix (this is my 2nd top cover by the way)? does anyone have a picture of the sleeves inside the transmission in the neutral position?

The old top cover left the transmission stuck in 3rd gear so instead of getting new parts I replaced it. Cover is from a late 60's transmission (has reverse switch), so is there a chance that this one is some how worn out?

There are still a few things left to rule out, but the reason the truck came into the shop was because I was testing out my new front axle parts and hopped the truck through a snow bank. Heard a clunk and then it was very hard to move the tires...

any help would be great, thanks guys!!

PS - I also checked the t-case for broken or offset parts, everything is well...

 

[Pin_Head]

Hard to tell what is going on from your description, but if it worked fine before you changed transfer cases and the top cover, then chances are that something didn't get reassembled correctly.

The shift covers are not interchangeable between early and late transmissions and I'm not sure what the cutoff date is. It should be obvious where the neutral position is. It is in the mid range of their slide and you can verify it by rotating the main shaft with the transfer case in neutral.

 

[KillerPea]

If you are only having issues on the lift, it sounds like your u joint for the rear shaft is binding. Did you bend the leaf springs?? Try putting the truck on jackstands and see if it still binds. BTW....the sm420 sticking in 3rd is common. You can remedy it by welding/grinding the 3-4 shift fork.

 

[Poser]

BTW....the sm420 sticking in 3rd is common. You can remedy it by welding/grinding the 3-4 shift fork.

Common?


Is there a thread/link/pictures about this welding/grinding?


I have owned and operated a number of SM420 trucks from Chevy/GMC Jeep and Toyotas with 420's in them and the only time I have run into the 'sticking in third gear' issue is when the top cover is not original to the transmission...


The trans in my Red truck right now will stick in third, and has me thinking about spending the money on a built unit from Novak, or, swapping out the manual all together, and installing a RamJet and 700R in front of the Atlas.....

 

[hondatech81]

my sm420 sticks in gear once it gets to operating temp. any suggestions or does it need a rebuild?

 

[Tigerstripe40]

My SM420 shifts every bit as nicely as my H42 did.
:dunno:

 

[fyresq]

x2 on the top covers not being interchangable. I had this problem. They seem to be fine when you put them on, but IIRC the shift forks are (engage) a little different. I would check there first.......

If you have the ability to get a later gearbox that's the route I would go and get rid of the older oil slinger. Where are you getting the top covers from? There are also places out there that sell new old stock 420's (like Novak).

 

[HunterMcC1]

top covers came from a junk yard where there is still a "stash" of other 420s, I still have the original top cover, I may try this weld/grind trick... did someone mention pictures of this??

 

[dx379]

BTW....the sm420 sticking in 3rd is common. You can remedy it by welding/grinding the 3-4 shift fork.

Common?


Is there a thread/link/pictures about this welding/grinding?


I have owned and operated a number of SM420 trucks from Chevy/GMC Jeep and Toyotas with 420's in them and the only time I have run into the 'sticking in third gear' issue is when the top cover is not original to the transmission...


The trans in my Red truck right now will stick in third, and has me thinking about spending the money on a built unit from Novak, or, swapping out the manual all together, and installing a RamJet and 700R in front of the Atlas.....

I have recently run into this 3rd hear problem myself. Out of nowhere, I can't get it to shift out of 3rd Wasn't sure if there was something that could be done about it. I also have 4th gear synchro problems so considering a rebuild, or just buying another used one without issues. I currently have the late 420 with the seal...

Any thoughts?

 

[I Tried]

Common?


Is there a thread/link/pictures about this welding/grinding?


I have owned and operated a number of SM420 trucks from Chevy/GMC Jeep and Toyotas with 420's in them and the only time I have run into the 'sticking in third gear' issue is when the top cover is not original to the transmission...


The trans in my Red truck right now will stick in third, and has me thinking about spending the money on a built unit from Novak, or, swapping out the manual all together, and installing a RamJet and 700R in front of the Atlas.....

Mine does the same thin, Just shift it like a grain truck and it works fine.

 

[72RatCamaro]

Yeah mine hangs in 4th everynow and then but I can usually beat it into submission without to much problems.

 

[dx379]

Mine does the same thin, Just shift it like a grain truck and it works fine.

LOL - define " shift it like a grain truck" ?

Yeah mine hangs in 4th everynow and then but I can usually beat it into submission without to much problems.

So you just wrestle it out?
Jonny

 

[Pin_Head]

Sticking in 3rd gear may be like a problem with the shift gates in the top cover. Maybe of the other gates (1-2 or R) is not in the N position and locking out the 3-4 shift rod. If it is not in the top cover, it won't be easy or cheap to fix. Novak has a list on their site of the cut off dates for the different transmissions and I would bet that the top covers fall in the same range. Each cover has a cast in date code.

 

[I Tried]

LOL - define " shift it like a grain truck" ?

Take your time and don't drive it like it is a drag car and I do not have problems with my 420. It is when I am trying to row the gears fast when I get stuck in 3rd.

 

[72RatCamaro]

yeah just hit it from the rear to the frt until it pops out.....if I don't get it out I have to take off in 4th which doesn't work.

 

[KillerPea]

Grind and weld on the bottom of the shifter so it engages 1st and 3rd less, second and 4th more. Do it in increments until it stops sticking and popping out of gear. As I remember, there was and updated shift fork that addressed the issue and there were even service bulletins on it way back. The shift fork engages 3rd too deeply and 4th shallow. So, you probably have an early shift tower that never got updated.
I can't find the bulletin anymore, and to the best of my knowledge, the parts arean't available. Either modify your shifter so it works better most of the time. or start tranmission tops.

 

[I Tried]

I will take a look at it. Where do you weld? Have any pictures of what to do?

 

[Poser]

I will take a look at it. Where do you weld? Have any pictures of what to do?

The tip of the cane where it engages the shifting gates.

Look at it; you will get it.