Just like the ***le, the sm420 in my 48 chevy 3600 won't shift into 3rd or 4th. I just got the truck so I have no idea how long it's been this way. It goes into 1st, 2nd and reverse with no problem. I pulled the shifter out and looked in the hole, all I see is a sliding plate that seems to glide left to right with no trouble using a screwdriver. The two little pins that hold the shifter "cup"/shifter hold down are chewed up pretty good. I can watch the pins walk on the outside of the shift tower while I move the shifter side to side, the spring doesn't do much I don't have to push the cup down hardly, just twist and the shifter pulls right out. Any ideas?
below the sliding plate are the notches that the end of the lever fits into. Are the notches all in line when it's in neutral? there should be three of them, one for R, one for 1-2, one for 3-4.
A lot of things can go wrong.You can try using a pry bar to shift the 3rd and fourth gears without the stick. I suspect the 3-4 syncro ***embly is broken preventing the slider from moving.
I still can't figure this thing out. Will the locating pins on the top of the tower make it not shift if they are worn out? These aren't even round, and wiggle in the holes really bad.
they shouldn't make it not shift. Does the lever move to the 3/4 gate? and you just can't move it up or down? Or does it not move over past the 1/2 gate? If you describe exactly what's happening when you try to shift, maybe someone could help you.
It shifts fine in the 1/2 gate, move it over to grab 3rd and it won't go past neutral. Same with 4th gear. Tomorrow I will pull the top plate off the trans in the daylight, truck is outside since my hot rod is in my shop taking up all the space.
The SM-420 was built from 48 through 67 and there are still tens of thousands of them running around and that many more sitting around. A lot of them are pretty well worn out too. The good is that rebuild kits aren't too spendy and most of the internal parts such as syncros and gears are available. https://www.transmissionpartsdistri...-48-67-gm-4-speed-with-10-5-case-1-pto-bk108/ https://store.fillingstation.com/detail/RW-812/Chevrolet_194867_4SPD_TRANSMISSION_OVERHAUL_KIT.html https://www.novak-adapt.com/catalog/transmission/transmission-parts/sm420-par Ae just a few. This is the link to the online version of the 48-51 Chevy truck shop manual but I highly suggest buying a copy. they are on Amazon, Ebay or most of the truck trinket vendors sell them. https://chevy.oldcarmanualproject.com/shop/1948_51truck/index.htm Click on 7 and then on 7-13 and it will take you to the 4 speed transmission index. The 3/4 gate should be in neutral if it shifts into reverse and 1-2 without too much of an issue and moves. I'm thinking badly worn or a broken part.
Taking the top cover off is what you need to do. My brother had something break on his years ago and a junkyard top cover fixed it up. IIRC, it was obvious once the bottom of the cover could be examined.
Going to finish my coffee and head out. Going to pop the cover and I'll get pics if I don't see anything obvious. One thing is, I really don't want to put any dollars in it as I am on the hunt for a T5 anyway. It would be super cool to fix this one for right now though, we'll see!
With the top off you should be able to knock the synchro hub into gear with a tap from a hammer and screwdriver. If you can do that then it’s got to be something in the top cover/gear lever.
Side note: Those transmissions are still popular with the Jeep CJ brigade to use in their crawlers. Meaning that you might have a buddy who has a buddy who knows them pretty well. The service manual pretty well tells how to work on them and the writers didn't ***ume that the guy reading the book had ever seen a transmission before.
OK figured it out. The 3/4 gear wouldn't slide, it was froze up. Penatrating oil and hammer taps got it moving again. Just had to line up the synchros once I got it sliding if I went too far one way. Took it for a drive and have all the gears! Thank you guys for all the help!
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