The Stovebolt Site Forums The Shop Area Engine Shop SM420 HELP???

Hi all, I have a SM420 in my 48 Chevy PU. It has stuck in 4th gear several times, and it usually requires pulling the top cover off, and tapping on the shifter forks to "unstick" the shifter. But this last time, the shifter was free when I pulled of the top cover, so I tried to move the larger main gear at the rear of the trans, and it wouldn't budge. So I carefully tapped it with a hammer, and it freed-up. and slid back by hand. I checked all the others and they all moved forward and back by hand. It is once again working...

Some back ground, I had this trans rebuilt 4 years ago in SoCal, at the cost of nearly $1000. He used many new gears and bearings, and all new synchros, but it wasn't until I drove it from San Diego to WA state pulling another truck, that I first has the sticking issue...

Any ideas to go and check so I don'y get stuck trying to drive home in 4th gear again...?
Thanks in advance, Garry K

Hy Garry K, forth gear is actually not a gear at all in a four speed, rather it is the act of coupling the transmission input shaft to the mainshaft, which takes place at the front [engine end] of the transmission. The gear you moved is the first/reverse sliding gear. Your problem might be in the shift tower, input shaft bearing, forth gear syncro drum, or somewhere else.

Thanks 3B,
What can I do to check out the shift tower. I am getting pretty good with removing and replacing it, but I am not an expert on this thing...
Thanks again for any help, Garry

Hy Garry K, with the shift tower removed make sure it will shift smoothly in and out of all the gear positions especially forth gear, if it doesn't hang up we have to look elsewhere. See if you can locate a maintenance manual for the truck, having that will help us walking you through stuff to check, like the third/forth clutch sleeve, and the condition of the input shaft forth gear clutching teeth.

Gary K, I just finished rebuilding my SM420, using a rebuild kit from Novak and other parts I aquired from some folks on this site. The problem could very well be with the shift tower as 3B stated. There are three shift shafts in the top cover assembly that move the shift forks when you move the shifter. On my SM420 these shift shafts were rusty and did not move very smoothly because of this. The rebuild kits do not contain any of the parts for the shift tower assembly, therefore there's a good chance that the rebuilder did nothing in this area during their rebuild process. I totally tore down my shift tower assembly and ended up replacing one of my shifter shafts and cleaned up the remaining two with a wire wheel. These rods slide within the top cover though very close tolerance holes and a rusted shaft will make the shifting difficult. To dissasemble the shifter shafts you will need to drive out the expansion plugs in the cover and the taper pins holding the shifter forks on the shift shafts. When removing the shift shafts turn them 180 degress and then carefully remove them from the cover, being careful to catch the spring and ball that are used to locate the shift shafts. I also removed the spring loaded reverse shift pin to clean it up as well (mine was quite rusty). I was able to locate the taper pins at my local hardware store and the expansion plugs were found at a local engine rebuilding shop. The NOS shift shaft I obtained was from someone on these boards. Good luck.

Jim

Jim, excellent explanation! Thank you.

Do you have the proper amount of gear oil in it????

3B, Thanks for the explanation. I will see if the weather holds out this weekend to go out and investigate that.
chevyb, yes, it has oil just up to the bottom of the upper fill plug.
I have owned this truck since 1982 when I turned 16. Before that it was my dad's back to 1978, and my grandfather bought it from the state of California at an auction around 1973.
I replaced the trans with a swapmeet find in 1983 during the restoration, and rebuild of the 55 235 that my grandfather had rebuilt and installed after he got the truck.
I have a 55 GMC center section, and an open drive-line rear yoke and rear cover plate to install on the back of the SM420 when I get rid of the torque tube and 4.10 gears. I love to drive my truck, and have put over 35000 miles on it since I restored it in 1984. Unfortunately being in the military, my truck sits for months or years at a time, then I get it out, get all the bugs worked out to drive it for a year or three, then when I go back to sea, it sits...
Right now I can't seam to get it started, as there seems to be no spark... So if I can fix the no spark issue, then I can move it in the garage, and check the trans... :-)
I have a weak spark at the points, but no spark at the wires/plugs. It is converted to 12V, and has a ballast resistor. I disconnected the condensor to make sure that it wasn't grounding out through a bad condensor. Well tomorrow is a new day...
Thanks again for the help, Garry K