The Stovebolt Site Forums The Shop Area Engine Shop SM420 Shifter Question

I have a SM420 Transmission in a '59 Apache. Since installing the transmission last October I have had 2 cases when the shifter handle disengages from the shifting arms inside the transmission. This occurs when shifting from 2nd going to reverse. The transmission will then "lock up" so you can not get into any gear. The roadside fix I do is take the top of the transmission off and then slide the gears back into neutral by hand and make sure the shifter arms are also in the proper position.

My theory is that the shifter handle might be worn down from use. Here are some pictures
http://www.flickr.com/photos/11159813@N00/141153523/
I included some dimensions if anyone knows what these measurments should be.

Has anyone else had similar problems with a SM420?

Thanks for any help
Adam

looks about right to me, and not worn much. The groves in the sides of the big ball can get very sloppy. I welded mine and re cut them.
Do you have the spring under the cap that keeps the shift lever tight in the cover at the ball socket sort of thing?

I do have a spring under the cap which does require about 10-15 lbs. of force down when installing and removing the shifter, by rotating the cover at the ball socket.

Could this be the wrong shifter for this transmission? The shifter handle does touch the steering wheel when moving the shifter into Reverse, but it still does get into reverse on most occasions. I got the tranny from a junk yard and the tranny and shifter were in 2 different locations.

Thanks
Adam

Your shifter is correct and complete than. I just pulled and measured mine, same thing.
It could be the two pins in the side of the tower that locate the shifter, are they worn badly? They should fit he slots in the shifter with no play idealy, although some is to be expected.
If not that then I would start looking at the shift rails and the slots that the shifter engages in them.

Is it actually locking it in two gears at once, or what exactly happens when it does this? start from there and see what it takes to do it again and that should show you what is wrong now that you know you have the correct shifter.

Thanks for your time and help Grigg.

The pins that locate the shifter fit fairly tightly.

The first time I had the trouble, the transmission was locked in 2nd and the shifter would move around, but would not move into neutral or any other gears. The truck would only go forward in 2nd.

The second time, the transmission seemed to be in 2nd and reverse at the same time. I would let the clutch out, load was placed on the engine, but it would not move forwards or backwards. Again the shifter handle would move around but would not switch gears.

The shift rails do seem to be tight and are positive when shifting in all gears, making a solid clicking sound. This would leave the problem occuring where the slots are where the end of the shifter rests.

Maybe if I am carefull I can avoid the problem. It takes me 15 minutes now to take the top of the transmission off and make the truck drivable again.

Adam

I am no tranny guy, and I may have said this before, but when I was looking to rebuild my SM420, I got some info that might be of interest. A local large tranny rebuilding shop offered a rebuilt exchange, but insisted that you had to have the factory top plate with the shifter forks that matched your tranny. They explained that they were a individual factory setup and if you didn't have the mated top plate, they would not exchange it. Just repeating what I was told.
Steve O

from the looks of the pic looking down into the tower, those grooves that the shifter rides in should be sqaure, looks like thats whats worn. and letting the shifter come out of the rails? floppin around means its out of the rails like you know, if its in a gear when it comes out then you try to shift it, it usually can slide another fork into gear. there should be detents and balls on the rails to keep it from going into two gears at once tho...id say the top cover is worn out more than the shifter itself... when the shifter itself used to wear out we used to build them back up brazing them back into shape,,it would last awhile... also, ive never heard of top cover matched to a transmission..think someone was tryin to sell something..??? ive mixed them with no problems..... and nothing in a manual says that....as long as they are both 420's..the only difference i can think of is maybe the post for emergency brake handle... and of course the rear cover can be different...

I had a jeep transmission(Warner gear T150 3-speed) that had enough wear on the face of the shift fork(where it rides in the slot to engage the synchronizer) to let it pop out of gear. I'd suppose to much wear on the other side of the fork would not pull it out fully. I assume wear of this type comes from operating with insufficient or very dirty gearoil. Only happening on the 2nd to reverse shift sounds like it may be the shift tower. Try another top if you can find one, as I have rebuilt several manual transmissions and never heard that a top was matched to the case.