The Stovebolt Site Forums The Shop Area Engine Shop SM 420 Will not downshift to 3rd...

I seem to be having trouble with the transmission in my truck ('55, 4400, 235 cu. in. 4 speed...)I can shift fine up through the gears, starting in 1st or second all the way to fourth (Double clutching, should this even be required on a synchronized transmission?). All is well till I need to down shift from 4th to 3rd, it will not go unless I manually synchronize the motor apeed and road speed, which I'm not very good at. I can downshift to 2nd if I slow down enough or go back in to 4th and bog the engine way down. I have tried revving the engine up to road speed but I'm not good enough yet I suppose, so that doesn't work. I have to slow down with the shifter in the 3rd gear gate and listen for the gears to synchronize (yes, they are grinding ever so lightly) and when they synch up engage the gears fully. It seems like something is not quite right in there for me to have this kind of trouble downshifting to 3rd gear. Any suggestions on either what could be wrong or how to execute this shft better?

Sounds like either the syncronizers are not working, or you've got a Crashbox.

A Crashbox Four-Speed can be identified by the little flippy lever thing on the shifter that operates the reverse safety lock-out. It's not original to your truck, but many old farmers swapped in whatever they had handy when something broke. (Just ask my grandpa. He's got more broken transmissions hanging out at his place than anyone else I know.)

Does it go from 3rd to 2nd okay, or do you need to double-clutch on that shift too? (Don't worry about not being able to downshift into first. Most people have problems with that.)

Going up you won't need to double-clutch on either transmission, just make sure that if you have a crashbox you wait for the tranny to come around. It WILL take the shifter out of your hand if you're not careful.

My guess would be that the clutch is dragging when it is disengaged. You should have only a small amount of free play in the pedal with the clutch released. If there is too much play at the top, the clutch pedal will not push the throw out bearing in enough and the clutch will drag.

You should not need to double clutch and your truck will have a synchromesh transmission.

Its not a crashbox, its the orginal transmission, If the synchronizers are out, a rebuild is in order correct? I always thought I had to double clutch it on every shift, if I can not shift with out doing this then the synchronizers are in fact toast, right? (I never tried shifting with out double clutching...)

Also I think the clutch is ok, although it makes a slight whinning noise whe the pedal is pushed in, but it always seems to completely disengage from the engine. It nevers puts a load on the engine, nor does it feel like its catching a little bit.

Sounds like your syncronizers are shot if you need to double-clutch to upshift. That's... just not natural.

Yes, you will need to rebuild that puppy.

Before you rebuild that puppy change the gear oil and put some Justin Brothers Friction Modifier in it and drive around for a while, I have had this problem before and some times it is just gummed up. Lucas Products makes a additive for automatics called "Stop Slip" it also works good, add a little to the steering box and you will notice the difference in a day or so. You only need about 1/2 pint in the tranny with new gear oil. Drive around a while before you pull the plug to drain the transmission, it will flow better.

asiliverblazer,

A relatively easy chore would be to pull the top cover and inspect the forks and rails. This just might reveal a problem not associated with the gears and synchros.

Stuart

I will definately try new fluid in it, forgot to mention I was thinking of doing that. I would think the forks and rails having a problem wouldn't cause a synchronisation issue, if the gears were synchronizing I could easily move it in gear, because when they are synchronized it shifts fine.

I can test this by getting moving up to about 35 mph or fast enough to get into 4th gear, then putting the clutch in taking it out of 4th into neutral and letting the clutch back out. With the engine idleling and the truck coasting, I put the clutch back in and push the shifter partially in to the 3rd gear gate so that I can barely detect the gears clashing through the feel in the shifter handle and audible noise. As the truck slows the clash gets less and less sever till my road speed matches my idle engine rpm, at which point I slip it fully in to 3rd gear. I know its not good to use the noise the gears make clashing like that to judge my synchronization but its all I got right now, and I'm not really "grinding" them... its done as gently and humanely as possible.

What tools will I need to rebuild the transmission other than a socket set? I have the factory manual but don't know a J-22456 from a J-59846...

After you let the clutch out that first time, goose the engine and then pop the clutch in. It should shift in a little quicker (and quieter).

The way the old pros double-clutch is: they jump on the gas when they're letting the clutch out that first time (like when you shift up) and then they shove it back in again and slide the shifter into gear.

I haven't managed to do it quite as well as them, but I can get it to downshift all the same.

I'm inclined to go wiht a syncronizer problem, too. I've seen SM420s break the syncros for 3rd and 4th gear before. Not too hard to change.
If you do decide to overhaul it, I'd suggest using Lucas Oil Stabilizer as an assembly lube, then run about a 25/75 mix of the stabilizer and Lucas synthetic gear oil in it. I did that wiht my 3 speed overdrive. Made it shift real slick.
PS for the tools, check with OTC or Snap-On. The may list them, and will say in the tool description the equivelant for the factory tool numbers/

I just rebuilt my sm420, still have not got it back in the truck yet. I ask some questions the other day. Got lots of good answers and a couple links, shows all the tools plus has instructions. Check it out, not more then two days or so ago, good luck with yours, Tom.

Sory its in the big bolts section, Tom.

I think I read the thread you mentioned. I will look at those tool sites too, I run into issues quite a bit not knowing what tool my service manuals are talking about...

I drove it last night to test a couple things, I can get to shift with out double clutching but not nearly as smooth or consistant as double clutching especially going from 3rd to 4th. It eventually goes but I have to take a lot of time between the shift.

Tom,
where did you get your rebuild parts from?

I got my rebuild kit from Novak conversions - www.novak-adapt.com Their web site was pretty good - but their techs were of almost no help. You might try Stick only transmission parts, the guys on my post said they were very good and their price was alittle better to - 1 (800) 225 - 8824
As far as tools, you really dont need to many. I have a fairly good amount but mostly for smaller stuff, I work with motorcycles and model T's for fun. Main things you might need snap ring plieres, some pullers - I got el cheapos from China, they worked fine, a big hammer and some good brass punches. They say a press is good and I bought one - but really I think all the parts would of come off with the hammer - though it said in there that the bushings dont take shock very well, better to press them off.
I skimp sometimes on cheap tools, though usually only on ones I will not use to often. I have lots of tools made in the good ol USA, even lots of Snap ons - but no more, cant afford em.

Tom,
thanks for that information and great link.